From Caltech: “Fifty Years Ago A Major Earthquake Shifted the Course of Seismology in SoCal”

Caltech Logo

From Caltech

February 08, 2021

Written by
Kimm Fesenmaier

Technical Contact
Robert Perkins
(626) 395‑1862

Credit: Caltech/USGS

The 1971 San Fernando quake led the USGS and Caltech to join forces, expanding seismic monitoring through the region

February 9 marks 50 years since the devastating 1971 San Fernando earthquake that rocked Los Angeles. The magnitude-6.6 temblor was the worst the region had experienced for decades. But out of the tragedy came a period of tremendous advances in earthquake science and also in increasing public safety during earthquakes in Southern California.

Just seconds after 6 a.m. on February 9, 1971, a 12-mile section of an under-appreciated fault along the San Gabriel Mountains suddenly and dramatically slipped. The entire Los Angeles region was rattled, but the shaking was particularly violent in the northeastern corner of the San Fernando Valley. By its end, two large hospitals (including one that was just months old) lay destroyed, powerlines had fallen, gas lines had exploded, freeway overpasses had collapsed, and many older buildings were damaged beyond repair. In the end, 65 people lost their lives, more than 2,000 other individuals were injured, and more than $500 million in property damage was apparent.

It might be hard to imagine today because good information is available at our fingertips almost immediately following any earthquake. But on that day, the network of seismometers that monitor ground shaking in Southern California was fledgling, and scientists knew very little about what actually happened during an earthquake.

“People hadn’t even started to ask the important questions about how earthquakes really happen,” says Thomas Heaton (PhD ’78), professor of engineering seismology, emeritus, at Caltech. “The 1971 San Fernando earthquake marked a major transition in earthquake science, and Caltech was very much in the lead in that transition.”

North-Trending fracture pattern near the Sylmar Converter Station above the upon Van Norman Dam. The fracture was due to a landslide and the dam’s setting in extensive fill material. Credit: USGS/Public domain.

The quake struck at the end of a period of significant urban expansion in Los Angeles, when the region’s first tall buildings had recently been constructed. One of the requirements for building such tall structures had been to keep a record of the shaking they experienced during earthquakes. As a result, the San Fernando earthquake was the first that was well-recorded by dozens of nearby seismometers.

“This was the first time we really had a glimpse of what the shaking was like around a major earthquake,” explains Heaton. “It allowed us to really begin to understand what the earthquake process was like.”

Heaton himself has made computer models of what happened during the 1971 quake—of what exactly happened along the fault. The models that best fit the actual records from the event turned out to be very different from what earthquake scientists would have expected at that time. Eventually those efforts, in combination with work on additional earthquakes, led to a completely new idea of earthquake physics: earthquakes unfold over time, with faults starting to slip in one place with the slip moving outward and migrating along the fault.

There was also a new realization among scientists after the 1971 earthquake that the thrust faults along the mountain ranges to the north of the L.A. region, such as the San Fernando and Sierra Madre faults, could produce large-magnitude quakes. The focus before San Fernando had been on the San Andreas and Newport-Inglewood faults. When the 1994 Northridge earthquake happened and was nearly a twin to the San Fernando event, scientists knew much more about what to expect.

Equally important after the quake, Heaton says, was the great sense among earthquake scientists and engineers that monitoring and reporting systems had to be improved. When the San Fernando quake hit, it knocked out power to most of the L.A. region. In the most badly damaged area in the San Fernando Valley, all communications went down, so it was difficult for emergency responders to know where to focus their efforts. Seismologists too were left virtually blind. Caltech’s Seismological Laboratory normally received records of shaking via the telephone lines, but those were down as well.

“Our inability to respond to that earthquake really had a strong impact on me and many of my colleagues to try to build a system that would provide information during the emergency to help emergency managers know what to do,” says Heaton.

Immediately after the 1971 earthquake, the U.S. Geological Survey (USGS), which had been operating in the Bay Area, was told to set up shop in Southern California. After all, the San Fernando earthquake had been by far the country’s most damaging earthquake since the 1906 San Francisco earthquake.

Caltech welcomed the USGS with open arms, and together, Caltech researchers and the USGS have put many systems in place to reveal where the shaking was during an earthquake and its strength. Now, those systems are so fast that Southern California has an earthquake early warning system that can warn that shaking is on its way.

“For the last 50 years we’ve had this incredibly strong relationship between the USGS and Caltech, and that has allowed the seismic networks in Southern California to both grow larger and to more naturally evolve to include the newest scientific ideas than they ever would have without it,” says Mike Gurnis, the John E. and Hazel S. Smits Professor of Geophysics and director of the Seismo Lab.

Another major piece of the developments following the 1971 earthquake was the creation by the federal government in 1977 of a multi-agency program called the National Hazards Earthquake Reduction Program (NHERP).

“It would be difficult to overstate the importance of NHERP for earthquake research, monitoring, and reporting in Southern California,” says Lucy Jones, a visiting associate in geophysics at Caltech who served with the USGS for more than 30 years. “It was created as part of the outcome of the 1971 earthquake, and it’s the main government program that’s funded earthquake work ever since, including the seismic network at Caltech and the USGS office in Pasadena. It’s also where the funding was added to bring about earthquake early warning.”

For the public, perhaps the most important outcomes of the 1971 San Fernando event were the laws and changes to building codes that were put into place to make buildings safer during major earthquakes. Because the damage during the quake had been so horrible, one of the first changes was the adoption of new seismic standards for hospitals.

Other changes took a bit longer. During the quake, the San Fernando Fault actually came to the surface of the earth and tore through people’s houses. Prior to the event there was nothing to prevent builders from constructing homes and businesses directly on top of active fault lines.

But as Jones notes, there are two types of damage associated with earthquakes. “The damage from shaking can be stopped by building stronger buildings,” she says. “The danger from the fault can’t be stopped because the fault itself is moving.”

Clarence Allen answers questions about the San Fernando Earthquake during a press conference at the Seismological Laboratory on February 10, 1971. Credit: Caltech.

After the 1971 earthquake, Clarence Allen (MS ’51, PhD ’54), the late Caltech geologist and geophysicist, went to Sacramento and explained to legislators that geologists know where the active faults are and that an earthquake like San Fernando would certainly happen again in California. In 1972, the California legislature passed the Alquist-Priolo Earthquake Fault Zoning Act, which prohibits building across active faults. “It was really because of Clarence spending the time and the effort to help people understand that geology could actually tell you where this was going to happen that this change was made,” says Jones.

It took a lot more fighting and time to get the City of Los Angeles to require a change that seismologists identified as sorely needed after the 1971 earthquake: the requirement to retrofit unreinforced masonry buildings. During the earthquake, many of these unreinforced buildings suffered damage, including tragic collapses at a homeless shelter in downtown Los Angeles and at the Veterans Administration Hospital in San Fernando, where 49 people died. In 1981, the city required that about 10,000 unreinforced buildings either be retrofitted or torn down. In 1986, the state of California passed a law requiring that all jurisdictions catalog unreinforced masonry buildings and develop retrofitting programs.

“In 1994, when the Northridge earthquake happened, nobody died in an unreinforced masonry building,” says Jones, “which is pretty amazing because that’s always been where people die in California earthquakes. So the 1971 earthquake certainly saved lives in the 1994 earthquake.”

Earthquake Alert


Earthquake Alert

Earthquake Network projectEarthquake 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.

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

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.


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

Learn more about EEW Research

ShakeAlert Fact Sheet

ShakeAlert Implementation Plan

See the full article here .

Please help promote STEM in your local schools.

Stem Education Coalition

The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

Caltech campus