NASA and Caltech Scientists Analyze the Californian Earthquake Sequence

It turns out the July sequence of earthquakes is part of a more complex situation.
Fabienne Lang

July of this year saw the largest sequence of earthquakes hit Southern California in twenty years. Scientists have now learned that these large quakes are far more complex than what was previously known. 

Geophysicists from NASA and Caltech partnered up to carry out an analysis of the Ridgecrest Earthquake Sequence.

Their study was published in the journal Science on Thursday.


The Ridgecrest Sequence

On July 4, California was shaken up by a magnitude 6.4 foreshock, followed by a magnitude 7.1 mainshock, and then came more than 100,000 aftershocks. The earthquake was most strongly felt north of Los Angeles, in the town of Ridgecrest.

Zachary Rossassistant professor of geophysics at Caltech and lead author of the paper, said: "This was a real test of our modern seismic monitoring system." 

He concluded that "it ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur."

The team discovered that this sequence of earthquakes that happened was much more complicated than previous ones. They were able to gather this information by using data from orbital radar satellites as well as on-the-ground seismometers. 

Typically, most earthquakes are believed to occur because of the rupture of a single long fault, for example, the 800-mile-long San Andreas Fault. 

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During their research, the team realized that the Ridgecrest Sequence was, in fact, generated by a plethora of smaller faults that, when triggered, ended up going through a domino effect. They ruptured one after the other. 

NASA and Caltech Scientists Analyze the Californian Earthquake Sequence
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. Source: USGS via NASA

This particular sequence involved around 20, previously undiscovered, faults. The sequence was also in an interesting and complex geometrical pattern. 

New technology

The team was only able to make these new discoveries thanks to a number of highly technical and scientific instruments, Ross noted.

Combining satellite images from above that covered a distance of 100 kilometers in every direction from the rupture, and an extensive network of seismometers caught the multiple waves emitted by the quakes.

The team themselves were impressed by what they could observe, as Ross stated: "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."

He continued, "It is remarkable that we now can resolve this level of detail."

This sequence will push scientists to rethink what they know about earthquakes and how they observe them.

He further explains just how little we currently understand about these natural phenomena. "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," Ross concluded.

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