California: July earthquake caused fault to move for first time

The massive earthquake that hit southern California during the mid-year has increased strain on a major nearby fault, making it move for the first time on record, scientists said on Thursday.

Ruptures in the Ridgecrest earthquake sequence in July ended a few miles from the Garlock fault. It runs east-west for 185 miles (300km) from the San Andreas fault to Death valley. The fault has been relatively quiet for 500 years but has now begun expanding slowly, according to a new study.

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Biggest in two decades 

Zachary Ross, the lead author of the paper and assistant professor of Geophysics at Caltech said that they had been monitoring earthquakes for a long time. The Ridgecrest earthquake sequence was the biggest in two decades in southern California. It started on July 4 in the Mojave desert around 120 miles (190km) north of Los Angeles. After a magnitude of 6.4 foreshocks, there was a magnitude of 7.1 mainshocks the following day, followed by 100,000 consequential aftershocks. The Garlock fault is capable of producing a magnitude 8 earthquake and has slipped 0.8in (2 cm) since July, according to a new study. It moved over a large enough length of the fault that radar satellite in space detected it, said Ross.

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The sequence included ruptures 

The study found that the Ridgecrest sequence included ruptures on a web of interconnected faults. The Ridgecrest sequence involved about 20 previously unknown faults which provide an example of how large earthquakes can be generated. A large quake on the Garlock fault has the potential to send strong shaking to the San Fernando Valley, Santa Clarita, Lancaster, Palmdale, Ventura, Oxnard, Bakersfield, and Kern County, one of the nation’s most productive regions for agriculture and oil.
A powerful earthquake on a stretch of the roughly 300-mile-long southern San Andreas fault could cause the worst shaking the Southern California region has felt since 1857, and send destructive tremors through Los Angeles and beyond. The southernmost tip of the San Andreas fault has generally crept in response to distant quakes including the extent 8.2 quakes off the coast of southern Mexico in 2017 nearly 2000 miles away.

The study was published on the 30th anniversary of the deadly Loma Prieta earthquake which badly damaged the San Francisco Bay area and the same day that California implemented a statewide earthquake early warning system for the public.

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