California Earthquake Swarm Caused Strain On A Major Fault Line That Has Been “Creeping” Ever Since

Researchers scan the surface rupture near the 'zone of maximum surface displacement' of the magnitude 7.1 earthquake that struck the Ridgecrest area. NASA JPL/USGS/Ben Brooks

An analysis of a large series of earthquakes that rocked Southern California earlier this summer shows that a major fault line quiet for 500 years is now slowly slipping.

Almost 5,000 earthquakes rocked the Los Angeles region in July, including a 6.4 foreshock on July 4 followed by a 7.1-magnitude mainshock just 36 hours later. The latter is the strongest to strike California in the last two decades and was felt east to Nevada and south to Mexico. It was so large that the resulting crack could be seen from space. One of the best-documented earthquake sequences in history, these shakes are now collectively named the Ridgecrest Earthquake Sequence and are giving us a greater understanding of earthquakes and their associated effects.

“The 2019 Ridgecrest sequence brought to an end the long earthquake silence in California,” wrote the authors in the journal Science. Stress from the sequence caused “substantial” changes to the Garlock Fault, an east-to-west fault that stretches 300 kilometers (185 miles) across Southern California from the San Andreas Fault to Death Valley. This region has been quiet for the last 500 years, but July’s earthquake sequence triggered it to start creeping, slipping by at least 2 centimeters in the last three months.

"This was a real test of our modern seismic monitoring system," said Zachary Ross, lead author of the paper, in a statement. "It ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur."

 The earthquake cluster hit California near the city of Ridgecrest, about 240 kilometers (150 miles) North-East of Los Angeles. By Sasa Kadrijevic/Shutterstock

To come to their findings, researchers analyzed satellite data that observed physical ruptures and deformities along the surface of the Earth that were caused by the Ridgecrest Earthquake Sequence. Automated computer analysis of the data, combined with measurements acquired through ground-based seismometers and highly precise location information allowed the team of researchers from Caltech and NASA's Jet Propulsion Laboratory (JPL) to create a model of subsurface fault slipping and to determine a relationship between slipping faults and the number of quakes that occurred before, between, and after the two largest.

Earthquake systems are much more complex than previously believed. Most are thought to occur along a single fault line, like the 1,300-kilometer-long (800 miles) San Andreas Fault that resulted in the 1906 San Francisco Earthquake. However, Ridgecrest involved 20 previously undiscovered small faults that simultaneously broke faults at right angles to each other. When ruptured, these can trigger a domino-like effect to create a web-like network of smaller, connected faults.

"We actually see that the magnitude-6.4 quake simultaneously broke faults at right angles to each other, which is surprising because standard models of rock friction view this as unlikely," Ross said. "It is remarkable that we now can resolve this level of detail."

Together, it shows how little we know about earthquakes

“It's going to force people to think hard about how we quantify seismic hazard and whether our approach to defining faults needs to change. We can't just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes," said Ross, adding that in the last century, the largest earthquakes in California have probably looked more like Ridgecrest than the devastating 1906 San Francisco earthquake, which was along a single fault system.

“It becomes an almost intractable problem to construct every possible scenario of these faults failing together – especially when you consider that the faults that ruptured during the Ridgecrest Sequence were unmapped in the first place,” Ross concluded. 

All earthquakes of magnitude 2.5 and greater in the Ridgecrest area July 4 to Aug. 15, 2019, are shown as gray circles. Red stars mark the two largest. The Garlock Fault south of the cluster of earthquakes has slipped almost an inch since July. USGS/NASA JPL
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