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August 2012 Brawley Earthquake Swarm

A ML5.5 earthquake occurred at 1:57PM on August 26th, 2012, and was preceeded by nearly 200 events ranging ML1.4-ML5.3.

CISN/SCSC Executive Summary (E. Hauksson, L. Jones, D. Given, K. Hutton, A. Guarino, N. Scheckel, R. Dollar, R. Graves, and K. Hudnut)

Version 3 - 12:00PM 8/27/2012

The swarm began on Saturday evening with six events of M<2.0, and activity picked up at 8:30 am today with three M2.5 events in a few minutes.  As of 12:00 pm on 8/28/12, CISN recorded more than 550 events. The largest two are M5.3 at 12:31 pm and a M5.5 at 1:57 pm. There have been eight events ranging between M4.0 and M5.0, and a total of 57 events M 3.0 or larger have occurred (for the list of M3 and larger events, see below, for a complete list of events, navigate to http://www.scsn.org/2012BrawleySwarmList.html).

The earthquakes are occurring on a northeast striking fault zone 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 significant swarm before this one occurred in 2005 along the south shore of the Salton Sea, with the largest event of M5.1.  In 1981 there was swarm-like activity called the Westmoreland sequence, with a largest event of M 5.8, which occurred 5 miles (8 km) to the northwest of todays activity.  Some of the aftershocks of the Mw6.4 1979 earthquake occurred in the same general area. 

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.

A field visit on 8/27/12 found no evidence of surface rupture, nor evidence of liquefaction. It would be surprising to find such earthquake phenomena associated with events of this magnitude, as surface rupture usually accompanies larger events with shallow depth.

Expected future development of this activity:  In the past these types of swarms have continued for many days.  Previously, events of up to M6.2 have occurred on northeast striking cross faults, but bigger events are possible on the major northwest striking late Quaternary faults in this vicinity. 

Current and Background Seismicity

 

Map of CISN/SCSN Relocations with 3D Model and HypoDD: Brawley Swarm, August 2012

3D Relocations

List of Brawley Swarm Events M3 and Larger, in descending order (for a complete list of events, click on the lists below: http://www.scsn.org/2012BrawleySwarmList.html,http://www.scsn.org/2012BrawleySwarmList2.html,http://www.scsn.org/2012BrawleySwarmList3.html)

5.5 8/26/2012 13:57:58 33.024N 115.549W 9
5 KM ( 3 MI) NNW OF BRAWLEY, CA
5.3 8/26/2012 12:31:22 33.019N 115.546W 12.3 5 KM ( 3 MI) N OF BRAWLEY, CA
4.9 8/26/2012 21:41:36 33.030N 115.531W 9 6 KM ( 4 MI) N OF BRAWLEY, CA
4.9 8/26/2012 12:33:00 33.021N 115.554W 14.5 5 KM ( 3 MI) NNW OF BRAWLEY, CA
4.6 8/26/2012 16:33:25 33.033N 115.531W 9.2 6 KM ( 4 MI) N OF BRAWLEY, CA
4.6 8/26/2012 12:20:04 33.019N 115.545W 13.1 5 KM ( 3 MI) N OF BRAWLEY, CA
4.3 8/26/2012 15:58:41 32.990N 115.590W 13 5 KM ( 3 MI) WNW OF BRAWLEY, CA
4.3 8/26/2012 12:40:12 32.990N 115.598W 13.8 6 KM ( 4 MI) SSE OF WESTMORLAND, CA
4.2 8/26/2012 14:15:29 33.042N 115.539W 9.1 7 KM ( 5 MI) N OF BRAWLEY, CA
4.0 8/26/2012 12:16:11 33.026N 115.547W 13.8 6 KM ( 3 MI) N OF BRAWLEY, CA
3.9 8/26/2012 14:17:26 32.986N 115.603W 11.8 6 KM ( 4 MI) SSE OF WESTMORLAND, CA
3.8 8/27/2012 10:47:51 33.010N 115.564W 12.2 4 KM ( 3 MI) NW OF BRAWLEY, CA
3.8 8/27/2012 10:47:51 33.010N 115.564W 12.2 4 KM ( 3 MI) NW OF BRAWLEY, CA
3.8 8/26/2012 16:36:58 33.052N 115.533W 7.7 8 KM ( 5 MI) N OF BRAWLEY, CA
3.8 8/26/2012 14:26:39 33.038N 115.527W 12.4 7 KM ( 4 MI) N OF BRAWLEY, CA
3.8 8/26/2012 10:02:12 33.019N 115.563W 13.2 5 KM ( 3 MI) NNW OF BRAWLEY, CA
3.7 8/26/2012 22:09:05 33.057N 115.536W 7.7 8 KM ( 5 MI) SSW OF CALIPATRIA, CA
3.6 8/26/2012 14:08:45 32.997N 115.587W 11.4 5 KM ( 3 MI) WNW OF BRAWLEY, CA
3.6 8/26/2012 12:58:17 33.025N 115.541W 13.6 5 KM ( 3 MI) N OF BRAWLEY, CA
3.5 8/26/2012 15:34:45 33.007N 115.589W 13.6 5 KM ( 3 MI) SE OF WESTMORLAND, CA
3.5 8/26/2012 14:23:23 32.999N 115.590W 14.3 5 KM ( 3 MI) SE OF WESTMORLAND, CA
3.5 8/26/2012 13:16:54 33.012N 115.564W 12.7 5 KM ( 3 MI) NW OF BRAWLEY, CA
3.5 8/26/2012 12:21:04 33.003N 115.546W 11 3 KM ( 2 MI) NNW OF BRAWLEY, CA
3.4 8/26/2012 23:31:27 33.023N 115.433W 7.4 11 KM ( 7 MI) ENE OF BRAWLEY, CA
3.4 8/26/2012 21:46:13 32.991N 115.463W 5 7 KM ( 4 MI) ENE OF BRAWLEY, CA
3.4 8/26/2012 17:58:45 32.998N 115.579W 13 5 KM ( 3 MI) WNW OF BRAWLEY, CA
3.4 8/26/2012 17:58:34 33.018N 115.539W 0.1 5 KM ( 3 MI) N OF BRAWLEY, CA
3.4 8/26/2012 16:34:50 33.001N 115.547W 5.2 3 KM ( 2 MI) NNW OF BRAWLEY, CA
3.4 8/26/2012 15:53:35 33.023N 115.553W 12.5 5 KM ( 3 MI) NNW OF BRAWLEY, CA
3.4 8/26/2012 15:52:09 32.998N 115.589W 12.4 5 KM ( 3 MI) WNW OF BRAWLEY, CA
3.4 8/26/2012 15:06:28 33.014N 115.539W 9.6 4 KM ( 3 MI) N OF BRAWLEY, CA
3.4 8/26/2012 13:20:24 33.026N 115.543W 11.9 5 KM ( 3 MI) N OF BRAWLEY, CA
3.4 8/26/2012 12:50:14 33.007N 115.586W 12.8 5 KM ( 3 MI) SE OF WESTMORLAND, CA
3.4 8/26/2012 12:35:51 32.995N 115.485W 7.9 5 KM ( 3 MI) ENE OF BRAWLEY, CA
3.4 8/26/2012 12:28:45 33.023N 115.561W 13.7 6 KM ( 3 MI) NNW OF BRAWLEY, CA
3.3 8/26/2012 10:16:02 33.023N 115.549W 13.4 5 KM ( 3 MI) NNW OF BRAWLEY, CA
3.2 8/26/2012 19:53:15 32.977N 115.609W 13.2 7 KM ( 4 MI) W OF BRAWLEY, CA
3.2 8/26/2012 16:53:14 33.033N 115.535W 13.5 6 KM ( 4 MI) N OF BRAWLEY, CA
3.2 8/26/2012 12:45:55 32.993N 115.575W 0.2 4 KM ( 2 MI) WNW OF BRAWLEY, CA
3.1 8/26/2012 20:03:33 32.993N 115.588W 13.6 5 KM ( 3 MI) WNW OF BRAWLEY, CA
3.1 8/26/2012 14:46:11 32.996N 115.591W 12.5 5 KM ( 3 MI) SSE OF WESTMORLAND, CA
3.1 8/26/2012 14:05:11 33.027N 115.535W 5.1 6 KM ( 3 MI) N OF BRAWLEY, CA
3.1 8/26/2012 13:31:13 33.024N 115.543W 4.7 5 KM ( 3 MI) N OF BRAWLEY, CA
3.1 8/26/2012 11:58:12 33.024N 115.546W 13.5 5 KM ( 3 MI) N OF BRAWLEY, CA
3.0 8/27/2012 11:05:58 32.961N 115.549W 12.8 2 KM ( 1 MI) SW OF BRAWLEY, CA
3.0 8/27/2012 0:32:06 32.979N 115.579W 14.9 4 KM ( 2 MI) W OF BRAWLEY, CA
3.0 8/27/2012 11:05:58 32.961N 115.549W 12.8 2 KM ( 1 MI) SW OF BRAWLEY, CA
3.0 8/27/2012 0:32:06 32.979N 115.579W 14.9 4 KM ( 2 MI) W OF BRAWLEY, CA
3.0 8/26/2012 22:47:24 33.040N 115.532W 12.3 7 KM ( 4 MI) N OF BRAWLEY, CA
3.0 8/26/2012 15:23:57 32.989N 115.428W 6 10 KM ( 6 MI) E OF BRAWLEY, CA
3.0 8/26/2012 15:01:59 33.028N 115.541W 13.4 6 KM ( 3 MI) N OF BRAWLEY, CA
3.0 8/26/2012 14:54:16 33.004N 115.586W 16.2 5 KM ( 3 MI) SE OF WESTMORLAND, CA
3.0 8/26/2012 14:53:58 32.971N 115.551W 14.8 2 KM ( 1 MI) WSW OF BRAWLEY, CA
3.0 8/26/2012 13:14:46 32.996N 115.582W 12.5 5 KM ( 3 MI) WNW OF BRAWLEY, CA
3.0 8/26/2012 12:43:41 33.023N 115.541W 10.6 5 KM ( 3 MI) N OF BRAWLEY, CA
3.0 8/26/2012 12:03:18 33.027N 115.538W 14.5 6 KM ( 3 MI) N OF BRAWLEY, CA

 

Hauksson/Shearer 3D Relocations, Demonstrating a NE Striking Trend. Both M5.3 and M5.5 Events Display Left-Lateral Strike Slip Motion Along a Strike the Sequence Follows.

Click here for a list of all related swarm events: http://www.scsn.org/2012BrawleySwarmList.html

(Data below are current through 8/26 at 5PM PDT)

b-Value, Cumulative Number vs. Magnitude

Event Distritubution and b Value
Figure:  b-value as of appox. 3 pm.  The b-value of 0.7 is low compared to the expect value of 1.0 for southern California. 

 

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