From temblor: “A tectonic squeeze may be loading three thrust faults beneath central Los Angeles”

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From temblor

September 17, 2018
Chris Rollins

Thrust-faulting earthquakes are a fact of life in Los Angeles and a threat to it. Three such earthquakes in the second half of the 20th century painfully etched this ongoing threat to life, limb and infrastructure into the memories and the backs of the minds of many who call this growing metropolis home. The first struck 40 seconds after 6:00 AM on a February morning in 1971 when a section of a thrust fault beneath the western San Gabriel Mountains ruptured in a magnitude 6.7 tremor. The earthquake killed 60 people, including 49 in the catastrophic collapse of the Veterans Administration Hospital in Sylmar, the closest town to the event (which is often referred to as the Sylmar earthquake). Among other structures hit hard were the newly built Newhall Pass interchange at the junction of Interstate 5 and California State Route 14, of which multiple sections collapsed, and the Van Norman Dam, which narrowly avoided failure in what could have been a cruel deja vu for a city that had been through deadly dam disasters in 1928 and 1963.

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Devastation at the Veterans Administration Hospital in the 1971 Sylmar earthquake. Photo courtesy of Los Angeles Times.

Sixteen years later, a section of the Puente Hills thrust fault ruptured in the magnitude 5.9 Whittier Narrows earthquake, killing eight people in East Los Angeles and bringing attention to a class of thrust faults that do not break the surface, called “blind” thrust faults, which will go on to form a key part of this story. Then early on another winter morning in 1994, an even more deeply buried blind thrust fault ruptured beneath the San Fernando Valley in the magnitude 6.7 Northridge earthquake, causing tens of billions of dollars in damage and taking 57 lives. One of the fatalities was Los Angeles police officer Clarence Wayne Dean, who died on his motorcycle when a span of the Newhall Pass interchange that had been rebuilt following the 1971 Sylmar earthquake collapsed again as he was riding across it in the predawn darkness.

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Collapse of the Newhall Pass (I-5/CA-14) interchange in the 1994 Northridge earthquake. Officer Dean died on the downed section of overpass at right. The interchange has since been renamed the Clarence Wayne Dean Memorial Interchange in his memory. Photo courtesy of CNN.

LA’s problem: The squeeze

Thrust earthquakes like these, in which the top side of the fault is thrust up and over the bottom side, will likely strike Los Angeles again in the 21st century. They may in fact pose a greater hazard to the city than earthquakes on the nearby San Andreas Fault because they can occur directly beneath the central metropolitan area. This means that a city that has found so much of its identity and place in history from being improvised as it went, and from being a cultural and economic melting pot, now faces the unwieldy task of readying its diverse infrastructure and populace for the strong shaking these kinds of earthquakes can produce.

One way that the earthquake science community has been assessing the seismic hazard in LA is by using geodesy – long-term, high-precision monitoring of the deformation of the Earth’s surface – to locate sections of faults that are stuck, or locked, causing the Earth’s crust to deform around them. It is this bending of the crust, or accumulated strain, that is violently released in earthquakes; therefore the locations where this bending is taking place might indicate where future earthquakes will occur, and perhaps how large and frequent they could be. Several decades of geodetic monitoring have shown that the greater Los Angeles area is being squeezed from north to south at roughly 8-9 millimeters per year (⅓ inch per year), about one-fourth the rate at which human fingernails grow. Thrust faults, such as those on which the Sylmar, Whittier Narrows and Northridge earthquakes struck, are ultimately driven by this compression.

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Geodetic data, tectonics and material properties relevant to the problem. Dark blue arrows show the north-south tectonic compression inferred by Argus et al. [2005] after removing deformation caused by aquifer and oil use. Black lines are faults, dashed where blind. Background shading is a measure of material stiffness at the surface based on the Community Velocity Model [Shaw et al., 2015]. “Beach balls” show the locations and senses of slip of the 1971 Sylmar, 1987 Whittier Narrows and 1994 Northridge earthquakes. Figure simplified from Rollins et al. [2018].

Why the science is still very much ongoing

The task of linking the north-south tectonic squeeze to specific faults encounters several unique challenges in Los Angeles. First, the city sits atop not only active faults but also several aquifers and oil fields that have long provided part of its livelihood and continue to be used today, which deforms the crust around them. Geodetic data are affected by this anthropogenic deformation, to the extent that a recent study used these data to observe Los Angeles “breathing” water from year to year and even to resolve key hydrological properties of particular sections of aquifers. This spectacular deformation, which furnishes science that can be used in resource management around the world, has the unfortunate effect of obscuring the more gradual north-south tectonic shortening in Los Angeles in these data.

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Animation from Riel et al. [2018] showing long-term subsidence of the Earth’s surface due to use of the Los Angeles and Santa Ana aquifers.

Second, the faults are a complex jumble. The crust underlying Los Angeles is cut by thrust faults, strike-slip faults like the San Andreas Fault and subparallel to it, and other strike-slip faults nearly perpendicular to it. Although these faults all take part in accommodating the gradual north-south squeeze, the relative contributions of the thrust and strike-slip faults in doing so has been the subject of debate. The problem of estimating strain accumulation on subsurface faults is also generally at the mercy of uncertainties as to how faults behave at depth in the Earth’s crust and how they intersect and link up.

Third, Los Angeles sits atop a deep sedimentary basin, created when a previous episode of extension created a “hole” in the crust that was gradually filled by sediments eroded off the surrounding mountain ranges. These sedimentary layers are more easily deformed than the stiffer rocks in the mountains around the basin, complicating the problem of estimating strain accumulation at depth from the way the surface is deforming. Finally, as in the case of the Puente Hills Fault, some of the major thrust faults in Los Angeles do not break the surface but are “blind.” This means that the bending of the crust around locked sections of these faults is buried and more difficult to detect at the surface.

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Basin sediments affect the relationship between fault slip and deformation at the surface by up to 50% for the cases of the Puente Hills Fault (left) and Compton Fault (right). For the same fault slip, the basin is more compliant and so the Earth’s surface is displaced more (red arrows) than if it were absent (blue arrows). Figure simplified from Rollins et al. [2018].

Three thrust faults may be doing a lot of the work

Several important advances over the past two decades have paved pathways towards overcoming these challenges. The signal of deformation due to water and oil management can be subtracted from the geodetic data to yield a clearer picture of the tectonic shortening. The geometries of faults at depth have also come into focus, as earth scientists at the Southern California Earthquake Center and Harvard University have compiled decades of oil well logs and seismic reflection data to build the Community Fault Model, a detailed 3D picture of these complex geometries. A parallel effort has yielded the Community Velocity Model, a 3D model of the structure and composition of the Southern California crust that is internally consistent with the fault geometries.

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A cross section of faults and earthquakes across central Los Angeles from Rollins et al. [2018]. Red lines are faults, dashed where uncertain; pairs of arrows along the thrust faults show their long-term sense of slip. White circles are earthquakes. Basin structure is from the Community Velocity Model.

Recently, a team of researchers from Caltech, JPL and USC (with contributions from many other earthquake scientists) has begun to put these pieces together. Their approaches and findings were published in the Journal of Geophysical Research (JGR) this summer. On the challenge presented by the complex array of faults, the study found that the strike-slip faults probably accommodate less than 20% of the total shortening at the max, leaving the rest to be explained by thrust faulting or other processes. Three thrust faults, the Sierra Madre, Puente Hills and Compton faults, stand out in particular as good candidates. All three appear to span the Los Angeles basin from west to east, and the Puente Hills and Sierra Madre faults have generated moderate earthquakes in the last three decades, including the Whittier Narrows shock and a magnitude 5.8 tremor in 1991. Paleoseismology (the study of prehistoric earthquakes) has also revealed that these three faults have each generated multiple earthquakes in the past 15,000 years whose magnitudes may have exceeded 7.0.

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Alternative models of how quickly strain is accumulating on the Compton, Puente Hills and Sierra Madre Faults, assuming that the transition between completely locked (stuck) and freely slipping patches of fault is gradual (left) or sharp (right), simplified from Rollins et al. [2018]. Gray lines are major highways.

How fast is stress building up on these faults?

Exploring a wide range of assumptions (such as whether the transitions between stuck and unstuck sections of faults may be gradual or abrupt), the team inferred that the Sierra Madre, Puente Hills and Compton faults appear to be partially or fully locked and building up stress on their upper (shallowest) sections. The estimated total rate of strain accumulation on the three faults is equivalent to a magnitude 6.7-6.8 earthquake like the Sylmar earthquake once every 100 years, or a magnitude 7.0 shock every 250 years. These back-of-the-envelope calculations, however, belie the fact that this strain is likely released by earthquakes across a wide range of magnitudes. The team is currently working to assess just how wide this range of magnitudes practically needs to be: whether the strain can be released as fast as it is accruing without needing to invoke earthquakes larger than Sylmar and Northridge, for example, or whether the M>7 thrust earthquakes inferred from paleoseismology are indeed a likely part of the picture over the long term.

This picture of strain accumulation will sharpen as the methods used to build it are improved, as community models of faults and structure continue to be refined, and especially as more high-resolution data, such as that used to observe LA “breathing” water, is brought to bear on the estimation problem. The tolls of the Sylmar, Whittier Narrows and Northridge earthquakes in lives and livelihoods are a reminder that we should work as fast as possible to understand the menace that lies beneath the City of Angels.

References

Argus, D. F., Heflin, M. B., Peltzer, G., Crampé, F., & Webb, F. H. (2005). Interseismic strain accumulation and anthropogenic motion in metropolitan Los Angeles. Journal of Geophysical Research: Solid Earth 110(B4).

Riel, B. V., Simons, M., Ponti, D., Agram, P., & Jolivet, R. (2018). Quantifying ground deformation in the Los Angeles and Santa Ana coastal basins due to groundwater withdrawal. Water Resources Research 54(5), 3557-3582.

Rollins, C., Avouac, J.-P., Landry, W., Argus, D. F., & Barbot, S. D. (2018). Interseismic strain accumulation on faults beneath Los Angeles, California. Journal of Geophysical Research: Solid Earth 123, doi: 10.1029/2017JB015387.

Shaw, J. H., Plesch, A., Tape, C., Suess, M. P., Jordan, T. H., Ely, G., Hauksson, E., Tromp, J., Tanimoto, T., & Graves, R. (2015). Unified structural representation of the southern California crust and upper mantle. Earth and Planetary Science Letters 415: 1-15.

See the full article here .


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Please help promote STEM in your local schools.

Stem Education Coalition

Earthquake Alert

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Earthquake Alert

Earthquake Network project

Earthquake Network is a research project which aims at developing and maintaining a crowdsourced smartphone-based earthquake warning system at a global level. Smartphones made available by the population are used to detect the earthquake waves using the on-board accelerometers. When an earthquake is detected, an earthquake warning is issued in order to alert the population not yet reached by the damaging waves of the earthquake.

The project started on January 1, 2013 with the release of the homonymous Android application Earthquake Network. The author of the research project and developer of the smartphone application is Francesco Finazzi of the University of Bergamo, Italy.

Get the app in the Google Play store.

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Smartphone network spatial distribution (green and red dots) on December 4, 2015

Meet The Quake-Catcher Network

QCN bloc

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).

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

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