From temblor: “Magnitude-5.1 earthquake rattles southeastern U.S”

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From temblor

August 10, 2020
John E. Ebel, Ph.D., Senior Research Scientist, Weston Observatory of Boston College

It was Sunday morning a little before 8:30 am when my phone beeped with a text message. A friend in Greer, South Carolina had texted me to say that she and her husband had just felt an earthquake “around 8:10 am for ~5 seconds.” By then, I had already gotten alerts from my seismic monitoring system in New England that it had detected earthquake signals and from the UGSS that a magnitude 5.1 earthquake had taken place in western North Carolina. I immediately texted my friends with the initial information about the earthquake location, magnitude and felt area.

For many people, the magnitude-5.1 earthquake at 8:07 am EDT on 9 August 2020 was a surprise. The epicenter, just southeast of the town of Sparta, North Carolina, was not in any recognized seismic zone. It was nowhere near the well-known New Madrid seismic zone beneath the Mississippi River in the vicinity of western Tennessee, northeastern Arkansas, southeastern Missouri and western Kentucky. It was well away from the seismic zone at Charleston, South Carolina. It was almost 100 miles east of the northern end of the somewhat diffuse eastern Tennessee seismic zone.

The takeaway: this earthquake was a rare but not unusual occurrence in both its location and magnitude.

A quake in a diffuse seismic zone

The highest concentration of earthquakes in this part of the southeastern U.S. is contained within the eastern Tennessee seismic zone, approximately outlined in the blue box in the following map. In this zone, earthquakes are occurring between 3 miles and 15 miles below the earth’s surface. Scientists have a poor understanding of why there is a concentration of earthquakes here. The seismic events are taking place in deep, 1-billion-year-old basements rocks, below the surface rocks that had been pushed atop the basement rocks by more recent collisions of tectonic plate (Wheeler, 1995). This makes it difficult to study the earthquakes of this seismic zone.

Even less well understood by seismologists is another band of diffuse earthquakes that runs parallel the eastern Tennessee seismic zone — about 50-100 miles east, just east of the western boundary of North Carolina. To date, there have been no scientific studies of this less active band seismicity in North Carolina. Yesterday’s earthquake is at the northeastern end of this diffuse band of earthquakes.

If before yesterday I had been asked where North Carolina’s next magnitude 5.1 earthquake would take place, I would have said somewhere in this earthquake band in the westernmost part of the state.

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Earthquakes of magnitude 2.5 and greater from 1 January 1980 to 9 August 2020. The eastern Tennessee seismic zone is outline in blue, and the blue dot shows the location of the 9 August 2020 earthquake.

Magnitude-5.0+ quakes do occur

From historical records, the largest earthquake that took place within the state of North Carolina was a shock on 21 February 1916. That earthquake was centered near Ashville, North Carolina in the western part of the state, and it caused some cracked plaster and chimneys in the area surrounding its epicenter. There are no seismographic recordings of this 1916 earthquake from which an instrumental measurement of the magnitude could be made. However, based on how far away residents reported shaking and the maximum intensity of shaking, scientists estimate this 1916 earthquake was a magnitude-5.2.

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A map of the felt area — the area in which residents reported shaking of various intensities, measured by the Mercalli scale — of the 21 February 1916 earthquake. Credit: North Carolina Department of Environmental Quality.

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“Did You Feel It” intensity reports from the first 12 hours after the 9 August 2020 earthquake. Credit: USGS.

Both the 1916 earthquake and yesterday’s tremor were felt over roughly comparable areas to the northeast, east and south. However, the 1916 earthquake seems to have been felt more strongly in eastern Tennessee and farther west in Tennessee than yesterday’s earthquake. For both, the strongest ground shaking corresponded to about modified Mercalli VI to VII shaking, meaning potentially damaging to damaging shaking. These data indicate that the two earthquakes are of comparable magnitude, with the 1916 earthquake probably being slightly larger.

Two large quakes in just over 100 years

Despite evidence suggesting that the location and magnitude of yesterday’s quake is not out of the ordinary, the occurrence of this and the 1916 quake is only 104 years apart. Would one expect two earthquakes in western North Carolina of magnitudes 5.2 and 5.1 to be separated by 104 years?

We can estimate the average time between earthquakes of a given magnitude if we know the total number of earthquakes of any magnitude that have occurred in the same area, by way of something called a Gutenberg-Richter distribution. From this mathematical distribution, scientists calculate the average time between earthquakes of different magnitudes. The average time between magnitude 5.1 earthquakes in western North Carolina is 203 years.

Globally, the time interval between earthquakes can vary greatly around the average value from the Gutenberg-Richter distribution. For the San Andreas fault near Los Angeles, the average time between major earthquakes is about 140 years. However, the shortest time interval between the known large earthquakes over the past 2,000 years is 45 years, whereas the longest time interval is over 300 years. Thus, the 104 years that separated the 1916 quake and yesterday’s quake is not unexpected given the calculated average time and the known variations from average earthquake repeat times in other parts of the world.

Earthquakes occur within plate interiors

Magnitude-5.1 earthquake rattles southeastern U.S.
Posted on August 10, 2020 by Temblor

Yesterday’s magnitude-5.1 quake in western North Carolina was felt throughout the southeastern U.S. Although it came as a shock, a quake of this magnitude is not unexpected.

By John E. Ebel, Ph.D., Senior Research Scientist, Weston Observatory of Boston College; Professor of Geophysics, Department of Earth and Environmental Sciences, Boston College

Citation: Ebel, J., 2020, Magnitude-5.1 earthquake rattles southeastern U.S., Temblor, http://doi.org/10.32858/temblor.109

The takeaway: this earthquake was a rare but not unusual occurrence in both its location and magnitude.

Further Reading

Dunn, Meredith M. and Martin C. Chapman. Fault orientation in the eastern Tennessee seismic zone: A study using the double-difference earthquake location algorithm, Seismological Research Letters, vol. 77, no. 4, pp. 494-504, July/August 2006.

Wheeler, Russell L. Earthquakes and the southeastern boundary of the intact Iapetan margin in eastern North America, Seismological Research Letters, vol. 67, no. 5, pp. 77-83, September/October 1996.

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