Geologists Tout Lessons Learned From Ridgecrest Quakes

Days after a 7.1 earthquake struck Searles Valley, California, a U.S Geological Survey crew scanning the area photographed huge swaths of surface rupture. (Ben Brooks / USGS)

(CN) – Months after Southern California experienced its largest earthquake sequence in over two decades, scientists have learned the temblors are more complex than previously thought.

The earthquake sequence near the Kern County town of Ridgecrest comprised of a magnitude 6.4 foreshock on July 4, followed by a magnitude 7.1 mainshock 34 hours later and more than 100,000 aftershocks. Based on data from orbiting radar satellites and ground-based seismometers, researchers built a distinct picture of an earthquake rupture far more complex than previous models could predict.

In a study published Thursday in the journal Science, a team of researchers analyzed the most comprehensive and detailed data gathered on an earthquake sequence of this size. Lead author Zachary Ross of Caltech said thanks to modern seismic monitoring systems, “it ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur.”

Major earthquakes were thought to be caused from ruptures on a single fault, resulting in a magnitude dependent on the size of the fault. In 1992, a 7.3 earthquake in Landers, California, included the rupture of multiple faults, challenging this model.

The Ridgecrest sequence demonstrated a domino effect of surrounding fault lines, triggering a web of tremors across Southern California. This included 20 previously undiscovered crisscrossing fault lines. “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 in a statement.

Water from a broken line runs down a crack on the ground in the aftermath of an earthquake along highway 178 Sunday, July 7, 2019, near Ridgecrest. Calif. (AP Photo/Marcio Jose Sanchez)

The authors stress this level of complete detail was only achieved through the multiple scientific instruments that studied this event, including satellites observing the ruptures and seismometers observing the seismic waves radiating from the quake. Thanks to both contributions, the team composed a model of the underground fault slipping, the relationships between the major slipping faults, and the massive number of small quakes that took place as a result.

Another consequence of this earthquake sequence is that the ruptures stopped just short of the 186-mile Garlock Fault on the outskirts of the Mojave Desert. It has been inactive for the past 500 years, but the Ridgecrest quakes have put a strain on it – causing it to slip nearly an inch since July.

Ross emphasized the study proves how little we understand about earthquakes.

“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,” he said, adding he hopes the study will make others rethink and possibly change the way scientists approach defining faults.

The report comes on the 30th anniversary of the Loma Prieta earthquake, which killed 63 people and injured over 3,700. The magnitude 6.9 quake also caused about $6 billion in damage, the equivalent of $12 billion today.

 

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