Gabriel Lotto and Ross Stein report that Hollister, CA, is a small city 65 km southeast of San Jose that sits over the Calaveras Fault and only a few kilometers from the San Andreas Fault, a good place to surf seismic waves. But Hollister has been in the news recently because of an earthquake swarm that started on July 17.
An earthquake swarm is a series of small earthquakes that occur roughly in the same place and same time. This swarm, 25 km (15 mi) south of Hollister, lasted for just a couple of days and extended over a 3-km-long linear region. One question you might ask ... why is this earthquake swarm not located on the San Andreas Fault? Actually, it probably is, because the San Andreas is inclined slightly to the southwest. The largest event in the earthquake swarm, a M4.2 temblor, caused mild shaking for thousands of people from San Jose to Salinas.
The earthquake swarm occurred where the San Andreas Fault transitions from a zone to the north that is fully locked, and a zone to the south that is fully creeping. The locked zone, extending north from San Juan Bautista, builds up stress over decades and releases that stress in large earthquakes like the 1906 event that demolished San Francisco. The creeping zone, extending from just south of San Juan Bautista to Parkfield in central California, is a better place to be ... Creeping faults continuously slide against each other at a speed of millimeters per year, often accompanied by a staccato chattering of small shocks. The creeping section does not accumulate stress and so cannot host large earthquakes. Instead, the fault here is lubricated by slippery minerals like serpentinite and talc.
Why, then, did these small earthquakes take place on an ostensibly creeping section of the San Andreas? The stress on faults has many spikes and saddles. Little quakes happen all the time, knocking down the spikes and filling in the saddles. So, there is no reason to believe that the area south of Hollister will go large.
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