Garlock fault moving for first time in centuries

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News from the CalTech Seismological Laboratory: the big one is coming. Not from the cinematic fault, although it could be effected, but from a fault many of us have not thought of for some time. Many may not have even heard of it until now. The Garlock fault is moving for the first time in over 500 years.

The Jet Propulsion Laboratory (JPL) contributed to the study as well.

Zachary Ross, assistant professor of geophysics at the California Institute of Technology (Caltech) and lead author of the paper, told the Los Angeles Times, “This is surprising, because we’ve never seen the Garlock fault do anything. Here, all of a sudden, it changed its behavior.” He added.

The fault line runs east-to-west, from the San Andreas fault to Death Valley, about 185 miles.

The fault creep began after July 4 of this year, after the Ridgecrest earthquakes hit the Kern County town. The first was 6.4 and the second was 7.1. Those were followed by more than 100,000 aftershocks.

One of the surprising aspects of the joint CalTech-JPL study is that the quakes caused smaller, connected fault lines that added strain to the Garlock fault. As a result, Ross said we may need to change the approach to defining the hazards of seismic activity. He said, “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.”

The abstract on the website for Science, the publication for the American Association for the Advance of Science (AAAS), put it into a short explanation:

“A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (Mw) of 6.4 followed by a Mw 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.”

This information was published just days before Californians felt two “minor” tremblors: shortly after noon Friday a 4.1 quake struck near Hollister, CA and another earlier this morning less than a mile from Compton that was a 3.5 magnitude.

Here’s a link to the Science article and then for the CalTech page on the Garlock fault and this one to track current quakes.

On the same day this study was published Governor Gavin Newsom signed into law the Shake Alert system that is designed to alert residents, by phone, of impending quakes. The expect the system will give people in affected areas second or even tens of seconds of notice so they can prepare, i.e. take cover. The Shake Alert system has been under development by the U.S. Geological Survey and various scientific institutions for years. UC-Berkeley developed the app with the Wire Emergency Alert system.