From temblor: “Faults slip slowly in Cascadia”

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From temblor

March 17, 2020
Noel Bartlow, Ph.D., Assistant Researcher, Berkeley Seismology Laboratory

The Cascadia Subduction Zone has occasional large earthquakes and frequent slow-slip events. A new study quantifies how these slow-slip events accommodate tectonic plate motion.

Subduction zones such as the one beneath the U.S. Pacific Northwest and British Columbia are capable of generating very large and destructive earthquakes. But not all of the tectonic motion accommodated in these areas causes earthquakes that can be felt. Episodic tremor and slip, a type of aseismic fault slip or slow slip, accounts for a large amount of fault motion on the deeper extent of the Cascadia Subduction Zone. A new study reveals what this means for future large earthquakes in the region.

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Seattle, Wash., sits on top of the Cascadia Subduction Zone. Credit: CommunistSquared

A quiet Cascadia comes to life

Just off the Pacific Northwest coast, the Juan de Fuca Plate collides with the North American Plate. Here, the Juan de Fuca Plate slides beneath North America, forming the Cascadia Subduction Zone. The contact between these two plates, called the plate interface, is stuck due to friction. Slip on the plate interface is necessary to accommodate the collision of the two plates. The Cascadia Subduction Zone plate interface slips every few hundred years in very large earthquakes with magnitudes approaching or even above 9.0. These earthquakes generate dangerous tsunamis similar to the 2011 magnitude-9.0 Tohoku-Oki earthquake and accompanying tsunami in Japan. The last such event in the Cascadia region occurred more than 320 years ago on January 26, 1700.

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Map showing the Cascadia subduction zone, the Gorda and Explorer “plates” are part of the larger Juan de Fuca tectonic plate, but move in slightly different directions and can be considered sub-plates.

In between large earthquakes, the plate interface is not silent. Instead it chatters to life every few months with episodic tremor and slip events. In these events, the plate interface slips as it would in an earthquake but takes much longer to do so, releasing the same energy as an earthquake with a magnitude of up to 6.8 over a period of a few days to weeks. These events do not produce dangerous shaking, but they do contain information about how the subduction zone is behaving. An episodic tremor and slip event differs from other slow-slip events in that episodic tremor and slip events recur frequently and are also accompanied by numerous tiny earthquakes called tremor, which are too small for humans to feel.

In a new study published in the journal Geophysical Research Letters, I reveal how much plate motion is accommodated by these events in the Cascadia Subduction Zone.

Using GPS satellites to observe plate motion

Knowing where and how much slip occurs during these events helps scientists understand how they may influence the location and timing of a future large earthquake.

Measuring slip on a plate interface is not as easy as pulling out a yard stick, however.

The plate interface lies below Earth’s surface, so to find out how much slip occurs during these events, we needed to look at what we can see—the ground beneath our feet. This is where satellites come in.

In this study, I used satellite GPS observations to determine how much ground motion occurs during each event. I then calculated how much slip had to occur along the plate interface at depth to account for that ground motion. I tallied up ground motion during these events to find the cumulative effect of all episodic tremor and slip events across the Cascadia Subduction Zone over the last 15-25 years, averaged over time. Applying this systematic approach across the region revealed that not all parts of the subduction zone are behaving the same.

A highly variable system

GPS observations and other data collected over the past few decades show that the Juan de Fuca and North American plates are moving toward each other at 40 millimeters per year in the northern part of the subduction zone near Seattle, and 31 millimeters per year in the southern part near Cape Mendocino, CA. The rate of motion between the two plates defines the “slip budget”, or the total amount of slip that must be accommodated everywhere on the plate interface. I compare this total to the amount released in episodic tremor and slip to understand its role in the overall accommodation of slip on the plate interface.

Episodic tremor and slip events accommodate a highly variable amount of slip along the length of the subduction zone. This has implications for how stress is distributed along the plate interface, and thus where future large earthquakes may nucleate.

In some areas, the slow-slip events account for all of the measured plate convergence. In the very southern part of the subduction zone, slow slip actually releases more slip than the expected convergence rate of the two plates. This might mean that the plates are moving together faster than previous estimates in this region. In other areas of the interface, the slow-slip events accommodate only a fraction of the convergence motion of the two plates—one-fourth or less of the motion in some places. This means that a lot of the motion between the two plates must be released in another way, most likely as steady creep of the plates past one another but potentially also in future earthquakes.

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Motions of GPS sites in the Cascadia region during episodic tremor and slip events, modified from Bartlow (2020). Each arrow represents one GPS station and its motion relative to the subducting plate. Motions are greatly exaggerated.

Identifying regions at risk

Previous work on the plate interface in this region revealed the locations where the plate is locked—that is, where friction prevents slip between the two plates (Schmalzle et al., 2014). Large earthquakes occur in these locked sections when the lock is abruptly broken. My results show that slow slip generally occurs in a region offset from the locked section of the plate interface. This means that at present, these events are less likely to trigger large earthquakes than if they were located right at the edge of the locked zone.

The main region of episodic tremor and slip in Cascadia is in an area with no locking. This means that the full slip budget is accommodated by episodic tremor and slip. In the majority of the subduction zone where episodic tremor and slip takes up less than the full slip budget, the plate interface is creeping along at a slower rate between these events.

It is possible that over time the episodic tremor and slip events will migrate closer to the locked zone over time. If this were to occur, it may indicate that the next big earthquake is on the horizon. It is also possible that slow-slip events will become larger or more frequent when a large earthquake is imminent. It is therefore important to monitor episodic tremor and slip in Cascadia over time. The method we applied here can be used to detect these changes and therefore remains an important tool in earthquake hazard monitoring.

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A) Time-averaged episodic tremor and slip rate (colors) and contours of density of tremor detections (brown lines) on the Cascadia plate interface (modified from Bartlow, 2020). B) Same as A, but with a comparison to the location of the locked zone (red and yellow colors) from Schmalzle et al. (2014).

See the full article here .


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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