NASA's DART impact permanently changed the shape and orbit of asteroid moon

When NASA's Double Asteroid Redirection Test (DART) spacecraft collided with an asteroid moon called Dimorphos in 2022, the moon was significantly deformed -- creating a large crater and reshaping it so dramatically that the moon derailed from its original evolutionary progression -- according to a new study. The study's researchers believe that Dimorphos may start to "tumble" chaotically in its attempts to move back into gravitational equilibrium with its parent asteroid named Didymos.

"For the most part, our original pre-impact predictions about how DART would change the way Didymos and its moon move in space were correct," said Derek Richardson, a professor of astronomy at the University of Maryland and a DART investigation working group lead. "But there are some unexpected findings that help provide a better picture of how asteroids and other small bodies form and evolve over time."

The paper published in Planetary Science Journal on August 23, 2024 by a team led by Richardson detailed notable post-impact observations and described possible implications for future asteroid research.

One of the biggest surprises was how much the impact with DART changed the shape of Dimorphos. According to Richardson, the asteroid moon was originally oblate (shaped like a hamburger) but became more prolate (stretched out like a football) after the DART spacecraft collided with it.

"We were expecting Dimorphos to be prolate pre-impact simply because that's generally how we believed the central body of a moon would gradually accumulate material that's been shed off a primary body like Didymos. It would naturally tend to form an elongated body that would always point its long axis toward the main body," Richardson explained. "But this result contradicts that idea and indicates that something more complex is at work here. Furthermore, the impact-induced change in Dimorphos' shape likely changed how it interacts with Didymos."

Richardson noted that although DART only hit the moon, the moon and the main body are connected through gravity. The debris scattered by the spacecraft on impact also played a role in the disturbed equilibrium between the moon and its asteroid, shortening Dimorphos' orbit around Didymos. Interestingly, Didymos' shape remained the same -- a finding that indicates that the larger asteroid's body is firm and rigid enough to maintain its form even after losing mass to create its moon.

According to Richardson, Dimorphos' changes have important implications for future exploration efforts, including the European Space Agency's follow-up mission to the Didymos system slated for October 2024.

"Originally, Dimorphos was probably in a very relaxed state and had one side pointing toward the main body, Didymos, just like how Earth's moon always has one face pointing toward our planet," Richardson explained. "Now, it's knocked out of alignment, which means it may wobble back and forth in its orientation. Dimorphos might also be 'tumbling,' meaning that we may have caused it to rotate chaotically and unpredictably."

The team is now waiting to find out when the ejected debris will clear from the system, whether Dimorphos is still tumbling in space and when it will eventually regain its previous stability.

"One of our biggest questions now is if Dimorphos is stable enough for spacecraft to land and install more research equipment on it," he said. "It could take a hundred years to see noticeable changes in the system, but it's only been a few years since the impact. Learning about how long it takes Dimorphos to regain its stability tells us important things about its internal structure, which in turn informs future attempts to deflect hazardous asteroids."

Richardson and his team hope that Hera will provide more information about DART's impact. By late 2026, Hera will arrive at the binary asteroid system containing Dimorphos and Didymos to assess the internal properties of both asteroids for the first time, providing a more detailed analysis of the DART mission and its implications for the future.

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Decoding mysterious seismic signals

For the decades since their discovery, seismic signals known as PKP precursors have challenged scientists. Regions of Earth's lower mantle scatter incoming seismic waves, which return to the surface as PKP waves at differing speeds.

The origin the precursor signals, which arrive ahead of the main seismic waves that travel through Earth's core, has remained unclear, but research led by University of Utah geophysicists sheds new light on this mysterious seismic energy.

PKP precursors appear to propagate from places deep below North America and the western Pacific and possibly bear an association with "ultra-low velocity zones," thin layers in the mantle where seismic waves significantly slow down, according to research published in AGU Advances, the American Geophysical Union's lead journal. (The AGU highlighted the research in its magazine Eos.)

"These are some of the most extreme features discovered on the planet. We legitimately do not know what they are," said lead author Michael Thorne, a U associate professor of geology and geophysics. "But one thing we know is they seem to end up accumulating underneath hotspot volcanoes. They seem like they may be the root of whole mantle plumes giving rise to hotspot volcanoes."

These plumes are responsible for the volcanism observed at Yellowstone, the Hawaiian Islands, Samoa, Iceland and the Galapagos Islands.

"These really, really big volcanoes seem to persist for hundreds of millions of years in roughly the same spot," Thorne said. In previous work, he also found one of the world's largest known ultra-low velocity zones.

"It sits right beneath Samoa, and Samoa is one of the biggest hotspot volcanoes," Thorne noted.

For nearly a century, geoscientists have used seismic waves to probe Earth's interior, leading to numerous discoveries that would not be otherwise possible. Other researchers at the U, for example, have characterized the structure of Earth's solid inner core and tracked its movement by analyzing seismic waves.

When an earthquake rattles Earth's surface, seismic waves shoot through the mantle -- the 2,900-kilometer-thick dynamic layer of hot rock between Earth's crust and metal core. Thorne's team is interested in those that get "scattered" when they pass through irregular features that pose changes in material composition in the mantle. Some of those scattered waves become PKP precursors.

Thorne sought to determine exactly where this scattering happens, especially since the waves travel through Earth's mantle twice, that is, before and after passing through Earth's liquid outer core. Because of that double journey through the mantle, it has been nearly impossible to distinguish whether the precursors originated on the source-side or receiver-side of the ray path.

Thorne's team, which included research assistant professor Surya Pachhai, devised a way to model waveforms to detect crucial effects that previously went unnoticed.

Using a cutting-edge seismic array method and new theoretical observations from earthquake simulations, the researchers developed, they analyzed data from 58 earthquakes that occurred around New Guinea and were recorded in North America after passing through the planet.

"I can put virtual receivers anywhere on the surface of the earth, and this tells me what the seismogram should look like from an earthquake at that location. And we can compare that to the real recordings that we have," Thorne said. "We're able to now back project where this energy's coming from."

Their new method allowed them to pinpoint where the scattering occurred along the boundary between the liquid metal outer core and the mantle, known as the core-mantle boundary, located 2,900 kilometers below Earth's surface.

Their findings indicate that the PKP precursors likely come from regions that are home to ultra-low velocity zones. Thorne suspects these layers, which are only 20 to 40 kilometers thick, are formed where subducted tectonic plates impinge on the core-mantle boundary in oceanic crust.

"What we've now found is that these ultra-low velocity zones do not just exist beneath the hotspots. They're spread out all across the core-mantle boundary beneath North America," Thorne said. "It really looks like these ULVZs are getting actively generated. We don't know how. But because we're seeing them near subduction, we think mid-ocean ridge basalts are getting melted, and that is how it's getting generated. And then the dynamics is pushing these things all across Earth, and ultimately they're going to accumulate beneath the hotspots."

"What we've now found is that these ultra-low velocity zones do not just exist beneath the hotspots. They're spread out all across the core-mantle boundary beneath North America," Thorne said. "It really looks like these ULVZs are getting actively generated. We don't know how. But because we're seeing them near subduction, we think mid-ocean ridge basalts are getting melted, and that may be how they're getting generated."

The dynamics is pushing these things all across Earth, and ultimately, they're going to accumulate against the boundaries of Large Low Velocity Provinces, which are compositionally distinct continent scale features beneath the Pacific and Africa, according to Thorne.

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Researchers find oldest undisputed evidence of Earth's magnetic field

A new study, led by the University of Oxford and MIT, has recovered a 3.7-billion-year-old record of Earth's magnetic field, and found that it appears remarkably similar to the field surrounding Earth today. The findings have been published today in the Journal of Geophysical Research.

Without its magnetic field, life on Earth would not be possible since this shields us from harmful cosmic radiation and charged particles emitted by the Sun (the 'solar wind'). But up to now, there has been no reliable date for when the modern magnetic field was first established.

In the new study, the researchers examined an ancient sequence of iron-containing rocks from Isua, Greenland. Iron particles effectively act as tiny magnets that can record both magnetic field strength and direction when the process of crystallization locks them in place. The researchers found that rocks dating from 3.7 billion years ago captured a magnetic field strength of at least 15 microtesla comparable to the modern magnetic field (30 microtesla).

These results provide the oldest estimate of the strength of Earth's magnetic field derived from whole rock samples, which provide a more accurate and reliable assessment than previous studies which used individual crystals.

Lead researcher Professor Claire Nichols (Department of Earth Sciences, University of Oxford) said: 'Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analysed these samples in the lab. This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging.'

Whilst the magnetic field strength appears to have remained relatively constant, the solar wind is known to have been significantly stronger in the past. This suggests that the protection of Earth's surface from the solar wind has increased over time, which may have allowed life to move onto the continents and leave the protection of the oceans.

Earth's magnetic field is generated by mixing of the molten iron in the fluid outer core, driven by buoyancy forces as the inner core solidifies, which create a dynamo. During Earth's early formation, the solid inner core had not yet formed, leaving open questions about how the early magnetic field was sustained. These new results suggest the mechanism driving Earth's early dynamo was similarly efficient to the solidification process that generates Earth's magnetic field today.

Understanding how Earth's magnetic field strength has varied over time is also key for determining when Earth's inner, solid core began to form. This will help us to understand how rapidly heat is escaping from Earth's deep interior, which is key for understanding processes such as plate tectonics.

A significant challenge in reconstructing Earth's magnetic field so far back in time is that any event which heats the rock can alter preserved signals. Rocks in the Earth's crust often have long and complex geological histories which erase previous magnetic field information. However, the Isua Supracrustal Belt has a unique geology, sitting on top of thick continental crust which protects it from extensive tectonic activity and deformation. This allowed the researchers to build a clear body of evidence supporting the existence of the magnetic field 3.7 billion years ago.

The results may also provide new insights into the role of our magnetic field in shaping the development of Earth's atmosphere as we know it, particularly regarding atmospheric escape of gases. A currently unexplained phenomenon is the loss of the unreactive gas xenon from our atmosphere more than 2.5 billion years ago. Xenon is relatively heavy and therefore unlikely to have simply drifted out of our atmosphere. Recently, scientists have begun to investigate the possibility that charged xenon particles were removed from the atmosphere by the magnetic field.

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One of the largest magnetic storms in history quantified: Aurorae covered much of the night sky from the Tropics to the Polar Regions

In early November of this year, aurora borealis were observed at surprisingly low latitudes, as far south as Italy and Texas. Such phenomena indicate the impacts of a solar coronal mass ejection on the Earth's magnetic field and atmosphere. Far more dramatic than this recent light show was, it was nothing compared to a huge solar storm in February 1872. The resulting auroral display from that event ringed the globe and produced auroras observed in sites as close to the equator as Bombay and Khartoum. An international team consisting of scientists from nine counties has now published a detailed study of this historically important event, tracing its solar origin and widespread terrestrial impacts. Telegraph communications were widely disrupted by this storm, but in today's technologically dependent society, such a storm would disrupt power grids and satellite communications. Their findings confirm that such extreme storms are more common than previously thought.

In the modern world, we are increasingly dependent on technological infrastructure such as power grids, communication systems, and satellites.

However, this dependency makes us increasingly vulnerable to the effects of large geomagnetic storms.

"The longer the power supply could be cut off, the more society, especially those living in urban areas, will struggle to cope," Designated Assistant Professor Hayakawa, the lead author of the study, explains.

Such storms could be big enough to knock out the power grid, communication systems, airplanes, and satellites in the worst case.

"Could we maintain our life without such infrastructure?" Hayakawa comments: "Well, let us just say that it would be extremely challenging."

Such extreme storms are rare. In recent studies, two such storms stand out: the Carrington storm in September 1859 and the New York Railroad storm in May 1921.

The new study suggests that another storm, the Chapman-Silverman storm in February 1872, should also be considered as one of these extreme events.

At the time, the storm was big enough to affect the technological infrastructure even in the tropics.

Telegraph communications on the submarine cable in the Indian Ocean between Bombay (Mumbai) and Aden were disrupted for hours.

Similar disturbances were reported on the land line between Cairo and Khartoum.

The multidisciplinary team, consisting of 22 scientists, was led by Nagoya University in Japan (Hisashi Hayakawa), the US National Solar Observatory (Edward Cliver), and the Royal Observatory of Belgium (Frédéric Clette). The 22 researchers used historical records and modern techniques to assess the Chapman-Silverman storm from its solar origin to its terrestrial impacts.

For the solar origin, the group turned to largely forgotten sunspot records from historical archives, especially Belgian and Italian records.

For terrestrial impacts, they used geomagnetic field measurements recorded in places as diverse as Bombay (Mumbai), Tiflis (Tbilisi), and Greenwich to assess temporal evolution and storm intensity.

They also examined hundreds of accounts of visual aurora in different languages caused by the storm.

One of the more interesting aspects of the 1872 storm was that it likely originated in a medium-sized, but complex, sunspot group near the solar disk centre as confirmed by analyses of solar records from Belgium and Italy.

These findings suggest that even a medium-sized sunspot group triggered one of the most extreme magnetic storms in history.

Hayakawa and his colleagues extended their investigations of the historical aurorae by combing through records in libraries, archives, and observatories around the world.

They identified more than 700 auroral records that indicated that the night sky was illuminated by magnificent auroral displays from the polar regions to the tropics (down to ≈ 20° in latitude in both hemispheres).

"Our findings confirm the Chapman-Silverman storm in February 1872 as one of the most extreme geomagnetic storms in recent history. Its size rivalled those of the Carrington storm in September 1859 and the NY Railroad storm in May 1921," Hayakawa said.

"This means that we now know that the world has seen at least three geomagnetic superstorms in the last two centuries. Space weather events that could cause such a major impact represent a risk that cannot be discounted."

Hayakawa said: "Such extreme events are rare. On the one hand, we are fortunate to have missed such superstorms in the modern time. On the other hand, the occurrence of three such superstorms in 6 decades shows that the threat to modern society is real. Therefore, the preservation and analysis of historical records is important to assess, understand, and mitigate the impact of such events."

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The remains of an ancient planet lie deep within Earth

In the 1980s, geophysicists made a startling discovery: two continent-sized blobs of unusual material were found deep near the center of the Earth, one beneath the African continent and one beneath the Pacific Ocean. Each blob is twice the size of the Moon and likely composed of different proportions of elements than the mantle surrounding it.

Where did these strange blobs -- formally known as large low-velocity provinces (LLVPs) -- come from? A new study led by Caltech researchers suggests that they are remnants of an ancient planet that violently collided with Earth billions of years ago in the same giant impact that created our Moon.

The study, published in the journal Nature on November 1, also proposes an answer to another planetary science mystery. Researchers have long hypothesized that the Moon was created in the aftermath of a giant impact between Earth and a smaller planet dubbed Theia, but no trace of Theia has ever been found in the asteroid belt or in meteorites. This new study suggests that most of Theia was absorbed into the young Earth, forming the LLVPs, while residual debris from the impact coalesced into the Moon.

The research was led by Qian Yuan, O.K. Earl Postdoctoral Scholar Research Associate in the laboratories of both Paul Asimow (MS '93, PhD '97), the Eleanor and John R. McMillan Professor of Geology and Geochemistry; and Michael Gurnis, the John E. And Hazel S. Smits Professor of Geophysics and Clarence R. Allen Leadership Chair, director of Caltech's Seismological Laboratory, and director of the Schmidt Academy for Software Engineering at Caltech.

Scientists first discovered the LLVPs by measuring seismic waves traveling through the earth. Seismic waves travel at different speeds through different materials, and in the 1980s, the first hints emerged of large-scale three-dimensional variations deep within the structure of Earth. In the deepest mantle, the seismic wave pattern is dominated by the signatures of two large structures near the Earth's core that researchers believe possess an unusually high level of iron. This high iron content means the regions are denser than their surroundings, causing seismic waves passing through them to slow down and leading to the name "large low velocity provinces."

Yuan, a geophysicist by training, was attending a seminar about planet formation given by Mikhail Zolotov, a professor at Arizona State University, in 2019. Zolotov presented the giant-impact hypothesis, while Qian noted that the Moon is relatively rich in iron. Zolotov added that no trace had been found of the impactor that must have collided with the Earth.

"Right after Mikhail had said that no one knows where the impactor is now, I had a 'eureka moment' and realized that the iron-rich impactor could have transformed into mantle blobs," says Yuan.

Yuan worked with multidisciplinary collaborators to model different scenarios for Theia's chemical composition and its impact with Earth. The simulations confirmed that the physics of the collision could have led to the formation of both the LLVPs and the Moon. Some of Theia's mantle could have become incorporated into the Earth's own, where it ultimately clumped and crystallized together to form the two distinct blobs detectable today at Earth's core-mantle boundary today; other debris from the collision mixed together to form the Moon.

Given such a violent impact, why did Theia's material clump into the two distinct blobs instead of mixing together with the rest of the forming planet? The researchers' simulations showed that much of the energy delivered by Theia's impact remained in the upper half of the mantle, leaving Earth's lower mantle cooler than estimated by earlier, lower-resolution impact models. Because the lower mantle was not totally melted by the impact, the blobs of iron-rich material from Theia stayed largely intact as they sifted down to the base of the mantle, like the colored masses of paraffin wax in a turned-off lava lamp. Had the lower mantle been hotter (that is, if it had received more energy from the impact), it would have mixed more thoroughly with the iron-rich material, like the colors in a stirred pot of paints.

The next steps are to examine how the early presence of Theia's heterogeneous material deep within the earth might have influenced our planet's interior processes, such as plate tectonics.

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Geomagnetic field protects Earth from electron showers

Tohoku University geophysicist Yuto Katoh led a study into the activity of high energy electrons and clarified the unexpected role of the geomagnetic field surrounding the Earth in protecting.

Understanding the ionosphere high in the Earth's atmosphere is important due to its effects on communications systems, satellites and crucial chemical features including the ozone layer. New insights into the activity of high energy electrons have come from a simulation study led by geophysicist Yuto Katoh at Tohoku University, reported in the journal Earth, Planets and Space.

"Our results clarify the unexpected role of the geomagnetic field surrounding the Earth in protecting the atmosphere from high energy electrons," says Katoh.

The ionosphere is a wide region between roughly 60 and more than 600 kilometers above the Earth's surface. It contains electrically charged particles that are a mixture of ions and free electrons generated by the interaction of the atmosphere with radiation from the sun.

Polar regions of the ionosphere are subjected to a particularly steady and energetic stream of incoming electrons in a process called electron precipitation. These 'relativistic' electrons move at close to the speed of light, where the effects of Einstein's relativity theory become ever more significant. They collide with gas molecules and contribute to many phenomena in the ionosphere, including colourful auroral displays. The processes are heavily influenced by the effects of the geomagnetic field on the charged particles involved.

The Tohoku team, with colleagues in Germany and other institutions in Japan, developed a sophisticated software code that focused particular attention on simulating the effects of a relatively unstudied 'mirror force' on the electron precipitation. This is caused by the magnetic force acting on charged particles under the influence of the geomagnetic field.

The simulations demonstrated how the mirror force causes relativistic electrons to bounce back upwards, to an extent dependent on the angles at which the electrons arrive. The predicted effects mean that electrons collide with other charged particles higher in the ionosphere than previously suspected.

Illustrating one example of the significance of this work, Katoh comments: "Precipitating electrons that manage to pass through the mirror force can reach the middle and lower atmosphere, contributing to chemical reactions related to variations in ozone levels." Decreased ozone levels at the poles caused by atmospheric pollution reduce the protection ozone offers living organisms from ultraviolet radiation.

Katoh emphasizes the key theoretical advance of the research is in revealing the surprising significance of the geomagnetic field and the mirror force in protecting the lower atmosphere from the effects of electron precipitation activities by keeping them further away.

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Magma on Mars likely

Since 2018, when the NASA InSight Mission deployed the SEIS seismometer on the surface of Mars, seismologists and geophysicists at ETH Zurich have been listening to the seismic pings of more than 1,300 marsquakes. Again and again, the researchers registered smaller and larger Mars quakes. A detailed analysis of the quakes' location and spectral character brought a surprise. With epicentres originating in the vicinity of the Cerberus Fossae -- a region consisting of a series of rifts or graben -- these quakes tell a new story. A story that suggests vulcanism still plays an active role in shaping the Martian surface.

Mars shows signs of life and youth

An international team of researchers, led by ETH Zurich, analysed a cluster of more than 20 recent marsquakes that originated in the Cerberus Fossae graben system. From the seismic data, scientists concluded that the low-frequency quakes indicate a potentially warm source that could be explained by present day molten lava, i.e., magma at that depth, and volcanic activity on Mars. Specifically, they found that the quakes are located mostly in the innermost part of Cerberus Fossae.

When they compared seismic data with observational images of the same area, they also discovered darker deposits of dust not only in the dominant direction of the wind, but in multiple directions surrounding the Cerebus Fossae Mantling Unit. "The darker shade of the dust signifies geological evidence of more recent volcanic activity -- perhaps within the past 50,000 years -- relatively young, in geological terms," explains Simon Staehler, the lead author of the paper, which has now been published in the journal Nature. Staehler is a Senior Scientist working in the Seismology and Geodynamics group led by Professor Domenico Giardini at the Institute of Geophysics, ETH Zurich.

Why study the terrestrial neighbour?

Exploring Earth's planetary neighbours is no easy task. Mars is the only planet, other than Earth, in which scientists have ground-based rovers, landers, and now even drones that transmit data. All other planetary exploration, so far, has relied on orbital imagery. "InSight's SEIS is the most sensitive seismometer ever installed on another planet," says Domenico Giardini. "It affords geophysicists and seismologists an opportunity to work with current data showing what is happening on Mars today -- both at the surface and in its interior." The seismic data, along with orbital images, ensures a greater degree of confidence for scientific inferences.

One of our nearest terrestrial neighbours, Mars is important for understanding similar geological processes on Earth. The red planet is the only one we know of, so far, that has a core composition of iron, nickel, and sulphur that might have once supported a magnetic field. Topographical evidence also indicates that Mars once held vast expanses of water and possibly a denser atmosphere. Even today, scientists have learned that frozen water, although possibly mostly dry ice, still exists on its polar caps. "While there is much more to learn, the evidence of potential magma on Mars is intriguing," Anna Mittelholz, Postdoctoral Fellow at ETH Zurich and Harvard University.

Last remnants of geophysical life

Looking at images of the vast dry, dusty Martian landscape it is difficult to imagine that about 3.6 billion years ago Mars was very much alive, at least in a geophysical sense. It spewed volcanic debris for a long enough time to give rise to Tharsis Montes region, the largest volcanic system in our solar system and the Olympus Mons -- a volcano nearly three times the elevation of Mount Everest. The quakes coming from the nearby Cerberus Fossae -- named for a creature from Greek mythology known as the "hell-hound of Hades" that guards the underworld -- suggest that Mars is not quite dead yet. Here the weight of the volcanic region is sinking and forming parallel graben (or rifts) that pull the crust of Mars apart, much like the cracks that appear on the top of a cake while its baking. According to, Staehler "it is possible that what we are seeing are the last remnants of this once active volcanic region or that the magma is right now moving eastward to the next location of eruption."

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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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Fate of sinking tectonic plates is revealed

Our world's surface is a jumble of jostling tectonic plates, with new ones emerging as others are pulled under. The ongoing cycle keeps our continents in motion and drives life on Earth. But what happens when a plate disappears into the planet's interior?

The question has long puzzled scientists because conventional wisdom said that sinking tectonic plates must remain intact to keep pulling on the portion behind it, but according to geophysical evidence, they are destroyed.

Now, in a study published Nov. 11 in Nature, scientists say they've found an answer that reconciles the two stories: Plates are significantly weakened as they sink but not so much that they break apart entirely.

The finding came after scientists put tectonic plates through a computer-generated gauntlet of destructive geologic forces. The model showed that as the plate enters the mantle, it bends abruptly downward, cracking its cold, brittle back. At the same time, the bending changes the fine grain structure of the rock along its underbelly, leaving it weakened. Combined, the stresses pinch the plate along its weak points, leaving it mostly intact but segmented like a slinky snake.

This means the plate continues to be pulled under despite becoming folded and distorted.

According to the researchers, the model predicted a scenario that matches observations from Japan. Studies of the region where the Pacific tectonic plate dives -- or subducts -- under Japan have turned up large cracks where the plate bends downward, and they have shown signs of weaker material underneath. Deep seismic imaging conducted by The University of Texas at Austin's Steve Grand has also revealed tectonic shapes in the Earth's mantle under Japan that appear a close match for the slinky snake in the model.

Co-author Thorsten Becker, a professor in UT's Jackson School of Geosciences, said that the study does not necessarily close the book on what happens to subducting plates, but it certainly gives a compelling case to explain several important geologic processes.

"It's an example of the power of computational geosciences," said Becker who assisted in developing the model and is a faculty associate at UT's Oden Institute for Computational Engineering & Sciences. "We combined these two processes that geology and rock mechanics are telling us are happening, and we learned something about the general physics of how the Earth works that we wouldn't have expected. As a physicist, I find that exciting."

The study's lead author, Taras Gerya, a professor of geophysics at ETH Zurich, added that until now, geophysicists had lacked a comprehensive explanation for how tectonic plates bend without breaking.

Things got interesting when the researchers ran their simulations with a hotter interior, similar to the early Earth. In these simulations, the tectonic snake segments made it only a few miles into the mantle before breaking off. That means that subduction would have occurred intermittently, raising the possibility that modern plate tectonics began only within the past billion years.

"Personally, I think there are a lot of good arguments for plate tectonics being much older," Becker said, "but the mechanism revealed by our model suggests things might be more sensitive to the temperature of the mantle than we thought, and that, I think, could lead to interesting new avenues of discussion."

Becker and Gerya were joined by David Bercovici, a geophysicist at Yale University whose investigation into how rock grains are altered in the deep mantle helped motivate the research. The study is based on a two-dimensional computer model of plate tectonics incorporating Bercovici's rock deformation research and other plate-weakening mechanics. The researchers are now studying the phenomena using 3D models and plan to investigate what those models can tell them about the occurrence of earthquakes.

Read more at Science Daily

Crushed resistance: Tectonic plate sinking into a subduction zone

The Earth's surface consists of a few large plates and numerous smaller ones that are continuously moving either away from or towards each other at an extremely slow pace. At the boundaries of two plates, the heavier oceanic plate sinks below the lighter continental plate in a process that experts call subduction. For a long time, though, those experts have been puzzling over what happens to the plate margin that dives into the Earth's mantle, known as the subducting slab. Some scientists assumed that the slab remains as rigid and strong as the plate itself and simply bends due to the gravity force and mechanical interaction with the Earth's mantle.

Heavily deformed plate margin

However, models of the Earth's interior constructed by scientists using seismic tomography revealed contradictory results: in the western United States, for example, the researchers observed anomalies at different depths on their tomographic images. These indicated that the slabs submerged beneath the Americas may be segmented. The scientists therefore concluded that the slabs in the mantle must be strongly deformed and are by no means rigid and immobile.

With the aid of computer models, other researchers, including ETH Professor Paul Tackley, confirmed that subducted slabs are indeed weak and deformable. And they formulated the subduction dichotomy hypothesis that can be expressed in simple terms: plates on the surface are rigid and strong (read: non-deformable), while the slabs in the mantle are soft and weak.

Seeking a plausible mechanism

"Until now, however, research has lacked a plausible mechanism to explain how this bending occurs and why sinking plate margins (slabs) become soft and weak," says Taras Gerya, Professor of Geophysics at ETH Zurich.

Observations revealed that numerous faults are found on the upper surface of a sinking plate where it meets the other plate. Seawater penetrates the plate through these faults and is in fact literally sucked in by suction forces. This weakens the plate on its upper side.

Yet this alone is not sufficient to explain the segmentation of the slab -- the anomalies observed on tomographic images. Another mechanism must also be at work to weaken the underside of the margin enough for segmentation to occur.

Gerya and his American colleagues David Bercovici and Thorsten Becker therefore suspected that compression of the underside of the plate at the point where it bends downward was "crushing" large and strong, millimetres size olivine crystals in the plate by forcing them to recrystallise into much weaker, micrometres size granular aggregate -- thereby reducing the plate's resistance and allowing it to bend.

Sinking plate margin divided into segments

Using a new two-dimensional computer model that integrated this grain reduction as a central mechanism, the three researchers then studied the process in silico. Their study was recently published in the journal Nature.

And indeed, the simulations revealed that sinking plates deform due to the massive reduction of olivine grains on their undersides, splitting into individual segments over time. These segments are rigid and stiff, but remain connected to each other by weak hinges made of ground grains.

In the simulations, parallel cracks appear at the segment boundaries on the plate's upper surface. Below these cracks are the zones with "crushed" mineral grains.

"Just imagine you're breaking a bar of chocolate," Gerya says with a grin. A bar of chocolate, too, can be divided into segments only along the specified weak points. The squares of chocolate are rigid, but the connecting pieces between them are weak. "That's why a sinking plate isn't uniformly bent or deformed, but segmented."

And here's how it might play out in reality: The heavier plate sinks under the lighter one. A weak spot with smaller mineral grains within the sinking plate allows it to bend. The bending stress causes the minerals to crumble in more places on the underside. The resulting weakness leads to a fracture, and a segment forms. As the plate margin sinks deeper and deeper into the mantle, it causes further segments to form at the bend. As a result, the slab eventually resembles a chain with rigid links and bendable connectors. At a depth of about 600 kilometres, the segmented plate margin slides onto what is known as the 670 km discontinuity in the Earth's mantle, from which point it moves horizontally.

Clues from nature support simulation

"The results of our simulations are consistent with observations in nature," Gerya explains. A great deal of research has been done on the natural situation along the Japan Trench, where the Pacific plate sinks below the Okhotsk plate. The pattern of faults found here is an exact match for the pattern produced in the simulations.

Researchers have also studied the seismic velocity structure of subducting Japan slab thoroughly using its recently produced high-resolution seismic tomography model. They found that the velocity of the seismic waves sent out by earthquakes was reduced at some nodes inside the slab. The pattern with which these nodes occur in reality coincides with that of the segment boundaries from the simulations. And both in nature and in the computer model, it is zones with very small crystals only micrometres across that are responsible for reducing the velocity of the seismic waves.

Read more at Science Daily

Earth's mantle, not its core, may have generated planet's early magnetic field

New research lends credence to an unorthodox retelling of the story of early Earth first proposed by a geophysicist at Scripps Institution of Oceanography at UC San Diego.

In a study appearing March 15 in the journal Earth and Planetary Science Letters, Scripps Oceanography researchers Dave Stegman, Leah Ziegler, and Nicolas Blanc provide new estimates for the thermodynamics of magnetic field generation within the liquid portion of the early Earth's mantle and show how long that field was available.

The paper provides a "door-opening opportunity" to resolve inconsistencies in the narrative of the planet's early days. Significantly, it coincides with two new studies from UCLA and Arizona State University geophysicists that expand on Stegman's concept and apply it in new ways.

"Currently we have no grand unifying theory for how Earth has evolved thermally," Stegman said. "We don't have this conceptual framework for understanding the planet's evolution. This is one viable hypothesis."

The trio of studies are the latest developments in a paradigm shift that could change how Earth history is understood.

It has been a bedrock tenet of geophysics that Earth's liquid outer core has always been the source of the dynamo that generates its magnetic field. Magnetic fields form on Earth and other planets that have liquid, metallic cores, rotate rapidly, and experience conditions that make the convection of heat possible.

In 2007, researchers in France proposed a radical departure from the long-held assumption that the Earth's mantle has remained entirely solid since the very beginnings of the planet. They argued that during the first half of the planet's 4.5-billion-year history, the bottom third of Earth's mantle would have had to have been molten, which they call "the basal magma ocean." Six years later, Stegman and Ziegler expanded upon that idea, publishing the first work showing how this once-liquid portion of the lower mantle, rather than the core, could have exceeded the thresholds needed to create Earth's magnetic field during that time.

The Earth's mantle is made of silicate material that is normally a very poor electrical conductor. Therefore, even if the lowermost mantle were liquid for billions of years, rapid fluid motions inside it wouldn't produce large electrical currents needed for magnetic field generation, similar to how Earth's dynamo currently works in the core. Stegman's team asserted the liquid silicate might actually be more electrically conductive than what was generally believed.

"Ziegler and Stegman first proposed the idea of a silicate dynamo for the early Earth," said UCLA geophysicist Lars Stixrude. The idea was met with skepticism because their early results "showed that a silicate dynamo was only possible if the electrical conductivity of silicate liquid was remarkably high, much higher than had been measured in silicate liquids at low pressure and temperature."

A team led by Stixrude used quantum-mechanical computations to predict the conductivity of silicate liquid at basal magma ocean conditions for the first time.

According to Stixrude, "we found very large values of the electrical conductivity, large enough to sustain a silicate dynamo." The UCLA study appeared in the Feb. 25 issue of Nature Communications.

In another paper, Arizona State geophysicist Joseph O'Rourke applied Stegman's concept to consider whether it's possible that Venus might have at one point generated a magnetic field within a molten mantle.

These new studies are signs that the premise is starting to take hold, but is still far from being widely accepted.

"No one is going to believe it until they do it themselves and now two other highly esteemed scientists have done it themselves," said Stegman.

"The pioneering studies of Dave Stegman and his collaborators directly inspired my work on Venus," said O'Rourke. "Their recent paper helps answer a question that vexed scientists for many years: How has Earth's magnetic field survived for billions of years?"

If Stegman's premise is correct, it would mean the mantle could have provided the young planet's first magnetic shield against cosmic radiation. It could also underpin studies of how tectonics evolved on the planet later in history.

"If the magnetic field was generated in the molten lower mantle above the core, then Earth had protection from the very beginning and that might have made life on Earth possible sooner," Stegman said.

Read more at Science Daily

NASA's treasure map for water ice on Mars

Mars illustration.

NASA has big plans for returning astronauts to the Moon in 2024, a stepping stone on the path to sending humans to Mars. But where should the first people on the Red Planet land?

A new paper published in Geophysical Research Letters will help by providing a map of water ice believed to be as little as an inch (2.5 centimeters) below the surface.

Water ice will be a key consideration for any potential landing site. With little room to spare aboard a spacecraft, any human missions to Mars will have to harvest what's already available for drinking water and making rocket fuel.

NASA calls this concept "in situ resource utilization," and it's an important factor in selecting human landing sites on Mars. Satellites orbiting Mars are essential in helping scientists determine the best places for building the first Martian research station. The authors of the new paper make use of data from two of those spacecraft, NASA's Mars Reconnaissance Orbiter (MRO) and Mars Odyssey orbiter, to locate water ice that could potentially be within reach of astronauts on the Red Planet.

"You wouldn't need a backhoe to dig up this ice. You could use a shovel," said the paper's lead author, Sylvain Piqueux of NASA's Jet Propulsion Laboratory in Pasadena, California. "We're continuing to collect data on buried ice on Mars, zeroing in on the best places for astronauts to land."

Buried Treasure on Mars

Liquid water can't last in the thin air of Mars; with so little air pressure, it evaporates from a solid to a gas when exposed to the atmosphere.

Martian water ice is locked away underground throughout the planet's mid-latitudes. These regions near the poles have been studied by NASA's Phoenix lander, which scraped up ice, and MRO, which has taken many images from space of meteor impacts that have excavated this ice. To find ice that astronauts could easily dig up, the study's authors relied on two heat-sensitive instruments: MRO's Mars Climate Sounder and the Thermal Emission Imaging System (THEMIS) camera on Mars Odyssey.

Why use heat-sensitive instruments when looking for ice? Buried water ice changes the temperature of the Martian surface. The study's authors cross-referenced temperatures suggestive of ice with other data, such as reservoirs of ice detected by radar or seen after meteor impacts. Data from Odyssey's Gamma Ray Spectrometer, which is tailor-made for mapping water ice deposits, were also useful.

As expected, all these data suggest a trove of water ice throughout the Martian poles and mid-latitudes. But the map reveals particularly shallow deposits that future mission planners may want to study further.

Picking a Landing Site

While there are lots of places on Mars scientists would like to visit, few would make practical landing sites for astronauts. Most scientists have homed in on the northern and southern mid-latitudes, which have more plentiful sunlight and warmer temperatures than the poles. But there's a heavy preference for landing in the northern hemisphere, which is generally lower in elevation and provides more atmosphere to slow a landing spacecraft.

A large portion of a region called Arcadia Planitia is the most tempting target in the northern hemisphere. The map shows lots of blue and purple in this region, representing water ice less than one foot (30 centimeters) below the surface; warm colors are over two feet (60 centimeters) deep. Sprawling black zones on the map represent areas where a landing spacecraft would sink into fine dust.

What's Next?

Piqueux is planning a comprehensive campaign to continue studying buried ice across different seasons, watching how the abundance of this resource changes over time.

The more we look for near-surface ice, the more we find," said MRO Deputy Project Scientist Leslie Tamppari of JPL. "Observing Mars with multiple spacecraft over the course of years continues to provide us with new ways of discovering this ice."

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

Scientists gain supervolcano insights from Wyoming granite

University of Wyoming researchers Davin Bagdonas and Carol Frost make observations on Lankin Dome, part of the Wyoming batholith, in central Wyoming's Granite Mountains.

Geophysical monitoring of the ground above active supervolcanoes shows that it rises and falls as magma moves beneath the surface of Earth. Silica-rich magmas like those in the Yellowstone region and along the western margin of North and South America can erupt violently and explosively, throwing vast quantities of ash into the air, followed by slower flows of glassy, viscous magma.

But what do the subterranean magma chambers look like, and where does the magma originate? Those questions can't be answered directly at modern, active volcanoes.

Instead, a new National Science Foundation (NSF)-funded study by University of Wyoming researchers suggests that scientists can go back into the past to study the solidified magma chambers where erosion has removed the overlying rock, exposing granite underpinnings. The study and its findings are outlined in a paper published in the June issue of American Mineralogist, the journal of the Mineralogical Society of America.

"Every geology student is taught that the present is the key to the past," says Carol Frost, director of the NSF's Division of Earth Sciences, on leave from UW, where she is a professor in the Department of Geology and Geophysics. "In this study, we used the record from past to understand what is happening in modern magma chambers."

One such large granite body, the 2.62 billion-year-old Wyoming batholith, extends more than 125 miles across central Wyoming. UW master's degree student Davin Bagdonas traversed the Granite, Shirley and Laramie mountains to examine the body, finding remarkable uniformity, with similar biotite granite throughout.

"It was monotonous," says Bagdonas, who worked on the project with Frost. "Only minor variations were observed in granite near the roof and margins of the intrusion."

This homogeneity indicates that the crystallizing magma was generally well-mixed. However, more subtle isotopic variations across the batholith show that the magma formed by melting of multiple rock sources that rose through multiple conduits, and that homogenization was incomplete.

Studies of the products of supervolcanoes and their possible batholithic counterparts at depth are a vibrant, controversial area of research, says Brad Singer, professor in the Department of Geoscience at the University of Wisconsin-Madison. He says the research by Frost and her colleagues offers "a novel perspective gleaned from the ancient Wyoming batholith, suggesting that it is the frozen portion of a vast magma system that could have fed supervolcanoes like those which erupted in northern Chile-southern Bolivia during the last 10 million years.

"The possibility of such a connection, while intriguing, does raise questions. The high silica and potassium contents of the Wyoming granites differ from the bulk magma compositions erupted by these huge Andean supervolcanos. This might mean that the Wyoming batholith records the complete solidification of potentially explosive magma at depth, without the eruption of much high-silica rhyolite," Singer says. "Notwithstanding, this paper will certainly provoke a deeper look into how ancient Archean granites can be used to leverage understanding of the 'volcanic-plutonic connection' at supervolcanoes."

Read more at Science Daily

Balanced Rocks Hint at San Andreas Secret

A mysterious group of balanced rocks that ought to have been knocked flat centuries ago may have let slip a deep, dark secret about the San Andreas Fault, according to a new study. 


For two decades a handful of researchers have been uncovering the power of centuries-old earthquakes by studying how easily it would be to tip the balanced rocks that dot the countryside: If precariously balanced rocks have stood for centuries, then the risk or frequency of large quakes is probably low in that area. But if only very sturdy balanced rocks can be found, then it could be that more frequent strong quakes knocked down everything else.

“So it's indirect evidence of what has not happened,” explained Lisa Grant Ludwig, the lead author of a paper about the balanced rocks in the latest issue of the journal Seismological Research Letters. And that's important for drawing up good earthquake hazard maps and establishing adequate building codes.

Then a few years ago researchers found a group of balanced rocks that seem to defy common sense, located in the San Bernadino Mountains, northeast of Los Angeles. These rocks are far too close to the large San Jacinto Fault, right where it edges near the even more notorious San Andreas Fault.

“Based on what we know about the physics of earthquakes and fault ruptures, these shouldn't be here,” said Ludwig. So Ludwig and her colleagues looked for the possible reasons the rocks have remained. “We kind of had a process of elimination.”

One possibility is that the balanced rocks are much younger than they appear and so they have not been around long enough to experience a less frequent, but strong earthquake. Maybe they are even younger than the great earthquakes of 1812 and 1857, the former of which famously toppled the big church, which still lies in ruins, at San Juan Capistrano.

To find out, the rocks were dated using cosmogenic dating techniques. These allow researchers to determine how long a rock surface has been exposed to the sky. That showed the rocks were in place for up to 18,000 years -- plenty of time to have experienced lots of San Andreas and San Jacinto quakes.

To find out, the rocks were dated using cosmogenic dating techniques. These allow researchers to determine how long a rock surface has been exposed to the sky. That showed the rocks were in place for up to 18,000 years -- plenty of time to have experienced lots of San Andreas and San Jacinto quakes.

Another possibility is that the rocks really aren't as fragile as they look. There are a few ways to estimate the fragility of the rocks, including directly pushing to see if they move and modeling the rocks and working out their center of gravity, to see how they might respond to shaking. But that was a dead end as well.

“They look fragile and the data came back that they're fragile too,” Ludwig said. “Finally it occurred to us that the San Andreas and San Jacinto faults are very close together -- just 2 to 2 1/2 kilometers apart there. We began to wonder if it was a step over.”

A step over is when one fault ruptures and then the rupture ends and jumps over to a nearby fault, no more than about 2 1/2 miles (4 kilometers) away. These have been known to happen, and usually suggest that the two faults are really connected deep underground.

Modeling of the faults by Ph.D. student Julian Lozos, a coauthor on the paper, showed that just such a thing could happen between the San Jacinto and the San Andreas, and that it could create a region of less shaking where the balanced rocks are found.

“The modeling shows that ... near the nucleation point (of a quake in that area) you can have rather low shaking,” said seismic hazard scientist Mark Stirling at GNS Science in New Zealand. “She might actually be identifying an area of relatively low shaking.”

Read more at Discovery News