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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New chip-based beam steering device lays groundwork for smaller, cheaper lidar

Researchers have developed a new chip-based beam steering technology that provides a promising route to small, cost-effective and high-performance lidar (or light detection and ranging) systems. Lidar, which uses laser pulses to acquire 3D information about a scene or object, is used in a wide range of applications such as autonomous driving, free-space optical communications, 3D holography, biomedical sensing and virtual reality.

"Optical beam steering is a key technology for lidar systems, but conventional mechanical-based beam steering systems are bulky, expensive, sensitive to vibration and limited in speed," said research team leader Hao Hu from the Technical University of Denmark. "Although devices known as chip-based optical phased arrays (OPAs) can quickly and precisely steer light in a non-mechanical way, so far, these devices have had poor beam quality and a field of view typically below 100 degrees."

In Optica, Optica Publishing Group's journal for high-impact research, Hu and co-author Yong Liu describe their new chip-based OPA that solves many of the problems that have plagued OPAs. They show that the device can eliminate a key optical artifact known as aliasing, achieving beam steering over a large field of view while maintaining high beam quality, a combination that could greatly improve lidar systems.

"We believe our results are groundbreaking in the field of optical beam steering," said Hu. "This development lays the groundwork for OPA-based lidar that is low cost and compact, which would allow lidar to be widely used for a variety of applications such as high-level advanced driver-assistance systems that can assist in driving and parking and increase safety."

A new OPA design

OPAs perform beam steering by electronically controlling light's phase profile to form specific light patterns. Most OPAs use an array of waveguides to emit many beams of light and then interference is applied in far field (away from the emitter) to form the pattern. However, the fact that these waveguide emitters are typically spaced far apart from each other and generate multiple beams in the far field creates an optical artifact known as aliasing. To avoid the aliasing error and achieve a 180° field of view, the emitters need to be close together, but this causes strong crosstalk between adjacent emitters and degrades the beam quality. Thus, until now, there has been a trade-off between OPA field of view and beam quality.

To overcome this trade-off, the researchers designed a new type of OPA that replaces the multiple emitters of traditional OPAs with a slab grating to create a single emitter. This setup eliminates the aliasing error because the adjacent channels in the slab grating can be very close to each other. The coupling between the adjacent channels is not detrimental in the slab grating because it enables the interference and beam formation in the near field (close to the single emitter). The light can then be emitted to the far field with the desired angle. The researchers also applied additional optical techniques to lower the background noise and reduce other optical artifacts such as side lobes.

High quality and wide field of view

To test their new device, the researchers built a special imaging system to measure the average far-field optical power along the horizontal direction over a 180° field of view. They demonstrated aliasing-free beam steering in this direction, including steering beyond ±70°, although some beam degradation was seen.

They then characterized beam steering in the vertical direction by tuning the wavelength from 1480 nm to 1580 nm, achieving a 13.5° tuning range. Finally, they showed the versatility of the OPA by using it to form 2D images of the letters "D," "T" and "U" centered at the angles of -60°, 0° and 60° by tuning both the wavelength and the phase shifters. The experiments were performed with a beam width of 2.1°, which the researchers are now working to decrease to achieve beam steering with a higher resolution and a longer range.

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Researchers use quantum-inspired approach to increase lidar resolution

Researchers have shown that a quantum-inspired technique can be used to perform lidar imaging with a much higher depth resolution than is possible with conventional approaches. Lidar, which uses laser pulses to acquire 3D information about a scene or object, is usually best suited for imaging large objects such as topographical features or built structures due to its limited depth resolution.

"Although lidar can be used to image the overall shape of a person, it typically doesn't capture finer details such as facial features," said research team leader Ashley Lyons from the University of Glasgow in the United Kingdom. "By adding extra depth resolution, our approach could capture enough detail to not only see facial features but even someone's fingerprints."

In the Optica Publishing Group journal Optics Express, Lyons and first author Robbie Murray describe the new technique, which they call imaging two-photon interference lidar. They show that it can distinguish reflective surfaces less than 2 millimeters apart and create high-resolution 3D images with micron-scale resolution.

"This work could lead to much higher resolution 3D imaging than is possible now, which could be useful for facial recognition and tracking applications that involve small features," said Lyons. "For practical use, conventional lidar could be used to get a rough idea of where an object might be and then the object could be carefully measured with our method."

Using classically entangled light

The new technique uses "quantum inspired" interferometry, which extracts information from the way that two light beams interfere with each other. Entangled pairs of photons -- or quantum light -- are often used for this type of interferometry, but approaches based on photon entanglement tend to perform poorly in situations with high levels of light loss, which is almost always the case for lidar. To overcome this problem, the researchers applied what they've learned from quantum sensing to classical (non-quantum) light.

"With quantum entangled photons, only so many photon pairs per unit time can be generated before the setup becomes very technically demanding," said Lyons. "These problems don't exist with classical light, and it is possible to get around the high losses by turning up the laser power."

When two identical photons meet at a beam splitter at the same time they will always stick together, or become entangled, and leave in the same direction. Classical light shows the same behavior but to a lesser degree -- most of the time classical photons go in the same direction. The researchers used this property of classical light to very precisely time the arrival of one photon by looking at when two photons simultaneously arrive at detectors.

Enhancing depth resolution

"The time information gives us the ability to perform depth ranging by sending one of those photons out onto the 3D scene and then timing how long it takes for that photon to come back," said Lyons. "Thus, two-photon interference lidar works much like conventional lidar but allows us to more precisely time how long it takes for that photon to reach the detector, which directly translates into greater depth resolution."

The researchers demonstrated the high depth resolution of two-photon interference lidar by using it to detect the two reflective surfaces of a piece of glass about 2 millimeters thick. Traditional lidar wouldn't be able to distinguish these two surfaces, but the researchers were able to clearly measure the two surfaces. They also used the new method to create a detailed 3D map of a 20-pence coin with 7-micron depth resolution. This shows that the method could capture the level of detail necessary to differentiate key facial features or other differences between people.

Two-photon interference lidar also works very well at the single-photon level, which could enhance more complex imaging approaches used for non-line-of-sight imaging or imaging through highly scattering media.

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Archaeologists reveal pre-Hispanic cities in Bolivia with laser technology

More than 20 years ago, Dr. Heiko Prümers from the German Archaeological Institute and Prof. Dr. Carla Jaimes Betancourt from the University of Bonn, at that time a student in La Paz, began archaeological excavations on two "mounds" near the village of Casarabe in Bolivia. The Mojos Plains is a southwestern fringe of the Amazon region. Even though the savannah plain, which flooded several months a year during rainy season, does not encourage permanent settlement, there are still many visible traces of the time before Spanish colonization at the beginning of the 16th century. Next to the "mounds," these traces include mainly causeways and canals that often lead for kilometers in a dead straight line across the savannahs.

"This indicated a relatively dense settlement in pre-Hispanic times. Our goal was to conduct basic research and trace the settlements and life there," says Heiko Prümers. In earlier studies, the researchers already found that the Casarabe culture -- named after the nearby village -- dates to the period between 500 and 1400 AD and, according to current knowledge, extended over a region of around 16,000 square kilometers. The "mounds" turned out to be eroded pyramid stumps and platform buildings.

Initial conventional surveys revealed a terraced core area, a ditch-wall enclosing the site, and canals. In addition, it became apparent that some of these pre-Hispanic settlements were enormous in size. "However, the dense vegetation under which these settlements were located prevented us from seeing the structural details of the monumental mounds and their surroundings," says Carla Jaimes Betancourt from the Department for the Anthropology of the Americas at the University of Bonn.

LIDAR technology used in the Amazon for the first time

To find out more, the researchers used the airborne laser technology LIDAR (Light Detection and Ranging) for the first time in the Amazon region. This involves surveying the terrain with a laser scanner attached to a helicopter, small aircraft or drone that transmits around 1.5 million laser pulses per second. In a subsequent evaluation step, the vegetation is digitally removed creating a digital model of the earth's surface, which can also be displayed as a 3D image. "The first results were excellent and showed how effective the technology was even in dense rainforest. From that moment on, the desire arose to map the large settlements of the Casarabe culture using LIDAR technology," says study leader Dr. Heiko Prümers.

For the current study, in 2019 the team together with Prof. Dr. José Iriarte and Mark Robinson from the University of Exeter, mapped a total of 200 square kilometers of the Casarabe cultural area. The evaluation done by the company ArcTron3 held a surprise. What came to light were two remarkably large sites of 147 hectares and 315 hectares in a dense four-tiered settlement system. "With a north-south extension of 1.5 kilometers and an east-west extension of about one kilometer, the largest site found so far is as large as Bonn was in the 17th century," says co-author Prof. Dr. Carla Jaimes Betancourt.

It is not yet possible to estimate how many people lived there. "However, the layout of the settlement itself tells us that planners and many active hands were at work here," says Heiko Prümers. Modifications made to the settlement, for example the expansion of the rampart-ditch system, also speak to a reasonable increase in population. "For the first time, we can refer to pre-Hispanic urbanism in the Amazon and show the map of the Cotoca site, the largest settlement of the Casarabe culture known to us so far," Prümers emphasizes. In other parts of the world similar agrarian cities with low population densities had already been found.

LIDAR shows anthropogenically altered landscape

LIDAR mapping reveals the architecture of the settlement's large squares. Stepped platforms topped by U-shaped structures, rectangular platform mounds, and conical pyramids (up to 22 meters high). Causeway-like paths and canals connect the individual settlements and indicate a tight social fabric. At least one other settlement can be found within five kilometers of each of the settlements known today. "So the entire region was densely settled, a pattern that overturns all previous ideas," says Carla James Betancourt, who is a member of the Transdisciplinary Research Area "Present Pasts" at the University of Bonn.

The researchers emphasize that for all the euphoria about the site mappings and the possibilities they offer for reinterpreting the settlements in their geographic setting, the real archaeological work is just beginning. The goal for the future, they say, is to understand how these large regional centers functioned.

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3D holographic head-up display could improve road safety

Researchers have developed the first LiDAR-based augmented reality head-up display for use in vehicles. Tests on a prototype version of the technology suggest that it could improve road safety by 'seeing through' objects to alert of potential hazards without distracting the driver.

The technology, developed by researchers from the University of Cambridge, the University of Oxford and University College London (UCL), is based on LiDAR (light detection and ranging), and uses LiDAR data to create ultra high-definition holographic representations of road objects which are beamed directly to the driver's eyes, instead of 2D windscreen projections used in most head-up displays.

While the technology has not yet been tested in a car, early tests, based on data collected from a busy street in central London, showed that the holographic images appear in the driver's field of view according to their actual position, creating an augmented reality. This could be particularly useful where objects such as road signs are hidden by large trees or trucks, for example, allowing the driver to 'see through' visual obstructions. The results are reported in the journal Optics Express.

"Head-up displays are being incorporated into connected vehicles, and usually project information such as speed or fuel levels directly onto the windscreen in front of the driver, who must keep their eyes on the road," said lead author Jana Skirnewskaja, a PhD candidate from Cambridge's Department of Engineering. "However, we wanted to go a step further by representing real objects in as panoramic 3D projections."

Skirnewskaja and her colleagues based their system on LiDAR, a remote sensing method which works by sending out a laser pulse to measure the distance between the scanner and an object. LiDAR is commonly used in agriculture, archaeology and geography, but it is also being trialled in autonomous vehicles for obstacle detection.

Using LiDAR, the researchers scanned Malet Street, a busy street on the UCL campus in central London. Co-author Phil Wilkes, a geographer who normally uses LiDAR to scan tropical forests, scanned the whole street using a technique called terrestrial laser scanning. Millions of pulses were sent out from multiple positions along Malet Street. The LiDAR data was then combined with point cloud data, building up a 3D model.

"This way, we can stitch the scans together, building a whole scene, which doesn't only capture trees, but cars, trucks, people, signs, and everything else you would see on a typical city street," said Wilkes. "Although the data we captured was from a stationary platform, it's similar to the sensors that will be in the next generation of autonomous or semi-autonomous vehicles."

When the 3D model of Malet St was completed, the researchers then transformed various objects on the street into holographic projections. The LiDAR data, in the form of point clouds, was processed by separation algorithms to identify and extract the target objects. Another algorithm was used to convert the target objects into computer-generated diffraction patterns. These data points were implemented into the optical setup to project 3D holographic objects into the driver's field of view.

The optical setup is capable of projecting multiple layers of holograms with the help of advanced algorithms. The holographic projection can appear at different sizes and is aligned with the position of the represented real object on the street. For example, a hidden street sign would appear as a holographic projection relative to its actual position behind the obstruction, acting as an alert mechanism.

In future, the researchers hope to refine their system by personalising the layout of the head-up displays and have created an algorithm capable of projecting several layers of different objects. These layered holograms can be freely arranged in the driver's vision space. For example, in the first layer, a traffic sign at a further distance can be projected at a smaller size. In the second layer, a warning sign at a closer distance can be displayed at a larger size.

"This layering technique provides an augmented reality experience and alerts the driver in a natural way," said Skirnewskaja. "Every individual may have different preferences for their display options. For instance, the driver's vital health signs could be projected in a desired location of the head-up display.

"Panoramic holographic projections could be a valuable addition to existing safety measures by showing road objects in real time. Holograms act to alert the driver but are not a distraction."

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