New dates for landslides reveal past Seattle fault earthquakes

New maps of more than 1,000 deep-seated landslides in the Puget Lowlands of Washington State provide evidence of the last major earthquake along the Seattle Fault about 1,100 years ago -- and may also hold traces of older earthquakes along the fault.

Clusters of landslides offer a potential record of earthquakes, if researchers can determine when the landslides occurred. The new study published in the Bulletin of the Seismological Society of America combines information about the location of these Puget Lowlands landslides along with new dates obtained from measuring the surface roughness of the landslides.

The combination of data helped Erich Herzig of the University of Washington and colleagues uncover strong evidence of the last known major Seattle Fault earthquake, thought to be a magnitude 7 to 7.5 event. (A recent study suggested that there may have even been a double earthquake at the time in the region.)

The researchers compared their new landslide map to ground motions generated by different Seattle Fault earthquake scenarios. The scenario that best matches the landslide clusters, they found, is one that produces the strongest shaking in a west to east band from west Seattle to Mercer Island and the bluffs bordering Puget Sound.

"While other studies have refined our understanding of the overall strength or timing of the 1,100-year-old Seattle Fault earthquake, to our knowledge, this is the first study that has attempted to characterize the locations of strong shaking," Herzig said.

Herzig and colleagues also uncovered other landslide clusters at 4600-4200 years ago, 4000-3800 years ago, 2800 to 2600 years ago, and 2200 to 2000 years ago that could be signs of older Seattle Fault earthquakes.

They began by mapping more than 1,000 deep-seated landslides across the Puget Lowlands -- deep-seated refers to landslides where the slide plane lays below the roots of the trees -- using a technique called airborne lidar.

"Airborne lidar is a technology where a plane mounted with lasers is used to measure the shape of the land surface in detail, even through vegetation," Herzig explained. "In the past few years this technology has been able to produce maps at one meter resolution or better, which is essential for measuring roughness as we do in the paper."

Measuring the roughness of a landslide surface is a relatively new technique used by scientists to estimate the age of a landslide, he noted. The general idea is that ground surfaces roughen after the mass movement of rocks and soil, so that landslide deposits are the roughest right after the landslide occurs and become smoother over time. By modeling this age-roughness relationship, with information from landslides dated by other means such as carbon dating, researchers can estimate when a particular landslide took place.

For the landslides in the BSSA study, Herzig and colleagues calculated roughness by measuring variations in the land surface in a 15-meter-wide circle. Carbon dating of wood at some landslides provided data for calibrating the landslide ages.

The researchers uncovered spatial patterns in the landslides that correlate with the ground motions predicted by models of Seattle Fault earthquakes. They also noted that the timing of landslides in the Puget Lowland fits better with a model of a pulse of landslides at the time of the last major Seattle Fault earthquake, rather than a model of landslides happening steadily through time.

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'Starquakes' could explain mystery signals

Fast radio bursts, or FRBs, are an astronomical mystery, with their exact cause and origins still unconfirmed. These intense bursts of radio energy are invisible to the human eye, but show up brightly on radio telescopes. Previous studies have noted broad similarities between the energy distribution of repeat FRBs, and that of earthquakes and solar flares. However, new research at the University of Tokyo has looked at the time and energy of FRBs and found distinct differences between FRBs and solar flares, but several notable similarities between FRBs and earthquakes. This supports the theory that FRBs are caused by "starquakes" on the surface of neutron stars. This discovery could help us better understand earthquakes, the behavior of high-density matter and aspects of nuclear physics.

The vastness of space holds many mysteries. While some people dream of boldly going where no one has gone before, there is a lot we can learn from the comfort of Earth. Thanks to technological advances, we can explore the surface of Mars, marvel at Saturn's rings and pick up mysterious signals from deep space. Fast radio bursts are hugely powerful, bright bursts of energy which are visible on radio waves. First discovered in 2007, these bursts can travel billions of light years but typically last mere thousandths of a second. It has been estimated that as many as 10,000 FRBs may happen every day if we could observe the whole sky. While the sources of most bursts detected so far appear to emit a one-off event, there are about 50 FRB sources which emit bursts repeatedly.

The cause of FRBs is unknown, but some ideas have been put forward, including that they might even be alien in origin. However, the current prevailing theory is that at least some FRBs are emitted by neutron stars. These stars form when a supergiant star collapses, going from eight times the mass of our sun (on average) to a superdense core only 20-40 kilometers across. Magnetars are neutron stars with extremely strong magnetic fields, and these have been observed to emit FRBs.

"It was theoretically considered that the surface of a magnetar could be experiencing a starquake, an energy release similar to earthquakes on Earth," said Professor Tomonori Totani from the Department of Astronomy at the Graduate School of Science. "Recent observational advances have led to the detection of thousands more FRBs, so we took the opportunity to compare the now large statistical data sets available for FRBs with data from earthquakes and solar flares, to explore possible similarities."

So far, statistical analysis of FRBs has focused on the distribution of wait times between two successive bursts. However, Totani and co-author Yuya Tsuzuki, a graduate student in the same department, point out that calculating only the wait-time distribution does not take into account correlations that might exist across other bursts. So the team decided to calculate correlation across two-dimensional space, analyzing the time and emission energy of nearly 7,000 bursts from three different repeater FRB sources. They then applied the same method to examine the time-energy correlation of earthquakes (using data from Japan) and of solar flares (using records from the Hinode international mission to study the sun), and compared the results of all three phenomena.

Totani and Tsuzuki were surprised that, in contrast to other studies, their analysis showed a striking similarity between FRBs and earthquake data, but a distinct difference between FRBs and solar flares. Totani explained: "The results show notable similarities between FRBs and earthquakes in the following ways: First, the probability of an aftershock occurring for a single event is 10-50%; second, the aftershock occurrence rate decreases with time, as a power of time; third, the aftershock rate is always constant even if the FRB-earthquake activity (mean rate) changes significantly; and fourth, there is no correlation between the energies of the main shock and its aftershock."

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How to distinguish slow and fast earthquakes

Researchers from the University of Tokyo and Stanford University show what differentiates slow and fast earthquakes and how their magnitudes vary with time.

Normally, earthquakes last up to a few minutes and radiate strong seismic waves. But around 23 years ago, scientists discovered an unusual slow-slip phenomena called slow earthquakes. Slow earthquakes last days or even months. Though they involve significant tectonic movement, you may never feel them. Since slow earthquakes could indicate future fast earthquakes, monitoring and understanding them helps accurately forecast devastating earthquakes and tsunamis.

Understanding them requires knowing how they change over time. For that, researchers use scaling laws which define the relationship between two quantities over a wide interval. In 2007, researchers proposed a controversial scaling law relating the magnitude and duration of earthquakes, which can help differentiate slow and fast earthquakes.

According to the scaling law, for slow earthquakes, as its magnitude (measured by a quantity called seismic moment) increases, the duration of the earthquake increases proportionately. For fast earthquakes, the relation is not linearly but cubically proportionate, which means the seismic moment increases very rapidly in a short time.

The scaling law received criticism from other researchers and raised questions about the likelihood of events in between slow and fast earthquakes that do not fall within the law. Seismologists Satoshi Ide of the University of Tokyo and Gregory Beroza of Stanford University now bolster the scaling law with more data, reinterpret the scaling relation, and address the controversy.

"Most of the challenges to the scaling law were problematic, but we have had no chance to disprove their challenges," says Ide. "A surprise was that totally erratic results were published in Nature, and believed by many scientists, who made further problematic numerical models."

With the advent of new seismic detection technology and data accumulated over 16 years, Ide and Beroza now reason that most arguments against the law had improper data calculations and were inconsistent given their data constraints. They suggest the presence of a speed limit to slow earthquakes and reveal physical processes that differentiate slow and fast earthquakes.

Many, but all the same

Since slow earthquakes include phenomena with different frequency bands, they are more diverse than fast earthquakes. They were named differently, such as low-frequency earthquakes, tectonic tremors, very low-frequency earthquakes, and slow slip events. So researchers observing one type of slow earthquake considered other types irrelevant. "Our study confirmed that all these phenomena are mutually connected, or rather regarded as a single phenomenon that radiates various signals," explains Ide.

Slow slips, but not so fast

Slow earthquakes are so subtle and inaccessible that detecting and monitoring them is challenging. Due to the detection bias, only large enough slow earthquakes are observed. That prompted Ide and Beroza to propose an upper limit to the speed of slow earthquakes. Based on that, the duo redefined the 2007 scaling law with the maximum value constraint. As they showed continuous evidence for the scaling law over a broad time scale of less than a second to more than a year, they put an end to the debate.

How are slow and fast earthquakes different?

When Ide's group proposed the scaling law in 2007, they were unsure of what makes these two earthquake types different. Now, with more data and theoretical models, Ide and Beroza show that their scaling differences dictate physical movement processes governing the events. Diffusion processes govern slow earthquakes, whereas seismic wave propagation dictates fast earthquakes. Because of this difference, the magnitude of slow earthquakes cannot be as large as fast earthquakes when the event lasts longer.

"We pointed out that 'diffusion' is important in slow earthquakes, but what is physically diffusing is not well understood," says Ide.

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Ridgecrest faults increasingly sensitive to solid Earth tides before earthquakes

Faults in the Ridgecrest, California area were very sensitive to solid earth tidal stresses in the year and a half before the July 2019 Ridgecrest earthquake sequence, researchers reported at the Seismological Society of America (SSA)'s 2023 Annual Meeting.

"The signal of tidal modulation becomes extremely strong" after 2018, said Eric Beauce of Lamont-Doherty Earth Observatory, who noted that the signal was identified with seismicity that occurred around the faults that broke in the 2019 magnitude 7.1 earthquake.

The link does not mean that tidal stresses -- which are very small compared to other tectonic stresses -- triggered the earthquake, however.

"We don't know if something started to happen in the fault zone, something that is an indicator of the upcoming earthquake," Beauce said. "Maybe that process changed the properties of the crust in a way that made the crust be more sensitive to tidal stresses."

Pulled by the same gravitational forces of sun and moon that create ocean tides, the solid earth also deforms in the same periodic way. People can't feel the changes, but the ground deforms between 10 to 20 centimeters a day.

These solid tides "induce very, very small stress changes in the crust," Beauce explains, "which can induce stress changes in all the faults within the crust."

Although researchers have known about these tiny stress changes for more than a century, it has been difficult to extract their signal from the seismic record, and to determine whether they modulate seismicity.

In the past ten years, however, better earthquake detection and analysis techniques have made it possible to search through earthquake catalogs to find the signal of tidal stresses, Beauce said.

He and his colleagues built a rich, high-resolution earthquake catalog, using machine learning algorithms along with other techniques, for the past decade of microseismicity in the Ridgecrest area. (Microseismicity usually refers to earthquakes of magnitude 2.0 or smaller).

They found that "there is suggestive evidence that peak seismicity happens when tidal stresses are maximum," Beauce said, "but this modulation is weak, and because it is weak, it is only suggested."

Other researchers looking at the 2004 Indian Ocean and 2011 Tohoku megathrust earthquakes have detected an increase in modulation of seismicity connected to tidal stresses, decades before the earthquakes, said Beauce. And some scientists have been able to generate similar results in lab-created earthquake experiments.

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Earthquake scientists have a new tool in the race to find the next big one

An everyday quirk of physics could be an important missing piece in scientists' efforts to predict the world's most powerful earthquakes.

In a study published in the journal Science, researchers at The University of Texas at Austin discovered that a frictional phenomenon could be key to understanding when and how violently faults move. That's because the phenomenon, which explains why it takes more effort to shove a heavy box from a standstill than it does to keep it moving, governs how quickly the fault surfaces bond together, or heal, after an earthquake. A fault that is slow to heal is more likely to move harmlessly, while one that heals quickly is more likely to stick until it breaks in a large, damaging earthquake.

The discovery could be key to understanding when, and how violently, faults move. That alone won't allow scientists to predict when the next big one will strike -- the forces behind large earthquakes are too complex -- but it does give researchers a valuable new way to investigate the causes and potential for a large, damaging earthquake to happen, the authors said.

"The same physics and logic should apply to all different kinds of faults around the world," said the study's co-lead author Demian Saffer, director of the University of Texas Institute for Geophysics at the Jackson School of Geosciences. "With the right samples and field observations we can now start to make testable predictions about how big and how often large seismic slip events might occur on other major faults, like Cascadia in the Pacific Northwest."

To make the discovery, researchers devised a test that combined rocks from a well-studied fault off the coast of New Zealand and a computer model, to successfully calculate that a harmless kind of "slow motion" earthquake would happen every few years because the clay-rich rocks within the fault are very slow to heal.

The rock samples the researchers tested were drilled from about half a mile under the seafloor in a fault in New Zealand. They squeezed the fault zone rocks in a hydraulic press and found that they were very slow to heal and slipped easily. When they plugged the rock data into a computer model of the fault, the result was a small, slow-motion tremor every two years, a near exact match with observations from the New Zealand fault.

The researchers think the clay-rich rocks, which are common at many large faults, could be regulating earthquakes by allowing plates to slip quietly past each other, which limits the buildup of stress. The discovery could be used to determine whether a fault is prone to slipping in large, damaging earthquakes, said study co-lead Srisharan Shreedharan, affiliate researcher at the University of Texas Institute for Geophysics and assistant professor at Utah State University.

"This doesn't get us any closer to actually predicting earthquakes, but it does tell us whether a fault is likely to slip silently with no earthquakes, or have large ground-shaking earthquakes," he said.

At Cascadia, there is little evidence of shallow, slow-motion tremors. That's one of the reasons the Pacific Northwest Seismic Network wants to place sensors across key areas of the fault. The new study gives them the framework to do so, said network Director Harold Tobin.

"We want to zero in on the processes in the shallow part of the fault because that's what governs the size of the tsunami," said Tobin, who was not part of the study. "Fault healing doesn't explain everything, but it does give us a window into the working of subduction zone faults that we didn't have before."

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Marsquake!

The largest earthquake ever detected on Mars has revealed layers in its crust that could indicate past collision with a massive object, such as a meteoroid. Previous data has suggested the past occurrence of a large impact, and the findings offer evidence that might support this hypothesis.

The research, led by UCLA planetary scientists and published in two papers in Geophysical Research Letters, could also indicate that alternating layers of volcanic and sedimentary rocks lie beneath the surface.

The 4.7 magnitude earthquake, or marsquake, happened in May 2022 and lasted more than four hours, releasing five times more energy than any previously recorded quake. Though moderate by Earth standards, the temblor was nonetheless powerful enough to send seismic surface waves completely around the planet's circumference, the first time this phenomenon has been observed on Mars.

The readings were taken from InSight, which landed on Mars in 2018. InSight is the first outer space seismometer to study in-depth the "inner space" of Mars: its crust, mantle and core.

"The seismometer aboard the InSight lander has recorded thousands of marsquakes but never one this large, and it took over three years after landing to record it," said corresponding author Caroline Beghein, a professor of Earth, planetary and space sciences. "This quake generated different kinds of waves, including two types of waves trapped near the surface. Only one of those two has been observed on Mars before, after two impact events, never during a marsquake."

Mapping the seismic activity, the location and frequency of impacts on Mars and the interior structure is important for future missions to the red planet as it will inform scientists and engineers where and how to build structures to ensure the safety of future human explorers.

As on Earth, studying how seismic waves travel through rocks can give scientists clues about the temperature and composition of the planet below the surface that help inform the search for underground water or magma. It also helps scientists understand the past forces that shaped the planet.

Beghein's group combined measurements from two types of surface waves, called Love and Rayleigh waves, to infer the speed of underground shear-waves, which travel horizontally and move rocks perpendicular to the direction of wave propagation. This is the first time Love waves have been observed in conjunction with Rayleigh waves on Mars.

The measurements showed that the shear-waves move faster in the crust when rocks between 10 and 25 kilometers underground oscillate in a direction almost parallel to the planet surface than if the rocks vibrate in the vertical direction.

"This wave speed information is related to deformations inside the crust," Beghein said. "Alternating volcanic rocks and sedimentary layers, which were deposited long ago, or a very large impact, such as a meteoroid, most likely account for the seismic wave measurements we observed."

These data also enabled Jiaqi Li, a UCLA postdoctoral researcher in Beghein's group, to learn that shear-waves move faster in the Martian southern highland areas than in the northern lowlands. The northern hemisphere of Mars has a lower elevation and is covered with more craters than the southern hemisphere. A large impact in the lowlands has been the prevalent theory to explain the origin of this difference.

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Impact that killed the dinosaurs triggered 'mega-earthquake' that lasted weeks to months

66 million years ago, a 10-kilometer asteroid hit Earth, triggering the extinction of the dinosaurs. New evidence suggests that the Chicxulub impact also triggered an earthquake so massive that it shook the planet for weeks to months after the collision. The amount of energy released in this "mega-earthquake" is estimated at 1023 joules, which is about 50,000 times more energy than was released in the magnitude 9.1 Sumatra earthquake in 2004.

Hermann Bermúdez will present evidence of this "mega-earthquake" at the upcoming GSA Connects meeting in Denver this Sunday, 9 October. Earlier this year, with support from a GSA Graduate Student Research Grant, Bermúdez visited outcrops of the infamous Cretaceous-Paleogene (K-Pg) mass extinction event boundary in Texas, Alabama, and Mississippi to collect data, supplementing his previous work in Colombia and Mexico documenting evidence of the catastrophic impact.

In 2014, while doing fieldwork on Colombia's Gorgonilla Island, Bermúdez found spherule deposits -- layers of sediment filled with small glass beads (as large as 1.1 mm) and shards known as 'tektites' and 'microtektites' that were ejected into the atmosphere during an asteroid impact. These glass beads formed when the heat and pressure of the impact melted and scattered the crust of the Earth, ejecting small, melted blobs up into the atmosphere, to then fall back to the surface as glass under the influence of gravity.

The rocks exposed on the coast of Gorgonilla Island tell a story from the bottom of the ocean -- roughly 2 km down. There, about 3,000-km southwest from the site of the impact, sand, mud, and small ocean creatures were accumulating on the ocean floor when the asteroid hit. Layers of mud and sandstone as far as 10-15 meters below the sea floor experienced soft-sediment deformation that is preserved in the outcrops today, which Bermúdez attributes to the shaking from the impact. Faults and deformation due to shaking continue up through the spherule-rich layer that was deposited post-impact, indicating that the shaking must have continued for the weeks and months it took for these finer-grained deposits to reach the ocean floor. Just above those spherule deposits, preserved fern spores signal the first recovery of plant-life after the impact.

Bermúdez explains, "The section I discovered on Gorgonilla Island is a fantastic place to study the K-Pg boundary, because it is one of the best-preserved and it was located deep in the ocean, so it was not affected by tsunamis."

Evidence of deformation from the mega-earthquake is also preserved in Mexico and the United States. At the El Papalote exposure in Mexico, Bermúdez observed evidence of liquefaction -- when strong shaking causes water-saturated sediments to flow like a liquid. In Mississippi, Alabama, and Texas, Bermúdez documented faults and cracks likely associated with the mega-quake. He also documents tsunami deposits at several outcrops, left by an enormous wave that was part of the cascading catastrophes resulting from the asteroid collision.

Bermúdez will deliver a talk about evidence for the mega-earthquake at the GSA Connects meeting in Denver on Sunday, 9 October. He will also present a poster about his observations of tsunami deposits and earthquake-related deformation on Monday, 10 October, which will be available in English, Spanish, Italian, French, and Chinese. In discussing his research, he emphasized the important role collaboration has played in visiting and studying so many outcrops that tell the story of this extreme event in Earth's history.

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Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal

A handful of ancient zircon crystals found in South Africa hold the oldest evidence of subduction, a key element of plate tectonics, according to a new study published today in AGU Advances, AGU's journal for high-impact, open-access research and commentary across the Earth and space sciences.

These rare time capsules from Earth's youth point to a transition around 3.8 billion years ago from a long-lived, stable rock surface to the active processes that shape our planet today, providing a new clue in a hot debate about when plate tectonics was set in motion.

Earth's crust and the top layer of mantle just under it are broken up into rigid plates that move slowly on top of viscous but mobile lower layers of mantle rock. Heat from Earth's core drives this slow but inexorable motion, responsible for volcanoes, earthquakes, and the uplift of mountain ranges.

Estimates for when this process revved up and modern crust formed range from over 4 billion years ago to just 800 million years ago. Uncertainty arises because the geologic record from Earth's youth is sparse, due to the surface recycling effect of plate tectonics itself. Almost nothing remains from the Hadean Eon, Earth's first 500 million years.

"The Hadean Earth is this big mystery box," said Nadja Drabon, a geologist at Harvard University and the lead author of the new study.

Tiny time capsules

In an exciting step forward in solving this mystery, in 2018 Drabon and her colleagues unearthed a chronological series of 33 microscopic zircon crystals from a rare, ancient block of crust in the Barberton Greenstone Belt in South Africa, that formed at different times over a critical 800-million-year span from 4.15 to 3.3 billion years ago.

Zircon is a relatively common accessory mineral in Earth's crust, but ancient representatives from the Hadean Eon, 4 to 4.56 billion years ago, are exceedingly rare, found in only 12 places on Earth, and usually in numbers fewer than three at each location.

Hafnium isotopes and trace elements preserved in the Greenstone Belt zircons told a story about the conditions on Earth at the time they crystalized. Zircons 3.8-billion-years-old and younger appeared to have formed in rock experiencing pressures and melting similar to modern subduction zones, suggesting the crust may have started moving.

"When I say plate tectonics, I'm specifically referring to an arc setting, when one plate goes under another and you have all that volcanism -- think of the Andes, for example, and the Ring of Fire," Drabon said, describing a classic example of subduction.

"At 3.8 billion years there is a dramatic shift where the crust is destabilized, we have new rocks forming and we see geochemical signatures becoming more and more similar to what we see in modern plate tectonics," Drabon said.

In contrast, the older zircons preserved evidence of a global cap of "protocrust" derived from remelting mantle rock that had remained stable for 600 million years, the study found.

Signs of global change

The new study found a similar transition to conditions resembling modern subduction in zircons from other locations around the world, dating to within about 200 million years of the South African zircons.

"We see evidence for a significant change on the Earth around 3.8 to 3.6 billion years ago and evolution toward plate tectonics is one clear possibility." Drabon said.

While not conclusive, the results suggest a global change may have begun, Drabon said, possibly starting and stopping in scattered locations before settling into the efficient global engine of constantly moving plates we see today.

Plate tectonics shapes Earth's atmosphere as well as its surface. Release of volcanic gasses and production of new silicate rock, which consumes large amounts carbon dioxide from the atmosphere, temper large temperature swings from too much or too little greenhouse gas.

"Without all of the recycling and new crust forming, we might be going back and forth between boiling hot and freezing cold," Drabon said. "It's kind of like a thermostat for the climate."

Plate tectonics has, so far, only been observed on Earth, and may be essential to making a planet livable, Drabon said, which makes the origins of plate motions of interest in research into the early development of life.

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Neural network model helps predict site-specific impacts of earthquakes

In disaster mitigation planning for future large earthquakes, seismic ground motion predictions are a crucial part of early warning systems and seismic hazard mapping. The way the ground moves depends on how the soil layers amplify the seismic waves (described in a mathematical site "amplification factor"). However, geophysical explorations to understand soil conditions are costly, limiting characterization of site amplification factors to date.

A new study by researchers from Hiroshima University published on April 5 in the Bulletin of the Seismological Society of America introduced a novel artificial intelligence (AI)-based technique for estimating site amplification factors from data on ambient vibrations or microtremors of the ground.

Subsurface soil conditions, which determine how earthquakes affect a site, vary substantially. Softer soils, for example, tend to amplify ground motion from an earthquake, while hard substrates may dampen it. Ambient vibrations of the ground or microtremors that occur all over the Earth's surface caused by human or atmospheric disturbances can be used to investigate soil conditions. Measuring microtremors provides valuable information about the amplification factor (AF) of a site, thus its vulnerability to damage from earthquakes due to its response to tremors.

The recent study from Hiroshima University researchers introduced a new way to estimate site effects from microtremor data. "The proposed method would contribute to more accurate and more detailed seismic ground motion predictions for future earthquakes," says lead author and associate professor Hiroyuki Miura in the Graduate School of Advanced Science and Engineering. The study investigated the relationship between microtremor data and site amplification factors using a deep neural network with the goal of developing a model that could be applied at any site worldwide.

The researchers looked into a common method known as Horizontal-to-vertical spectral ratios (MHVR) which is usually used to estimate the resonant frequency of the seismic ground. It can be generated from microtremor data; ambient seismic vibrations are analyzed in three dimensions to figure out the resonant frequency of sediment layers on top of bedrock as they vibrate. Previous research has shown, however, that MHVR cannot reliably be used directly as the site amplification factor. So, this study proposed a deep neural network model for estimating site amplification factors from the MHVR data.

The study used 2012-2020 microtremor data from 105 sites in the Chugoku district of western Japan. The sites are part of Japan's national seismograph network that contains about 1700 observation stations distributed in a uniform grid at 20 km intervals across Japan. Using a generalized spectral inversion technique, which separates out the parameters of source, propagation, and site, the researchers analyzed site-specific amplifications.

Data from each site were divided into a training set, a validation set, and a test set. The training set were used to teach a deep neural network. The validation set were used in the network's iterative optimization of a model to describe the relationship between the microtremor MHVRs and the site amplification factors. The test data were a completely unknown set used to evaluate the performance of the model.

The model performed well on the test data, demonstrating its potential as a predictive tool for characterizing site amplification factors from microtremor data. However, notes Miura, "the number of training samples analyzed in this study (80) sites is still limited," and should be expanded before assuming that the neural network model applies nationwide or globally. The researchers hope to further optimize the model with a larger dataset.

Rapid and cost-effective techniques are needed for more accurate seismic ground motion prediction since the relationship is not always linear. Explains Miura, "By applying the proposed method, site amplification factors can be automatically and accurately estimated from microtremor data observed at arbitrary site." Going forward, the study authors aim to continue to refine advanced AI techniques to evaluate the nonlinear responses of the ground to earthquakes.

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Contorted oceanic plate caused complex quake off New Zealand’s East Cape

Subduction zones, where a slab of oceanic plate is pushed beneath another tectonic plate down into the mantle, cause the world's largest and most destructive earthquakes. Reconstructing the geometry and stress conditions of the subducted slabs at subduction zones is crucial to understanding and preparing for major earthquakes. However, the tremendous depths of these slabs make this challenging -- seismologists rely mainly on the rare windows into these deeply buried slabs provided by the infrequent but strong earthquakes, termed intraslab earthquakes, that occur within them.

In a new study published in Geophysical Research Letters, a research team led by the University of Tsukuba used seismic data generated by a magnitude 7.3 earthquake that occurred off the northeasternmost tip of New Zealand's North Island on March 4, 2021, detected by seismometers around the world, to investigate the particularly unusual geometry and stress states of the subducted slab deep below the surface in this region.

"The 2021 East Cape earthquake showed a complex rupture process, likely because of its location at the boundary between the Kermadec Trench to the north and the Hikurangi Margin to the south," lead author of the study Assistant Professor Ryo Okuwaki explains. "To investigate the geometry of the stress field and earthquake rupture process, we used a novel finite-fault inversion technique that required no pre-existing knowledge of the area's faults."

This investigation revealed multiple episodes of rupture, generated by both compression and extension in the subsurface at different depths. These episodes included shallow (~30 km) rupture due to extension perpendicular to the trench as would typically be expected in a subduction zone. Unexpectedly, however, the deep (~70 km) rupture occurred with compression parallel to the subduction trench.

"Two alternative or inter-related factors may explain the unique rupture geometry of the 2021 East Cape earthquake," senior author Professor Yuji Yagi explains. "First, subduction of a seamount or multiple seamounts along with the subducted slab could contort the slab and create local changes in the stress field. Second, the transition from the Kermadec Trench to the Hikurangi Margin, where the subducted oceanic crust is considerably thicker, could create the local conditions responsible for the unusual faulting pattern."

Because of the rarity of deep intraslab earthquakes in this region, distinguishing between these two possibilities is currently challenging, and indeed both factors might play significant roles in creating the complex stress field revealed by the East Cape earthquake. Additional earthquakes off the northeast coast of New Zealand in the future may shed further light on this deep tectonic mystery.

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Simulating 800,000 years of California earthquake history to pinpoint risks

 Massive earthquakes are, fortunately, rare events. But that scarcity of information blinds us in some ways to their risks, especially when it comes to determining the risk for a specific location or structure.

"We haven't observed most of the possible events that could cause large damage," explained Kevin Milner, a computer scientist and seismology researcher at the Southern California Earthquake Center (SCEC) at the University of Southern California. "Using Southern California as an example, we haven't had a truly big earthquake since 1857 -- that was the last time the southern San Andreas broke into a massive magnitude 7.9 earthquake. A San Andreas earthquake could impact a much larger area than the 1994 Northridge earthquake, and other large earthquakes can occur too. That's what we're worried about."

The traditional way of getting around this lack of data involves digging trenches to learn more about past ruptures, collating information from lots of earthquakes all around the world and creating a statistical model of hazard, or using supercomputers to simulate a specific earthquake in a specific place with a high degree of fidelity.

However, a new framework for predicting the likelihood and impact of earthquakes over an entire region, developed by a team of researchers associated with SCEC over the past decade, has found a middle ground and perhaps a better way to ascertain risk.

A new study led by Milner and Bruce Shaw of Columbia University, published in the Bulletin of the Seismological Society of America in January 2021, presents results from a prototype Rate-State earthquake simulator, or RSQSim, that simulates hundreds of thousands of years of seismic history in California. Coupled with another code, CyberShake, the framework can calculate the amount of shaking that would occur for each quake. Their results compare well with historical earthquakes and the results of other methods, and display a realistic distribution of earthquake probabilities.

According to the developers, the new approach improves the ability to pinpoint how big an earthquake might occur in a given location, allowing building code developers, architects, and structural engineers to design more resilient buildings that can survive earthquakes at a specific site.

"For the first time, we have a whole pipeline from start to finish where earthquake occurrence and ground-motion simulation are physics-based," Milner said. "It can simulate up to 100,000s of years on a really complicated fault system."

Applying massive computer power to big problems

RSQSim transforms mathematical representations of the geophysical forces at play in earthquakes -- the standard model of how ruptures nucleate and propagate -- into algorithms, and then solves them on some of the most powerful supercomputers on the planet. The computationally-intensive research was enabled over several years by government-sponsored supercomputers at the Texas Advanced Computing Center, including Frontera -- the most powerful system at any university in the world -- Blue Waters at the National Center for Supercomputing Applications, and Summit at the Oak Ridge Leadership Computing Facility.

"One way we might be able to do better in predicting risk is through physics-based modeling, by harnessing the power of systems like Frontera to run simulations," said Milner. "Instead of an empirical statistical distribution, we simulate the occurrence of earthquakes and the propagation of its waves."

"We've made a lot of progress on Frontera in determining what kind of earthquakes we can expect, on which fault, and how often," said Christine Goulet, Executive Director for Applied Science at SCEC, also involved in the work. "We don't prescribe or tell the code when the earthquakes are going to happen. We launch a simulation of hundreds of thousands of years, and just let the code transfer the stress from one fault to another."

The simulations began with the geological topography of California and simulated over 800,000 virtual years how stresses form and dissipate as tectonic forces act on the Earth. From these simulations, the framework generated a catalogue -- a record that an earthquake occurred at a certain place with a certain magnitude and attributes at a given time. The catalog that the SCEC team produced on Frontera and Blue Waters was among the largest ever made, Goulet said. The outputs of RSQSim were then fed into CyberShake that again used computer models of geophysics to predict how much shaking (in terms of ground acceleration, or velocity, and duration) would occur as a result of each quake.

"The framework outputs a full slip-time history: where a rupture occurs and how it grew," Milner explained. "We found it produces realistic ground motions, which tells us that the physics implemented in the model is working as intended." They have more work planned for validation of the results, which is critical before acceptance for design applications.

The researchers found that the RSQSim framework produces rich, variable earthquakes overall -- a sign it is producing reasonable results -- while also generating repeatable source and path effects.

"For lots of sites, the shaking hazard goes down, relative to state-of-practice estimates" Milner said. "But for a couple of sites that have special configurations of nearby faults or local geological features, like near San Bernardino, the hazard went up. We are working to better understand these results and to define approaches to verify them."

The work is helping to determine the probability of an earthquake occurring along any of California's hundreds of earthquake-producing faults, the scale of earthquake that could be expected, and how it may trigger other quakes.

Support for the project comes from the U.S. Geological Survey (USGS), National Science Foundation (NSF), and the W.M. Keck Foundation. Frontera is NSF's leadership-class national resource. Compute time on Frontera was provided through a Large-Scale Community Partnership (LSCP) award to SCEC that allows hundreds of U.S. scholars access to the machine to study many aspects of earthquake science. LSCP awards provide extended allocations of up to three years to support long-lived research efforts. SCEC -- which was founded in 1991 and has computed on TACC systems for over a decade -- is a premier example of such an effort.

The creation of the catalog required eight days of continuous computing on Frontera and used more than 3,500 processors in parallel. Simulating the ground shaking at 10 sites across California required a comparable amount of computing on Summit, the second fastest supercomputer in the world.

"Adoption by the broader community will be understandably slow," said Milner. "Because such results will impact safety, it is part of our due diligence to make sure these results are technically defensible by the broader community," added Goulet. But research results such as these are important in order to move beyond generalized building codes that in some cases may be inadequately representing the risk a region face while in other cases being too conservative.

Read more at Science Daily

COVID-19 lockdown caused 50 percent global reduction in human-linked Earth vibrations

Seismograph

The lack of human activity during lockdown caused human-linked vibrations in the Earth to drop by an average of 50% between March and May 2020.

This quiet period, likely caused by the total global effect of social distancing measures, closure of services and industry, and drops in tourism and travel, is the longest and most pronounced quiet period of seismic noise in recorded history.

The new research, led by the Royal Observatory of Belgium and five other institutions around the world including Imperial College London, showed that the dampening of 'seismic noise' caused by humans was more pronounced in more densely populated areas.

The relative quietness allowed researchers to listen in to previously concealed earthquake signals, and could help us differentiate between human and natural seismic noise more clearly than ever before.

Co-author Dr Stephen Hicks, from Imperial's Department of Earth Science and Engineering, said: "This quiet period is likely the longest and largest dampening of human-caused seismic noise since we started monitoring the Earth in detail using vast monitoring networks of seismometers.

"Our study uniquely highlights just how much human activities impact the solid Earth, and could let us see more clearly than ever what differentiates human and natural noise."

The paper is published today in Science.

Anthropause

Measured by instruments called seismometers, seismic noise is caused by vibrations within the Earth, which travel like waves. The waves can be triggered by earthquakes, volcanoes, and bombs -- but also by daily human activity like travel and industry.

Although 2020 has not seen a reduction in earthquakes, the drop in human-caused seismic noise is unprecedented. The strongest drops were found in urban areas, but the study also found signatures of the lockdown on sensors buried hundreds of metres underground and in more remote areas.

Human-generated noise usually dampens during quiet periods like over the Christmas/New Year period and Chinese New Year, and during weekends and overnight. However, the drop in vibrations caused by COVID-19 lockdown measures eclipse even those seen during these periods.

Some researchers are dubbing this drop in anthropogenic (human-caused) noise and pollution the 'anthropause'.

Dr Hicks said: "This is the first global study of the impact of the coronavirus anthropause on the solid Earth beneath our feet."

To gather the data, researchers looked at seismic data from a global network of 268 seismic stations in 117 countries and found significant noise reductions compared to before any lockdown at 185 of those stations. Beginning in China in late January 2020, and followed by Europe and the rest of the world in March to April 2020, researchers tracked the 'wave' of quietening between March and May as worldwide lockdown measures took hold.

The largest drops in vibrations were seen in the most densely populated areas, like Singapore and New York City, but drops were also seen in remote areas like Germany's Black Forest and Rundu in Namibia. Citizen-owned seismometers, which tend to measure more localised noise, noted large drops around universities and schools around Cornwall, UK and Boston, USA -- a drop in noise 20 per cent larger than seen during school holidays. Countries like Barbados, where lockdown coincided with the tourist season, saw a 50 per cent decrease in noise. This coincided with flight data that suggested tourists returned home in the weeks before official lockdown.

Listening in

Over the past few decades, seismic noise has gradually increased as economies and populations have grown.

The drastic changes to daily life caused by the pandemic have provided a unique opportunity to study their environmental impacts, such as reductions in emissions and pollution in the atmosphere. The changes have also given us the opportunity to listen in to the Earth's natural vibrations without the distortions of human input.

The study reports the first evidence that previously concealed earthquake signals, especially during daytime, appeared much clearer on seismometers in urban areas during lockdown.

The researchers say the lockdown quietening could also help them differentiate between human-caused noise and natural signals that might warn of upcoming natural disasters.

Lead author Dr Thomas Lecocq from the Royal Observatory of Belgium said: "With increasing urbanisation and growing global populations, more people will be living in geologically hazardous areas. It will therefore become more important than ever to differentiate between natural and human-caused noise so that we can 'listen in' and better monitor the ground movements beneath our feet. This study could help to kick-start this new field of study."

The study's authors hope that their work will spawn further research on the seismic lockdown, as well as finding previously hidden signals from earthquakes and volcanoes.

Read more at Science Daily

The sixth sense of animals: An early warning system for earthquakes?

Even today, nobody can reliably predict when and where an earthquake will occur. However, eyewitnesses have repeatedly reported that animals behave unusually before an earthquake. In an international cooperation project, researchers from the Max Planck Institute of Animal Behavior in Konstanz/Radolfzell and the Cluster of Excellence Centre for the Advanced Study of Collective Behaviour at the University of Konstanz, have investigated whether cows, sheep, and dogs can actually detect early signs of earthquakes. To do so, they attached sensors to the animals in an earthquake-prone area in Northern Italy and recorded their movements over several months. The movement data show that the animals were unusually restless in the hours before the earthquakes. The closer the animals were to the epicentre of the impending quake, the earlier they started behaving unusually. The movement profiles of different animal species in different regions could therefore provide clues with respect to the place and time of an impending earthquake.

Experts disagree about whether earthquakes can be exactly predicted. Nevertheless, animals seem to sense the impending danger hours in advance. For example, there are reports that wild animals leave their sleeping and nesting places immediately before strong quakes and that pets become restless. However, these anecdotal accounts often do not stand up to scientific scrutiny because the definition of unusual behaviour is often too unclear and the observation period too short. Other factors could also explain the behaviour of the animals.

In order to be able to use animal activity patterns as a kind of early warning system for earthquakes, the animals would have to show measurable behavioural changes. Moreover, if they do indeed react to weak physical changes immediately before an earthquake, they should react more strongly the closer they are to the epicentre of the quake.

In an international cooperation project, researchers from the Max Planck Institute of Animal Behavior in Radolfzell/Konstanz and the Centre for the Advanced Study of Collective Behaviour, a Cluster of Excellence at the University of Konstanz, have investigated whether animals really do this. On an Italian farm in an earthquake-prone area, they attached accelerometers to the collars of six cows, five sheep, and two dogs that had already displayed unusual behaviour before earthquakes. The researchers then recorded their movements continuously over several months. During this period, official authorities reported about 18,000 earthquakes in the region. In addition to many small and hardly noticeable quakes, there were also 12 earthquakes with a strength of 4 or higher on the Richter scale.

The researchers then selected the quakes that triggered statistically relevant earth movements on the farm. These included strong quakes up to 28 km away as well as weaker quakes, the epicentres of which were very close to the farm. However, instead of explicitly looking for abnormal behaviours in the period before these events, the researchers chose a more cautious approach. They first marked all behavioural changes of the animals that were unusual according to objective, statistical criteria. "In this way, we ensure that we not only establish correlations retrospectively but also that we really do have a model that can be used for predictions," says Martin Wikelski, director at the Max Planck Institute of Animal Behavior and Principal Investigator at the Centre for the Advanced Study of Collective Behaviour.

The data -- measured as body acceleration of each farm animal (indicating activity level) -- were evaluated using statistical models drawn from financial econometrics. "Because every animal reacts differently in size, speed and according to species, the animal data resemble data on heterogenous financial investors," explains co-author Winfried Pohlmeier, Professor of Econometrics at the University of Konstanz and Principal Investigator at the Centre for the Advanced Study of Collective Behaviour. The scientists also considered other disturbance factors such as natural changes in animal activity patterns over the day.

In this way, the researchers discovered unusual behavioural patterns up to 20 hours before an earthquake. "The closer the animals were to the epicentre of the impending shock, the earlier they changed their behaviour. This is exactly what you would expect when physical changes occur more frequently at the epicentre of the impending earthquake and become weaker with increasing distance," explains Wikelski. However, this effect was clear only when the researchers looked at all animals together. "Collectively, the animals seem to show abilities that are not so easily recognized on an individual level," says Wikelski.

It is still unclear how animals can sense impending earthquakes. Animals may sense the ionization of the air caused by the large rock pressures in earthquake zones with their fur. It is also conceivable that animals can smell gases released from quartz crystals before an earthquake.

Real-time data measured by the researchers and recorded since December 2019 show what an animal earthquake early warning system could look like: a chip on the collar sends the movement data to a central computer every three minutes. This triggers a warning signal if it registers a significantly increased activity of the animals for at least 45 minutes.

The researchers have once received such a warning. "Three hours later, a small quake shook the region," says Wikelski. "The epicentre was directly below the stables of the animals."

However, before the behaviour of animals can be used to predict earthquakes, researchers need to observe a larger number of animals over longer periods of time in different earthquake zones around the world. For this, they want to use the global animal observation system Icarus on the International Space Station ISS, which will start its scientific operation in a few weeks.

Read more at Science Daily

New model to accurately date historic earthquakes

Three earthquakes in the Monterey Bay Area, occurring in 1838, 1890 and 1906, happened without a doubt on the San Andreas Fault, according to a new paper by a Portland State University researcher.

The paper, "New Insights into Paleoseismic Age Models on the Northern San Andreas Fault: Charcoal In-built ages and Updated Earthquake Correlations," was recently published in the Bulletin of the Seismological Society of America.

Assistant Professor of Geology at PSU Ashley Streig said the new research confirms what her team first discovered in 2014: three earthquakes occurred within a 68-year period in the Bay Area on the San Andreas Fault.

"This is the first time there's been geologic evidence of a surface rupture from the historic 1838 and 1890 earthquakes that we knew about from newspapers and other historical documents," Streig said. "It basically meant that the 1800s were a century of doom."

Building on the 2014 study, Streig said they were able to excavate a redwood slab from a tree felled by early Europeans, from one meter below the surface in the Bay Area. The tree was toppled before the three earthquakes in question occurred. That slab was used to determine the precise date logging first occurred in the area, and pinpointed the historic dates of the earthquakes. Further, they were able use the slab to develop a new model for determining recurrence intervals and more exact dating.

Streig used the dating technique wiggle matching for several measured carbon 14 samples from the tree slab and compared them with fluctuations in atmospheric carbon 14 concentrations over time to fingerprint the exact death of the tree and confirm the timing of the earthquakes. Because the researchers had an exact age from the slab, they were able to test how well the most commonly used material, charcoal, works in earthquake age models.

Charcoal is commonly used for dating and to constrain the ages of prehistoric earthquakes and develop an earthquake recurrence interval, but Streig said the charcoal can be hundreds of years older than the stratigraphic layer containing it, yielding an offset between what has been dated and the actual age of the earthquake. The new technique accounts for inbuilt charcoal ages -- which account for the difference in time between the wood's formation and the fire that generated said charcoal -- and can better estimate the age of the event being studied.

"We were able to evaluate the inbuilt age of the charcoal incorporated in the deposits and find that charcoal ages are approximately 322 years older than the actual age of the deposit -- so previous earthquake age models in this area using detrital charcoal would be offset roughly by this amount," she said.

New earthquake age modeling using a method to correct for this charcoal inbuilt age, and age results from the tree stump are what give Streig absolute certainly that the 1838 and 1890 earthquakes in question occurred on the San Andreas Fault and during those years.

Read more at Science Daily

Earthquakes disrupt sperm whales' ability to find food

Tail of sperm whale off coast of New Zealand

Otago scientists studying sperm whales off the coast of Kaikōura discovered earthquakes affect their ability to find food for at least a year.

The University of Otago-led research is the first to examine the impact of a large earthquake on a population of marine mammals, and offers new insight into how top predators such as sperm whales react and adapt to a large-scale natural disturbance.

Changes in habitat use by a deep-diving predator in response to a coastal earthquake, has recently been published in Deep Sea Research Part I.

Earthquakes and aftershocks can affect sperm whales in several ways, the study explains.

The whales depend on sound for communication, detection of prey and navigation and are also highly sensitive to noise.

Earthquakes produce among the loudest underwater sounds which can induce injuries, hearing damage, displacement and behavioural modifications.

While earthquakes and other extreme natural events are rare occurrences, they can really shift the state of ecosystems by wiping out animals and plants, lead author and Marine Sciences Teaching Fellow Dr Marta Guerra says.

"Understanding how wild populations respond to earthquakes helps us figure out their level of resilience, and whether we need to adjust management of these populations while they are more vulnerable."

The fatal 7.8 magnitude Kaikōura earthquake on November 14, 2016 produced strong ground shaking which triggered widespread underwater mudslides in the underwater canyon off the coastline.

This caused what's known as 'canyon flushing', which in the case of the Kaikōura earthquake, involved high-energy currents flushing 850 tonnes of sediment from the underwater canyon into the ocean.

The Kaikōura canyon is an important year-round foraging ground for sperm whales, which have an important ecological role as top predators and are a key attraction for the local tourism industry -- the main driver of the town's economy.

Just why the canyon is important to sperm whales is "a piece of the puzzle we are still trying to nut out," says Dr Guerra.

"But it's likely related to the immense productivity of the canyon's seabed, and a combination of how the currents interact with the steep topography of the submarine canyon."

Scientists examined data collected on the behaviour of 54 sperm whales between January 2014 and January 2018 -- a timeframe which allowed an opportunity to determine any significant changes in pre and post-earthquake whale foraging behaviour.

"We really didn't know what to expect, as there is so little known about how marine animals react to earthquakes," Dr Guerra says.

The researchers found clear changes in the whales' behaviour in the year following the earthquake: most noticeably whales spent about 25 per cent more time at the surface -- which potentially meant they needed to spend more effort searching for prey, either by diving deeper or for longer times

There are two main reasons the whales may have expanded their search effort, the study explains.

Firstly, benthic invertebrate communities which lived in the upper canyon may have been removed by the canyon flushing event, resulting in sparser prey and reduced foraging abilities.

Secondly, sediment deposition and erosion may have required sperm whales to 're-familiarise' with a modified habitat, increasing the effort to navigate and locate prey whose location may have changed.

"The flushing of almost 40,000 tonnes of biomass from the canyon's seabed probably meant that the animals that normally fed on the seabed had a short supply of food, possibly moving away," Dr Guerra says.

"This would have indirectly affected the prey of sperm whales (deep-water fish and squid), becoming scarce and making it harder for the whales to find food."

Scientists were particularly surprised by how clear the changes were, especially in terms of where the sperm whales were feeding.

"The head of the Kaikōura canyon, where we used to frequently find sperm whales foraging, was quiet as a desert," Dr Guerra says.

Although earthquakes happen relatively frequently in areas where marine mammals live, this study was the first to document the impact on a population, thanks to a long-term monitoring programme which has been in place since 1990.

Globally, there have been punctual observations, such as a fin whale displaying an 'escape response' after an earthquake on the Gulf of California, or particularly low sightings of humpback whales coinciding with the months following an earthquake off Alaska, Dr Guerra says.

"Deep-sea systems are so out of sight that we rarely consider the consequences of them being disturbed, whether by natural of human impacts.

"I think our results emphasise how far-reaching the impacts to the sea bed can be, affecting even animals at the top of the food chain such as sperm whales."

The study found the whales' behavioural changes lasted about a year after the 2016 earthquake and returned to normal levels in the summer of 2017-18.

Read more at Science Daily

Data mining digs up hidden clues to major California earthquake triggers

A historic image of quake damage in Long Beach, California, 1933.

A powerful computational study of southern California seismic records has revealed detailed information about a plethora of previously undetected small earthquakes, giving a more precise picture about stress in the earth's crust. A new publicly available catalog of these findings will help seismologists better understand the stresses triggering the larger earthquakes that occasionally rock the region.

"It's very difficult to unpack what triggers larger earthquakes because they are infrequent, but with this new information about a huge number of small earthquakes, we can see how stress evolves in fault systems," said Daniel Trugman, a post-doctoral fellow at Los Alamos National Laboratory and coauthor of a paper published in the journal Science today. "This new information about triggering mechanisms and hidden foreshocks gives us a much better platform for explaining how big quakes get started," Trugman said.

Crunching the Numbers

Trugman and coauthors from the California Institute of Technology and Scripps Institution of Oceanography performed a massive data mining operation of the Southern California Seismic Network for real quakes buried in the noise. The team was able to detect, understand, and locate quakes more precisely, and they created the most comprehensive earthquake catalog to date. The work identified 1.81 million quakes -- 10 times more earthquakes occurring 10 times more frequently than quakes previously identified using traditional seismology methods.

The team developed a comprehensive, detailed earthquake library for the entire southern California region, called the Quake Template Matching (QTM) catalog. They are using it to create a more complete map of California earthquake faults and behavior. This catalog may help researchers detect and locate quakes more precisely.

The team analyzed nearly two decades of data collected by the Southern California Seismic Network. The network, considered one of the world's best seismic systems, amasses a catalog of quakes from 550 seismic monitoring stations in the region. The SCSN catalog is based entirely on the traditional approach: manual observation and visual analysis. But Trugman says this traditional approach misses many weak signals that are indicators of small earthquakes.

Matching Templates Is Key

The team improved on this catalog with data mining. Using parallel computing, they crunched nearly 100 terabytes of data across 200 graphics processing units. Zooming in at high resolution for a 10-year period, they performed template matching using seismograms (waveforms or signals) of previously identified quakes. To create templates, they cut out pieces of waveforms from previously recorded earthquakes and matched those waveforms to patterns of signals recorded simultaneously from multiple seismic stations. Template matching has been done before, but never at this scale.

"Now we can automate it and search exhaustively through the full waveform archive to find signals of very small earthquakes previously hidden in the noise," Trugman explained.

Applying the templates found events quake precursors, foreshocks and small quakes that had been missed with manual methods. Those events often provide key physical and geographic details to help predict big quakes. The team also identified initiation sequences that reveal how quakes are triggered.

New details also revealed three-dimensional geometry and fault structures, which will support development of more realistic models.

Recently, Trugman and Los Alamos colleagues have applied machine learning to study earthquakes created in laboratory quake machines. That works has uncovered important details about earthquake behavior that may be used to predict quakes.

Read more at Science Daily

Massive Bolivian earthquake reveals mountains 660 kilometers below our feet

Graphic showing the Transition Zone inside the Earth Princeton seismologist Jessica Irving worked with then-graduate student Wenbo Wu and another collaborator to determine the roughness at the top and bottom of the transition zone, a layer within the mantle, using scattered earthquake waves. They found that the top of the transition zone, a layer located 410 kilometers down, is mostly smooth, but the base of the transition zone, 660 km down, in some places is much rougher than the global surface average. “In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,” said Wu. NOTE: This graphic is not to scale.

Most schoolchildren learn that the Earth has three (or four) layers: a crust, mantle and core, which is sometimes subdivided into an inner and outer core. That's not wrong, but it does leave out several other layers that scientists have identified within the Earth.

In a study published this week in Science, Princeton geophysicists Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni from the Institute of Geodesy and Geophysics in China, used data from an enormous earthquake in Bolivia to find mountains and other topography on a layer located 660 kilometers (410 miles) straight down, which separates the upper and lower mantle. (Lacking a formal name for this layer, the researchers simply call it "the 660-km boundary.")

To peer deep into the Earth, scientists use the most powerful waves on the planet, which are generated by massive earthquakes. "You want a big, deep earthquake to get the whole planet to shake," said Irving, an assistant professor of geosciences.

Big earthquakes are vastly more powerful than small ones -- energy increases 30-fold with every step up the Richter scale -- and deep earthquakes, "instead of frittering away their energy in the crust, can get the whole mantle going," Irving said. She gets her best data from earthquakes that are magnitude 7.0 or higher, she said, as the shockwaves they send out in all directions can travel through the core to the other side of the planet -- and back again. For this study, the key data came from waves picked up after a magnitude 8.2 earthquake -- the second-largest deep earthquake ever recorded -- that shook Bolivia in 1994.

"Earthquakes this big don't come along very often," she said. "We're lucky now that we have so many more seismometers than we did even 20 years ago. Seismology is a different field than it was 20 years ago, between instruments and computational resources."

Seismologists and data scientists use powerful computers, including Princeton's Tiger supercomputer cluster, to simulate the complicated behavior of scattering waves in the deep Earth.

The technology depends on a fundamental property of waves: their ability to bend and bounce. Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogenous rocks but reflect or refract when they encounter any boundary or roughness.

"We know that almost all objects have surface roughness and therefore scatter light," said Wu, the lead author on the new paper, who just completed his geosciences Ph.D. and is now a postdoctoral researcher at the California Institute of Technology. "That's why we can see these objects -- the scattering waves carry the information about the surface's roughness. In this study, we investigated scattered seismic waves traveling inside the Earth to constrain the roughness of the Earth's 660-km boundary."

The researchers were surprised by just how rough that boundary is -- rougher than the surface layer that we all live on. "In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary," said Wu. Their statistical model didn't allow for precise height determinations, but there's a chance that these mountains are bigger than anything on the surface of the Earth. The roughness wasn't equally distributed, either; just as the crust's surface has smooth ocean floors and massive mountains, the 660-km boundary has rough areas and smooth patches. The researchers also examined a layer 410 kilometers (255 miles) down, at the top of the mid-mantle "transition zone," and they did not find similar roughness.

"They find that Earth's deep layers are just as complicated as what we observe at the surface," said seismologist Christine Houser, an assistant professor at the Tokyo Institute of Technology who was not involved in this research. "To find 2-mile (1-3 km) elevation changes on a boundary that is over 400 miles (660 km) deep using waves that travel through the entire Earth and back is an inspiring feat. ... Their findings suggest that as earthquakes occur and seismic instruments become more sophisticated and expand into new areas, we will continue to detect new small-scale signals which reveal new properties of Earth's layers."

What it means

The presence of roughness on the 660-km boundary has significant implications for understanding how our planet formed and continues to function. That layer divides the mantle, which makes up about 84 percent of the Earth's volume, into its upper and lower sections. For years, geoscientists have debated just how important that boundary is. In particular, they have investigated how heat travels through the mantle -- whether hot rocks are carried smoothly from the core-mantle boundary (almost 2,000 miles down) all the way up to the top of the mantle, or whether that transfer is interrupted at this layer. Some geochemical and mineralogical evidence suggests that the upper and lower mantle are chemically different, which supports the idea that the two sections don't mix thermally or physically. Other observations suggest no chemical difference between the upper and lower mantle, leading some to argue for what's called a "well-mixed mantle," with both the upper and lower mantle participating in the same heat-transfer cycle.

"Our findings provide insight into this question," said Wu. Their data suggests that both groups might be partially right. The smoother areas of the 660-km boundary could result from more thorough vertical mixing, while the rougher, mountainous areas may have formed where the upper and lower mantle don't mix as well.

In addition, the roughness the researchers found, which existed at large, moderate and small scales, could theoretically be caused by heat anomalies or chemical heterogeneities. But because of how heat in transported within the mantle, Wu explained, any small-scale thermal anomaly would be smoothed out within a million years. That leaves only chemical differences to explain the small-scale roughness they found.

What could cause significant chemical differences? The introduction of rocks that used to belong to the crust, now resting quietly in the mantle. Scientists have long debated the fate of the slabs of sea floor that get pushed into the mantle at subduction zones, the collisions happening found all around the Pacific Ocean and elsewhere around the world. Wu and Irving suggest that remnants of these slabs may now be just above or just below the 660-km boundary.

Read more at Science Daily

Earthquakes can be weakened by groundwater

Around 100,000 earthquakes are recorded worldwide every year, but not all are naturally occurring. Some of the weaker ones are triggered by human activity underground -- this is referred to as induced seismicity. Researchers from EPFL's Laboratory of Experimental Rock Mechanics (LEMR) and the Ecole Normale Supérieure in Paris have just completed a study into the role of fluids in the propagation of induced earthquakes in an effort to decipher the underlying mechanisms. Their findings include the extremely counterintuitive discovery that highly pressurized water in the vicinity of an earthquake tends to limit -- rather than increase -- its intensity. These results were published today in Nature Communications.

Induced earthquakes can be the result of activities like mining, gas and oil extraction, toxic waste or CO2 storage, and the construction of tunnels and dam reservoirs. The generation of geothermal energy is another potential source of induced earthquakes -- and the main one in Switzerland. According to the Swiss Seismological Service, a geothermal project near Basel caused a 3.4 magnitude earthquake in 2006, and one in St. Gallen triggered a 3.5 magnitude trembler in 2013.

Geothermal energy is captured by tapping into subterranean heat. Highly pressurized water is pumped into the earth's crust at a depth of between two and four kilometers. The water is then recovered as steam and used to drive an electricity-producing turbine. "Injecting water can affect water-rock equilibria and disrupt nearby faults, thus triggering earthquakes in the area," says Marie Violay, who runs LEMR.

This type of earthquake is a thorn in the side of geothermal proponents, notes Mateo Acosta, a PhD student at LEMR and the study's lead author: "These earthquakes may be low in intensity, but they can cause damage and affect public opinion -- to the point of derailing projects."

Heat absorption

Acosta ran tests in which he sought to replicate earthquake conditions in order to study the impact of different levels of underground water pressure on fault dynamics. He focused mainly on earthquake propagation, which is when the two plates in a fault rub against each other, sending seismic waves out into the surrounding area.

"Rock friction generates a significant amount of heat, which further fuels the propagation effect," says the PhD student. "Some of this heat is absorbed by the water in the surrounding rock, and the amount absorbed depends to a large extent on the water's thermodynamic parameters. What we learned from our experiments is that the closer the fluid's initial pressure is to the critical pressure of water, the weaker the earthquake will be."

"This research shows that the initial fluid pressure in the rocks is crucial, especially at depths commonly reached by geothermal activities. Geothermal models need to take this into account," says François-Xavier Passelègue, an LEMR researcher and the study's second author.

Read more at Science Daily

500-year-old Leaning Tower of Pisa mystery unveiled by engineers

Leaning tower of Pisa, Italy.

Why has the Leaning Tower of Pisa survived the strong earthquakes that have hit the region since the middle ages? This is a long-standing question a research group of 16 engineers has investigated, including a leading expert in earthquake engineering and soil-structure interaction from the University of Bristol.

Professor George Mylonakis, from Bristol's Department of Civil Engineering, was invited to join a 16-member research team, led by Professor Camillo Nuti at Roma Tre University, to explore this Leaning Tower of Pisa mystery that has puzzled engineers for many years.

Despite leaning precariously at a five-degree angle, leading to an offset at the top of over five metres, the 58-metre tall Tower has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280.

Given the vulnerability of the structure, which barely manages to stand vertically, it was expected to sustain serious damage or even collapse because of moderate seismic activity. Surprisingly this hasn't happened and until now this has mystified engineers for a long time. After studying available seismological, geotechnical and structural information, the research team concluded that the survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI).

The considerable height and stiffness of the Tower combined with the softness of the foundation soil, causes the vibrational characteristics of the structure to be modified substantially, in such a way that the Tower does not resonate with earthquake ground motion. This has been the key to its survival. The unique combination of these characteristics gives the Tower of Pisa the world record in DSSI effects.

Professor Mylonakis, Chair in Geotechnics and Soil-Structure Interaction, and Head of Earthquake and Geotechnical Engineering Research Group in the Department of Civil Engineering at the University of Bristol, said: "Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events."

Results from the study have been presented to international workshops and will be formally announced at the 16th European Conference in Earthquake Engineering taking place in Thessaloniki, Greece next month [18 to 21 June 2018].

From Science Daily

AI Earthquake Tracker Is Inspired by Speech Recognition Technology

The state of Oklahoma has witnessed a stunning rise in the frequency of earthquakes, which has been linked to an increase in the use of fracking technology in the oil and gas sector. Starting in 2009, the annual number of quakes measuring above magnitude 3.0 in the state exploded from fewer than three to as many as 903 in 2015.

Now, all this seismic activity has prompted scientists to develop a new tool for tracking it — drawing on speech recognition technology.

The result is a system dubbed ConvNetQuake that’s designed to detect even the tiniest earthquake against background geological noise in the same way that a smartphone can discern a human voice inside a car that’s rumbling down the highway.

The system represents an upgrade in sensitivity and detection-speed from current methods, according to its designers. When tested against historical field data, the new approach uncovered 17 times more quakes than were recorded in the Oklahoma Geological Survey standard earthquake catalog.

“We’ve trained the algorithm to understand what’s just noise and what’s an earthquake, and also where the earthquake is coming from,” Thibaut Perol, lead author of a new paper describing the system, told Seeker. Perol works on voice-recognition and artificial intelligence at a startup in Washington DC called *gramLabs.

Fracking is a relatively new form of crude oil and natural gas production that’s dramatically revived US hydrocarbon output.

The process involves blasting chemical-laced water below ground to fracture rock formation and withdraw oil or natural gas, opening up previously inaccessible reserves. But excess water is seeping out into dormant faults, and is thought to be causing them to slip, resulting in earthquakes.

Most existing earthquake-detection methods are designed to detect moderate-to-large events. As a consequence, they miss many low-magnitude earthquakes that get masked by background seismic noise.

But picking up the smaller quakes allows researchers to paint a more precise picture of all the earthquake activity in a place like Oklahoma, yielding a better understanding of the location of the quakes, whether they might be shifting, and whether the frequency is rising or falling. The extra data could eventually yield insight into whether a big one is coming, Perol said.

That’s because the art of predicting earthquakes remains essentially one of modeling likely future risk based on the patterns that have come before. In spite of some promising new research in the field of earthquake forecasting, the state-of-the-art is still limited, essentially, to an understanding of how many quakes have come before, and how often.

Existing platforms for detecting earthquakes use three stations to triangulate the source of the rumbling. The new method isn’t just more sensitive, but requires only one detection location.

Read more at Seeker