Scientists use ‘virtual earthquakes’ to forecast Los Angeles quake risk

Stanford scientists are using weak vibrations generated by the Earth’s oceans to produce “virtual earthquakes” that can be used to predict the ground movement and shaking hazard to buildings from real quakes.

The new technique, detailed in the Jan. 24 issue of the journal Science, was used to confirm a prediction that Los Angeles will experience stronger-than-expected ground movement if a major quake occurs south of the city.

“We used our virtual earthquake approach to reconstruct large earthquakes on the southern San Andreas Fault and studied the responses of the urban environment of Los Angeles to such earthquakes,” said lead author Marine Denolle, who recently received her PhD in geophysics from Stanford and is now at the Scripps Institution of Oceanography in San Diego.

The new technique capitalizes on the fact that earthquakes aren’t the only sources of seismic waves. “If you put a seismometer in the ground and there’s no earthquake, what do you record? It turns out that you record something,” said study leader Greg Beroza, a geophysics professor at Stanford.

What the instruments will pick up is a weak, continuous signal known as the ambient seismic field. This omnipresent field is generated by ocean waves interacting with the solid Earth. When the waves collide with each other, they generate a pressure pulse that travels through the ocean to the sea floor and into the Earth’s crust. “These waves are billions of times weaker than the seismic waves generated by earthquakes,” Beroza said.

Scientists have known about the ambient seismic field for about 100 years, but it was largely considered a nuisance because it interferes with their ability to study earthquakes. The tenuous seismic waves that make up this field propagate every which way through the crust. But in the past decade, seismologists developed signal-processing techniques that allow them to isolate certain waves; in particular, those traveling through one seismometer and then another one downstream.

Denolle built upon these techniques and devised a way to make these ambient seismic waves function as proxies for seismic waves generated by real earthquakes. By studying how the ambient waves moved underground, the researchers were able to predict the actions of much stronger waves from powerful earthquakes.

She began by installing several seismometers along the San Andreas Fault to specifically measure ambient seismic waves.

Employing data from the seismometers, the group then used mathematical techniques they developed to make the waves appear as if they originated deep within the Earth. This was done to correct for the fact that the seismometers Denolle installed were located at the Earth’s surface, whereas real earthquakes occur at depth.

In the study, the team used their virtual earthquake approach to confirm the accuracy of a prediction, made in 2006 by supercomputer simulations, that if the southern San Andreas Fault section of California were to rupture and spawn an earthquake, some of the seismic waves traveling northward would be funneled toward Los Angeles along a 60-mile-long (100-kilometer-long) natural conduit that connects the city with the San Bernardino Valley. This passageway is composed mostly of sediments, and acts to amplify and direct waves toward the Los Angeles region.

Until now, there was no way to test whether this funneling action, known as the waveguide-to-basin effect, actually takes place because a major quake has not occurred along that particular section of the San Andreas Fault in more than 150 years.

The virtual earthquake approach also predicts that seismic waves will become further amplified when they reach Los Angeles because the city sits atop a large sedimentary basin. To understand why this occurs, study coauthor Eric Dunham, an assistant professor of geophysics at Stanford, said to imagine taking a block of plastic foam, cutting out a bowl-shaped hole in the middle, and filling the cavity with gelatin. In this analogy, the plastic foam is a stand-in for rocks, while the gelatin is like sediments, or dirt. “The gelatin is floppier and a lot more compliant. If you shake the whole thing, you’re going to get some motion in the Styrofoam, but most of what you’re going to see is the basin oscillating,” Dunham said.

As a result, the scientists say, Los Angeles could be at risk for stronger, and more variable, ground motion if a large earthquake – magnitude 7.0 or greater – were to occur along the southern San Andreas Fault, near the Salton Sea.

“The seismic waves are essentially guided into the sedimentary basin that underlies Los Angeles,” Beroza said. “Once there, the waves reverberate and are amplified, causing stronger shaking than would otherwise occur.”

Beroza’s group is planning to test the virtual earthquake approach in other cities around the world that are built atop sedimentary basins, such as Tokyo, Mexico City, Seattle and parts of the San Francisco Bay area. “All of these cities are earthquake threatened, and all of them have an extra threat because of the basin amplification effect,” Beroza said.

Because the technique is relatively inexpensive, it could also be useful for forecasting ground motion in developing countries. “You don’t need large supercomputers to run the simulations,” Denolle said.

In addition to studying earthquakes that have yet to occur, the technique could also be used as a kind of “seismological time machine” to recreate the seismic signatures of temblors that shook the Earth long ago, according to Beroza.

“For an earthquake that occurred 200 years ago, if you know where the fault was, you could deploy instruments, go through this procedure, and generate seismograms for earthquakes that occurred before seismographs were invented,” he said.

New model of Earth’s interior reveals clues to hotspot volcanoes

This is a map view of seismic shear-wave speed in the earth's upper mantle, highlighting the slow wave-speed channels (warm colors) imaged in this study. Where present, the channels align with the direction of tectonic-plate motion (dashed lines). -  Berkeley Seismological Laboratory, UC Berkeley
This is a map view of seismic shear-wave speed in the earth’s upper mantle, highlighting the slow wave-speed channels (warm colors) imaged in this study. Where present, the channels align with the direction of tectonic-plate motion (dashed lines). – Berkeley Seismological Laboratory, UC Berkeley

Scientists at the University of California, Berkeley, have detected previously unknown channels of slow-moving seismic waves in Earth’s upper mantle, a discovery that helps explain “hotspot volcanoes” that give birth to island chains such as Hawaii and Tahiti.

Unlike volcanoes that emerge from collision zones between tectonic plates, hotspot volcanoes form in the middle of the plates. The prevalent theory for how a mid-plate volcano forms is that a single upwelling of hot, buoyant rock rises vertically as a plume from deep within Earth’s mantle the layer found between the planet’s crust and core and supplies the heat to feed volcanic eruptions.

However, some hotspot volcano chains are not easily explained by this simple model, suggesting that a more complex interaction between plumes and the upper mantle is at play, said the study authors.

The newfound channels of slow-moving seismic waves, described in a paper to be published Thursday, Sept. 5, in Science Express, provide an important piece of the puzzle in the formation of these hotspot volcanoes and other observations of unusually high heat flow from the ocean floor.

The formation of volcanoes at the edges of plates is closely tied to the movement of tectonic plates, which are created as hot magma pushes up through fissures in mid-ocean ridges and solidifies. As the plates move away from the ridges, they cool, harden and get heavier, eventually sinking back down into the mantle at subduction zones.

But scientists have noticed large swaths of the seafloor that are significantly warmer than expected from this tectonic plate-cooling model. It had been suggested that the plumes responsible for hotspot volcanism could also play a role in explaining these observations, but it was not entirely clear how.

“We needed a clearer picture of where the extra heat is coming from and how it behaves in the upper mantle,” said the study’s senior author, Barbara Romanowicz, UC Berkeley professor of earth and planetary sciences and a researcher at the Berkeley Seismological Laboratory. “Our new finding helps bridge the gap between processes deep in the mantle and phenomenon observed on the earth’s surface, such as hotspots.”

The researchers utilized a new technique that takes waveform data from earthquakes around the world, and then analyzed the individual “wiggles” in the seismograms to create a computer model of Earth’s interior. The technology is comparable to a CT scan.

The model revealed channels dubbed “low-velocity fingers” by the researchers where seismic waves traveled unusually slowly. The fingers stretched out in bands measuring about 600 miles wide and 1,200 miles apart, and moved at depths of 120-220 miles below the seafloor.

Seismic waves typically travel at speeds of 2.5 to 3 miles per second at these depths, but the channels exhibited a 4 percent slowdown in average seismic velocity.

“We know that seismic velocity is influenced by temperature, and we estimate that the slowdown we’re seeing could represent a temperature increase of up to 200 degrees Celsius,” said study lead author Scott French, UC Berkeley graduate student in earth and planetary sciences.

The formation of channels, similar to those revealed in the computer model, has been theoretically suggested to affect plumes in Earth’s mantle, but it has never before been imaged on a global scale. The fingers are also observed to align with the motion of the overlying tectonic plate, further evidence of “channeling” of plume material, the researchers said.

“We believe that plumes contribute to the generation of hotspots and high heat flow, accompanied by complex interactions with the shallow upper mantle,” said French. “The exact nature of those interactions will need further study, but we now have a clearer picture that can help us understand the ‘plumbing’ of Earth’s mantle responsible for hotspot volcano islands like Tahiti, Reunion and Samoa.”

Earthquakes and tectonics in Pamir Tien Shan

Earthquake damage to buildings is mainly due to the existing shear waves which transfer their energy during an earthquake to the houses. These shear waves are significantly influenced by the underground and the topography of the surrounding area. Detailed knowledge of the landform and the near-surface underground structure is, therefore, an important prerequisite for a local seismic hazard assessment and for the evaluation of the ground-effect, which can strongly modify and increase local ground motion.

As described in the latest issue of Geophysical Journal International, a team of scientists from the GFZ German Research Center for Geosciences could show that it is possible to map complex shear wave velocity structures almost in real time by means of a newly developed tomgraphic approach.

The method is based on ambient seismic noise recordings and analyses. “We use small, hardly noticeable amplitude ground motions as well as anthropogenic ground vibrations”, Marco Pilz, a scientist at GFZ, explains. “With the help of these small signals we can obtain detailed images of the shallow seismic velocity structure”. In particular, images and velocity changes in the underground due to earthquakes and landslides can be obtained in almost real time.

“What is new about our method is the direct calculation of the shear wave velocity. Moreover, we are working on a local, small-scale level — compared to many other studies”, Marco Pilz continues.

This method has already been successfully applied: Many regions of Central Asia are threatened by landslides. Since the shear wave velocity usually drops significantly before a landslide slip this technique offers the chance to monitor changes in landslide prone areas almost in real time.

Further application can be used in earthquake research. The authors were able to map the detailed structure of a section of the Issyk-Ata fault, Kyrgyzstan, which runs along the southern border of the capital city, Bishkek, with a population of approx. 900.000 inhabitants. They showed that close to the surface of the mapped section a splitting into two different small fault branches can be observed. This can influence the pace of expansion or also an eventual halting of the propagation on the main fault.

Central Asia is extensively seismically endangered; the accompanying processes and risks are investigated by the Central-Asian Institute of Applied Geosciences (CAIAG) in Bishkek, a joint institution established by the GFZ and the Kyrgyz government.

Why do these earthquakes occur?

The Pamir and Tien Shan are the result of the crash of two continental plates: the collision of India and Eurasia causes the high mountain ranges. This process is still ongoing today and causes breaking of the Earths crust, of which earthquakes are the consequence.

A second group of GFZ-scientists has investigated together with colleagues from Tajikistan and CAIAG the tectonic process of collision in this region. They were, for the first time, able to image continental crust descending into the Earth’s mantle. In the scientific journal Earth and Planetary Sciences Letters the scientists report that this subduction of continental crust has, to date, never been directly observed. To make their images, the scientists applied a special seismological method (so-called receiver function-analysis) on seismograms that had been collected in a two years long field experiment in the Tien Shan-Pamir-Hindu Kush area. Here, the collision of the Indian and Eurasian plates presents an extreme dimension.

“These extreme conditions cause the Eurasian lower crust to subduct into the Earth’s mantle”, explains Felix Schneider from the GFZ German Research Centre for Geosciences.” Such a subduction can normally be observed during the collision of ocean crust with continental crust, as the ocean floors are heavier than continental rock.”

Findings at the surface of metamorphic rocks that must have arisen from ultra-high pressures deep in the Earth’s mantle also provide evidence for subduction of continental crust in the Pamir region. Furthermore, the question arises, how the occurrence of numerous earthquakes at unusual depths of down to 300 km in the upper mantel can be explained. Through the observation of the subducting part of the Eurasian lower crust, this puzzle could, however, be solved.

Earthquake seer wins accolade from star gazers

An ANU seismologist whose work could help forecast the damage path of future earthquakes has been honoured by one of the world’s top scientific organisations.

The Royal Astronomical Society (RAS) in London has awarded its 2008 Gold Medal for Geophysics to Professor Brian Kennett, Director of the Research School of Earth Sciences at ANU. In the citation, the researcher is described as “one of the most complete seismologists of his generation”.

Professor Kennett works on determining the structure of the Earth using the waves generated by earthquakes and man-made sources. His reference models for the structure of the Earth have been adopted as standards for the location of earthquakes across the globe, and are widely used in imaging the planet’s interior.

“It’s an honour to be recognised for my work by the Royal Astronomical Society, but the best reward is learning more about the powerful forces at play within the Earth,” Professor Kennett said.

“Some of my recent work in cooperation with Japanese colleagues has shown how the ground motion generated by a large earthquake is affected by Earth’s structure.

“With three-dimensional computational models it’s now possible to simulate the effect of possible earthquakes to assess likely patterns of damage. For example, puzzling observations – such as strong ground motion on the east coast of Japan from earthquakes at 600 km almost beneath China – can be explained by guided waves trapped with the descending Pacific Sea plate.

Professor Kennett’s work combines theoretical, computational and observational approaches to gain comprehensive information about the three-dimensional structure within the Earth on scales from local, through regional to global. He has worked on the techniques of seismic tomography, where structure is reconstructed from the properties of the seismic waves passing through it a similar way to a CAT scan in medical work. “Such studies reveal the detailed properties of the subduction zones that generate great earthquakes, such as the 2004 Sumatran-Andaman event that produced the devastating tsunami across Southeast Asia,” Professor Kennett said.

He has also led a major program of work on the structure of beneath the Australian region using deployments of portable instruments that provide high-fidelity recordings of earthquakes. The analysis of the seismograms recorded by these instruments has revealed strong contrasts at depth beneath Australia that link to the ages of rocks exposed at surface.