Earthquake Swarm in Imperial County Increases Chance for San Andreas Quake This week

Published : Saturday, October 1, 2016 | 5:21 AM

Map of earthquake swarm. Image: USGS

An earthquake swarm near Bombay Beach, California, which began Monday means there is a higher than usual risk of an earthquake along the powerful San Andreas fault in the immediate future which could impact Pasadena, geogologists said Friday.

The United States Geological Survey said preliminary calculations indicate that, as of 12:00 pm on Sept. 30 there is 0.006% to 0.2% chance (less than 1 in 10,000 to 1 in 500) of a magnitude 7 or greater earthquake being triggered on the Southern San Andreas Fault within the next seven days through October 7, with the likelihood decreasing over time.

This range is estimated using several models developed in California to assess foreshock/aftershock probabilities, and the lower bound is about equal to the average chance of a magnitude 7 earthquake occurring on the Southern San Andreas Fault in any given week.. according to the USGS.

Scientists said swarm-like activity in this region has occurred in the past, so this week’s activity, in and of itself, is not necessarily cause for alarm.

The swarm includes 96 earthquakes above magnitude 2 so far (as of 12:00 pm Friday. Relocations of these events show that they are occurring in the depth range 4 to 9 km. The largest of these events were two M4.3 earthquakes and a M4.1 earthquake on Sept. 26.

The earthquakes are occurring near a set of north-northeast trending cross-faults beneath the Salton Sea. The cross-faults are part of a fault network that connect the southernmost end of the San Andreas Fault with the Imperial Fault. Some of the cross-faults are oriented such that they add stress to the San Andreas Fault and the San Jacinto Fault system when they rupture in small earthquakes like those in the ongoing swarm.

These revised probabilities are lower than those made earlier this week, due to decreasing swarm activity. The probabilities may change if the swarm activity increases or decreases.

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