Sunday, August 26, 2012

Earthquake Swarm in Southern California

A rather extensive earthquake swarm is shaking up the southernmost part of California near the agricultural town of Brawley. At my last count, there have been two magnitude 5+ tremors, with the largest at 5.5. There have been five magnitude 4 quakes, and around twenty that have exceeded magnitude 3. The quakes are occurring between the south end of the San Andreas system and the San Jacinto fault zone just south of the Salton Sea. Some minor damage is reported in Brawley.

Swarms like this are not unusual for the region. As to whether this means some kind of bigger event is coming, one can say it is possible, but many other swarms have occurred without bigger, more damaging shocks. That said, big quakes have occurred in this region, including the 2010 Sierra El Mayor quake that clocked in at magnitude 7.2. The southern stretch of the San Andreas is also a threat for large quakes, in the range of 7.8 magnitude. This is the kind of event that can cause catastrophic damage and injure or kill thousands of people. We are not immune to quakes...

Even if nothing more comes of this swarm (which is the most likely outcome), the event should remind people that southern California does live under the threat of large damaging earthquakes, and that anyone who lives there owes it to themselves to be prepared. Have water, food, batteries, first aid supplies, and an emergency plan in place. Check out these resources for more information.

Postscript: USGS Field Office in Pasadena released this statement: (below the fold...)

 August 25, 2012 Brawley swarm (version 1 at 3:50 p.m. PDT)

The swarm began at 8:30 am (PDT) with three M2.5 events in a few minutes. As of 3:00 pm, CISN recorded 160 events. The largest two are M5.3 at 12:31 pm and M5.5 at 1:57 pm, and thirty events over M 3.5 have occurred.

The earthquakes are occurring on a northeast striking fault located about 6 km north of the northwest end of the mapped Imperial Valley fault. This is within the area called the Brawley Seismic Zone. The BSZ extends from the northern end of the Imperial fault to the southern end of the San Andreas fault. It has been characterized by swarms that have occurred in the 1930s, the 1960s, and 1970s. The last swarm before this one occurred in 2005 with the largest event of M5.1 In 1981, at almost the same location, there was swarm-like activity called the Westmoreland sequence, with a largest event of M 5.8.

In the 1960s and 1970s, the southern part of the BSZ was very active. Several swarms included many thousands of events, but with a largest magnitude of only M5.0 or less. From 1981 to 2000, the Brawley Seismic Zone was quiet. In the last decade several swarms smaller than today's were recorded in the northern part of the zone.


Randy A. said...

Concerning earthquake safety, everybody should be aware of the "Great California Shakeout", Oct. 18 at 10:18 am. Many other states and regions are having similar events, for those of you who are not Californians.

Concerning the geology of the region -- what is the sense of movement in the Brawley Seismic Zone?

Garry Hayes said...

The quakes are in sort of a stepover zone with NE trending faults, which leads to left lateral motion as the best interpretation.

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