From temblor: “Dispute: Do Magnitude 7-8 mainshocks commonly trigger immediate damaging aftershocks up to 300 km (180 mi) away?”

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temblor

December 29, 2017
Ross Stein, Ph.D, Temblor

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By analyzing a series of great earthquakes off the coast of Sumatra, Yue et al. argue that the seismic signals interpreted by Fan and Shearer as aftershocks are actually water reverberations in near-shore regions. (Photo from: trekkersblog.com)

In September 2016, Wenyuan Fan and Peter Shearer, from the Scripps Institution of Oceanography at U.C. San Diego, published an astonishing discovery in Science. Fan and Shearer detected nearly 50 previously unidentified M≥5.5 aftershocks up to 300 kilometers (200 mi) away from their M=7-8 mainshocks during the brief passage of the seismic ‘surface’ waves from the mainshock, or within 3 minutes. The authors concluded that remote dynamic triggering commonly exists and fundamentally promotes aftershock occurrence throughout the globe.

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This is an annotated version of Fig. 2b of Fan and Shearer (2016), for the case of a 2013 mainshock off the Japan coast. The contours give location uncertainties of the mainshock(magenta) and aftershock (turquoise). The aftershock locates between the trench and the coast, where the seafloor begins to shallow. Virtually all the discovered aftershocks occur between oceanic trenches and the coast.

How could these large aftershocks have been overlooked?

There are no large, immediate, and remote aftershocks in any seismic catalog, and so most of us had concluded that this must be exceedingly rare, or is non-existent. That’s because today, any M≥4.5 shock anywhere on Earth can be reliably detected.

But, during the first few minutes after a large mainshock, its seismic wave train could obscure aftershocks, and so during this period, large shocks could conceivably have been hidden. To overcome this obstacle, Fan and Shearer used a technique in which a continent’s worth of seismometers are retroactively trained on the site of a single large earthquake halfway around the globe, and then used to track where the seismic energy was released in time. The method, called ‘beam back-projection,’ was introduced by Miaki Ishii, Peter Shearer, Heidi Houston and John Vidale in 2005 (Ishii et al., 2005). Although remote dynamic triggering of tiny aftershocks is well known (Velasco et al., 2008; Parsons et al., 2014), there are only a few examples of M≥5.5 aftershocks (Johnson et al., 2015), the most impressive of which was the 2012 M=8.6 Indian Ocean shock, which triggered large aftershocks all over the globe (Pollitz et al., 2012). But those aftershocks struck over several days—not minutes—long after the seismic waves had vanished.

Implications of the Fan-Shearer hypothesis

If they are correct, the hazard after a large mainshock would be more widespread than understood today, and the first several minutes after a large mainshock are more dangerous than we currently assume. But there is another, equally important, implication: For historical quakes, whose magnitudes and locations are interpreted from contemporary intensity reports (descriptions of shaking), we might be overestimating their magnitudes and blurring their locations, because widespread shaking in aftershocks would be misconstrued as caused by the mainshock.

A Challenge by Yue et al.

In October 2017, Han Yue, from Beijing University, Jorge C. Castellanos, Chunquan Yu, and Lingsen Meng from UCLA and Zhongwen Zhan from Caltech published a rebuttal of the Fan-Shearer hypothesis in Geophysical Research Letters. In a nutshell, Yue et al. argue that the seismic signals interpreted by Fan and Shearer as aftershocks are actually water reverberations in near-shore regions. The reverberations are triggered by the seismic waves launched by the mainshock. Fan and Shearer had raised this possibility in their paper, but ultimately dismissed it. Yue et al. present a series of falsification tests, but I am going to focus on what I consider the two most persuasive.

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This is a simplified and annotated version of Fig. 3 of Yue et al. (2017). P waves transmit through rock and water, but S waves only through rock. So, if the energy pulses northeast (landward) of the trench were indeed aftershocks, they should appear in both panels, but they do not. ‘Seismic energy’ is the beam back-projection amplitude. The ‘+’ signs refer to the pulses in time shown in the figure below.

In the figure above, energy from a M=7.2 mainshock southwest of the trench is imaged by P waves. The energy is spread over about 100 km because this is roughly the rupture area of the shock. There are also strong energy pulses landward of the trench, near the ‘10 s resonance contour.’ These are the pulses identified by Fan and Shearer as aftershocks. But Yue et al. point out that if these were aftershocks, they should also appear when using S waves. But they are absent in the right-hand panel above. If, instead, they were water reverberations, they should appear in the P wave panel but not in the S wave panel, because S waves do not transmit in water. So, this would seem to be a very strong test, which the Fan-Shearer hypothesis does not pass.

Singing seismograms

In a second falsification test, Yue et al lined up seismograms of the M=7.2 mainshock recorded throughout the hemisphere. It takes about 25 s for a M=7.2 earthquake to rupture, and in those first 20-30 seconds, one sees the somewhat chaotic signature of the rupture. But beginning at 61 s (and perhaps at 51 s) one can see a coherent pulse on all the records (the red ‘+’ signs in the figure below). This pattern repeats at least three times at 10 s intervals (green, blue, and cyan ‘+’ signs in the figure below).

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This is a simplified and annotated version of Yue et al. Fig. 2. Seismograms from throughout the hemisphere show coherent reverberations every 10 s. This becomes evident 61 s after the mainshock, and lasts at least until 92 s. This rhythmic ringing is unlikely to be caused by an earthquake, whose oscillations would normally be much more irregular. Yue et al. located the source of the ringing; those ‘+’ icons are shown in the preceding figure.

Yue et al argue that earthquakes do not produce such simple and periodic pulses. When Yue et al. located the source of the pulses, they land next to the beam back-projection energy pulses that Fan and Shearer identified as aftershocks. In addition, the pulses are very close to the seafloor depth contour that would produce the observed 10 s resonance. So, it would be hard to argue that water reverberation was not occurring, and occurring right where Fan and Shearer identified aftershocks.

Dueling posters at the Fall Meeting of the American Geophysical Union

Fortunately, Wenyuan Fan (now a Post-Doctoral Scholar at Woods Hole Oceanographic Institution) and Han Yue presented side-by-side posters at AGU Meeting in New Orleans two weeks ago. This gave everyone the chance to see both sides of the story, and it also enabled me to pose questions to each author based on the arguments and rebuttals of the other.

Fan and Shearer believe that with more tuning, an aftershock energy pulse might emerge in the S wave analysis. They now concede that water reverberations are evident in the signals, but they argue that these are water reverberations from the remote aftershock, not the mainshock. So, while remote dynamic aftershocks might be less common than they originally proposed, it still occurs. Han Yue says that he cannot (yet) eliminate this possibility, and so the debate continues.

So, who’s right?

In addition to the falsification tests, two other factors lead me to believe that Yue et al. are likely correct, and that few if any of the signals are actually aftershocks. Why would different types of mainshocks (thrust, extensional, and strike-slip) all trigger aftershocks at about the same water depth between the trench and the coast? This just seems very unlikely. Beyond that, if there is a simpler, quotidian explanation for a phenomenon (water reverberation), then it should be favored over a more exotic interpretation (heretofore unseen aftershocks).

With that said, debates like this are essential to science, which only advances when bold new ideas are promulgated, and promulgated in a manner that can be unambiguously tested. And for that we can thank Fan and Shearer. All we can really do in science is falsify hypotheses; proving something right is extremely difficult.

Here is a video of seismic wave propagation through rock and water by Yue et al., 2017

The first ring-like wave launched from the 20-km deep hypocenter is the P wave, traveling at about 7 km/sec; the second is S wave, traveling at about half that speed. The video is moving at about realtime. The thick black line is the seafloor. At the site of the epicenter, the seafloor is about 5 km (3 mi) deep. The camera moves with the advancing waves toward the coast. Water reverberations become most pronounced when the seafloor shallows to about 2 km deep, at a distance of about 220 km. The P waves bounce back and forth every 10 s or so in the water.

References
[Unfortunately the writer does not simply give us the links, but you can retrieve them with the references that they do give.]

Wenyuan Fan and Peter M. Shearer (2016), Local near instantaneously dynamically triggered aftershocks of large earthquakes, Science, 353, 1133-1136, DOI: 10.1126/science.aag0013.

Miaki Ishii, Peter M. Shearer, Heidi Houston, and John E. Vidale, Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the HI-Net array (2005), Nature, DOI: 10.1038/nature03675.

Christopher W. Johnson, R. Bürgmann, and F. F. Pollitz (2015), Rare dynamic triggering of remote M≥ 5.5 earthquakes from global catalog analysis, J. Geophys. Res., 120, 1748–1761, doi:10.1002/ 2014JB011788.

Tom Parsons, Margaret Segou, Warner Marzocchi (2014), The global aftershock zone, Tectonophysics, 618, 1–34, .doi.org/10.1016/j.tecto.2014.01.038

Fred F. Pollitz, Ross S. Stein, Volkan Sevilgen, and Roland Bürgmann (2012), The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide, Nature, 490, 250–253, DOI:10.1038/nature11504.

Aaron A. Velasco, S. Hernandez, T. Parsons, and K. Pankow (2008), Global ubiquity of dynamic earthquake triggering, Nature Geoscience, 1, 375–379, doi:10.1038/ngeo204

Han Yue, Jorge C. Castellanos, Chunquan Yu, Lingsen Meng, and Zhongwen Zhan (2017), Localized water reverberation phases and its impact on backprojection images, Geophys. Res. Letts., DOI: 10.1002/2017GL073254.

See the full article here .

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You can help many citizen scientists in detecting earthquakes and getting the data to emergency services people in affected area.
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Quake-Catcher Network

The Quake-Catcher Network is a collaborative initiative for developing the world’s largest, low-cost strong-motion seismic network by utilizing sensors in and attached to internet-connected computers. With your help, the Quake-Catcher Network can provide better understanding of earthquakes, give early warning to schools, emergency response systems, and others. The Quake-Catcher Network also provides educational software designed to help teach about earthquakes and earthquake hazards.

After almost eight years at Stanford, and a year at CalTech, the QCN project is moving to the University of Southern California Dept. of Earth Sciences. QCN will be sponsored by the Incorporated Research Institutions for Seismology (IRIS) and the Southern California Earthquake Center (SCEC).

The Quake-Catcher Network is a distributed computing network that links volunteer hosted computers into a real-time motion sensing network. QCN is one of many scientific computing projects that runs on the world-renowned distributed computing platform Berkeley Open Infrastructure for Network Computing (BOINC).

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The volunteer computers monitor vibrational sensors called MEMS accelerometers, and digitally transmit “triggers” to QCN’s servers whenever strong new motions are observed. QCN’s servers sift through these signals, and determine which ones represent earthquakes, and which ones represent cultural noise (like doors slamming, or trucks driving by).

There are two categories of sensors used by QCN: 1) internal mobile device sensors, and 2) external USB sensors.

Mobile Devices: MEMS sensors are often included in laptops, games, cell phones, and other electronic devices for hardware protection, navigation, and game control. When these devices are still and connected to QCN, QCN software monitors the internal accelerometer for strong new shaking. Unfortunately, these devices are rarely secured to the floor, so they may bounce around when a large earthquake occurs. While this is less than ideal for characterizing the regional ground shaking, many such sensors can still provide useful information about earthquake locations and magnitudes.

USB Sensors: MEMS sensors can be mounted to the floor and connected to a desktop computer via a USB cable. These sensors have several advantages over mobile device sensors. 1) By mounting them to the floor, they measure more reliable shaking than mobile devices. 2) These sensors typically have lower noise and better resolution of 3D motion. 3) Desktops are often left on and do not move. 4) The USB sensor is physically removed from the game, phone, or laptop, so human interaction with the device doesn’t reduce the sensors’ performance. 5) USB sensors can be aligned to North, so we know what direction the horizontal “X” and “Y” axes correspond to.

If you are a science teacher at a K-12 school, please apply for a free USB sensor and accompanying QCN software. QCN has been able to purchase sensors to donate to schools in need. If you are interested in donating to the program or requesting a sensor, click here.

BOINC is a leader in the field(s) of Distributed Computing, Grid Computing and Citizen Cyberscience.BOINC is more properly the Berkeley Open Infrastructure for Network Computing, developed at UC Berkeley.

Earthquake safety is a responsibility shared by billions worldwide. The Quake-Catcher Network (QCN) provides software so that individuals can join together to improve earthquake monitoring, earthquake awareness, and the science of earthquakes. The Quake-Catcher Network (QCN) links existing networked laptops and desktops in hopes to form the worlds largest strong-motion seismic network.

Below, the QCN Quake Catcher Network map
QCN Quake Catcher Network map

Earthquake country is beautiful and enticing

Almost everything we love about areas like the San Francisco bay area, the California Southland, Salt Lake City against the Wasatch range, Seattle on Puget Sound, and Portland, is brought to us by the faults. The faults have sculpted the ridges and valleys, and down-dropped the bays, and lifted the mountains which draw us to these western U.S. cities. So, we enjoy the fruits of the faults every day. That means we must learn to live with their occasional spoils: large but infrequent earthquakes. Becoming quake resilient is a small price to pay for living in such a great part of the world, and it is achievable at modest cost.

A personal solution to a global problem

Half of the world’s population lives near active faults, but most of us are unaware of this. You can learn if you are at risk and protect your home, land, and family.

Temblor enables everyone in the continental United States, and many parts of the world, to learn their seismic, landslide, tsunami, and flood hazard. We help you determine the best way to reduce the risk to your home with proactive solutions.

Earthquake maps, soil liquefaction, landslide zones, cost of earthquake damage

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ShakeAlert: An Earthquake Early Warning System for the West Coast of the United States

The U. S. Geological Survey (USGS) along with a coalition of State and university partners is developing and testing an earthquake early warning (EEW) system called ShakeAlert for the west coast of the United States. Long term funding must be secured before the system can begin sending general public notifications, however, some limited pilot projects are active and more are being developed. The USGS has set the goal of beginning limited public notifications in 2018.

Watch a video describing how ShakeAlert works in English or Spanish.

The primary project partners include:

United States Geological Survey
California Governor’s Office of Emergency Services (CalOES)
California Geological Survey
California Institute of Technology
University of California Berkeley
University of Washington
University of Oregon
Gordon and Betty Moore Foundation

The Earthquake Threat

Earthquakes pose a national challenge because more than 143 million Americans live in areas of significant seismic risk across 39 states. Most of our Nation’s earthquake risk is concentrated on the West Coast of the United States. The Federal Emergency Management Agency (FEMA) has estimated the average annualized loss from earthquakes, nationwide, to be $5.3 billion, with 77 percent of that figure ($4.1 billion) coming from California, Washington, and Oregon, and 66 percent ($3.5 billion) from California alone. In the next 30 years, California has a 99.7 percent chance of a magnitude 6.7 or larger earthquake and the Pacific Northwest has a 10 percent chance of a magnitude 8 to 9 megathrust earthquake on the Cascadia subduction zone.

Part of the Solution

Today, the technology exists to detect earthquakes, so quickly, that an alert can reach some areas before strong shaking arrives. The purpose of the ShakeAlert system is to identify and characterize an earthquake a few seconds after it begins, calculate the likely intensity of ground shaking that will result, and deliver warnings to people and infrastructure in harm’s way. This can be done by detecting the first energy to radiate from an earthquake, the P-wave energy, which rarely causes damage. Using P-wave information, we first estimate the location and the magnitude of the earthquake. Then, the anticipated ground shaking across the region to be affected is estimated and a warning is provided to local populations. The method can provide warning before the S-wave arrives, bringing the strong shaking that usually causes most of the damage.

Studies of earthquake early warning methods in California have shown that the warning time would range from a few seconds to a few tens of seconds. ShakeAlert can give enough time to slow trains and taxiing planes, to prevent cars from entering bridges and tunnels, to move away from dangerous machines or chemicals in work environments and to take cover under a desk, or to automatically shut down and isolate industrial systems. Taking such actions before shaking starts can reduce damage and casualties during an earthquake. It can also prevent cascading failures in the aftermath of an event. For example, isolating utilities before shaking starts can reduce the number of fire initiations.

System Goal

The USGS will issue public warnings of potentially damaging earthquakes and provide warning parameter data to government agencies and private users on a region-by-region basis, as soon as the ShakeAlert system, its products, and its parametric data meet minimum quality and reliability standards in those geographic regions. The USGS has set the goal of beginning limited public notifications in 2018. Product availability will expand geographically via ANSS regional seismic networks, such that ShakeAlert products and warnings become available for all regions with dense seismic instrumentation.

Current Status

The West Coast ShakeAlert system is being developed by expanding and upgrading the infrastructure of regional seismic networks that are part of the Advanced National Seismic System (ANSS); the California Integrated Seismic Network (CISN) is made up of the Southern California Seismic Network, SCSN) and the Northern California Seismic System, NCSS and the Pacific Northwest Seismic Network (PNSN). This enables the USGS and ANSS to leverage their substantial investment in sensor networks, data telemetry systems, data processing centers, and software for earthquake monitoring activities residing in these network centers. The ShakeAlert system has been sending live alerts to “beta” users in California since January of 2012 and in the Pacific Northwest since February of 2015.

In February of 2016 the USGS, along with its partners, rolled-out the next-generation ShakeAlert early warning test system in California joined by Oregon and Washington in April 2017. This West Coast-wide “production prototype” has been designed for redundant, reliable operations. The system includes geographically distributed servers, and allows for automatic fail-over if connection is lost.

This next-generation system will not yet support public warnings but does allow selected early adopters to develop and deploy pilot implementations that take protective actions triggered by the ShakeAlert notifications in areas with sufficient sensor coverage.

Authorities

The USGS will develop and operate the ShakeAlert system, and issue public notifications under collaborative authorities with FEMA, as part of the National Earthquake Hazard Reduction Program, as enacted by the Earthquake Hazards Reduction Act of 1977, 42 U.S.C. §§ 7704 SEC. 2.

For More Information

Robert de Groot, ShakeAlert National Coordinator for Communication, Education, and Outreach
rdegroot@usgs.gov
626-583-7225

Learn more about EEW Research

ShakeAlert Fact Sheet

ShakeAlert Implementation Plan