Dormant For 500 Years, A Potentially Deadly Fault In California Has Started To Move

A USGS Earthquake Science Centre Mobile Laser Scanning truck analyses a surface rupture caused by the M7.1 Ridgecrest earthquake in July. (Image: USGS/Ben Brooks)

This past US summer, southern California experienced a significant earthquake swarm. Analysis of the event suggests earthquakes unravel in a more complicated manner than is typically appreciated. What’s more, this event has perturbed a major, previously idle fault nearby—and scientists aren’t entirely sure about the potential consequences.

New research published this week in Science details the Ridgecrest earthquake sequence, a seismic storm that unleashed two large tremors and thousands of aftershocks in southern California this past July and August. The new study suggests earthquakes are more multifaceted and complex than we thought; like dominoes, rupturing faults can prompt the movement of neighbouring faults, including nearby faults that don’t immediately appear to be connected, according to the new research, which involved scientists from Caltech and NASA’s Jet Propulsion Laboratory in Pasadena, California.

The Ridgecrest Earthquake Sequence started on July 4, 2019, when a magnitude 6.4 earthquake—a foreshock—struck southern California. The magnitude 7.1 mainshock occurred 36 hours later (the effect of which could be seen from space), followed by approximately 100,000 aftershocks.

It was the most significant earthquake storm to hit the region in two decades. The shaking earth was felt across much of southern California, but the areas around Ridgecrest, a town located 190 kilometers (120 miles) north of Los Angeles, had it the worst. Incredibly, the swarm included 20 smaller faults that were previously unknown to scientists.

“It ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur,” said Zachary Ross, a geologist at Caltech and the lead author of the paper, in a press release.

People crossing Highway 178 next to a crack left created by the Ridgecrest earthquake in July. (Image: AP/Marcio Jose Sanchez)

Alarmingly, the shifting and settling of the numerous faults has added pressure to the nearby Garlock Fault. This major east-west fault extends for 300 kilometers (185 miles) along the northern boundary of the Mojave Desert and intersects with the San Andreas Fault to the west.

Garlock Fault has been dormant for the past 500 years, but the Ridgecrest quakes placed considerable strain upon it, causing the fault to move in a process known as fault creep. The scientists report in the new paper that Garlock Fault has slipped 2 centimeters (0.8 inches) at the surface since July.

“This is surprising, because we’ve never seen the Garlock fault do anything. Here, all of a sudden, it changed its behaviour,” Ross told the LA Times. “We don’t know what it means.”

This doesn’t mean a major earthquake is inevitable, but should Garlock Fault continue to move, it could destabilise the San Andreas Fault, which runs along a different tectonic system.

For the new study, Ross and his colleagues combined data gathered from ground-based seismometers and orbiting radar satellites operated by NASA and Japan’s space agency, JAXA. They used an automated computing process to make sense of the tremendous amount of data generated by the earthquake swarm, which allowed them to produce a map showing the precise location of the fault ruptures and to create a new model showing how the faults slipped beneath the surface. The research is improving our understanding of the relationship between big faults and the copious number of smaller quakes associated with the largest shocks.

“I was surprised to see how much complexity there was and the number of faults that ruptured,” said JPL co-author Eric Fielding in the press release.

Map showing all earthquakes stronger than M2.5 in the Ridgecrest area from July 4 to August 15, 2019 (grey circles). The red stars show the location of the biggest two quakes. The Garlock Fault south of the earthquake cluster is also shown. (Image: USGS)

Indeed, the model displayed a web of tangled inter-causality not typically associated with large seismic events. Big quakes, it was previously thought, were triggered by a rupture along a long prominent fault, and the maximum strength of the ensuing quake was related to the total length of the fault. But observations of quakes over the past three decades, including the 1992 Landers earthquake in California, cast doubt on this assumption, with the new paper now adding further evidence to the contrary.

As shown in the new map, the smaller faults intersected with each other at surprising angles, in what’s considered to be geologically young fault zone.

“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,” said Ross in the Caltech press release. “It is remarkable that we now can resolve this level of detail,” but the event shows how much we still need to learn about earthquakes, he said, adding that scientists “can’t just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes.”

Frustratingly, this also means that predicting the timing and impacts of earthquakes could be next to impossible.

“Over the last century, the largest earthquakes in California have probably looked more like Ridgecrest than the 1906 San Francisco earthquake, which was along a single fault,” said Ross. “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.”

Again, a major earthquake along the Garlock Fault is not a certainty, so there’s no need for panic. At the same time, however, Californians need to be mindful of the grim possibly.

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