These events are the largest so far in a swarm that started on 26 Sep 2016, 04:03AM PDT, and is occurring in the Brawley Seismic Zone near the southern terminus of the San Andreas Fault.
M4.3, 26 Sep 2016 07:31:08 PDT, (33.298, -115.713), depth 2.4km, 6km SSE of Bombay Beach, California
M4.3, 26 Sep 2016 20:23:58 PDT, (33.300, -115.712), depth 4.8km, 6km SSE of Bombay Beach, California
M4.1, 26 Sep 2016 20:36:15 PDT, (33.305, -115.702), depth 2.5km, 6km SSE of Bombay Beach, California
This swarm is noteworthy because it is happening near the southern terminus of the San Andreas Fault. This is the same are as the two previous swarms in this region, which occurred in 2009 and 2001. No swarms prior to 2001 with a M4.0 have been recorded in the area since 1933.
Events in the swarm show a NE-SW trend, consistent with the 2009 and 2001 swarms. This trend is in alignment with the faults located in the northern Brawley Seismic Zone, and orthogonal to the San Andreas fault. Before 2001, there were few events that had occurred in the northern Brawley Seismic Zone. The southern Brawley Seismic Zone is much more active, with events occurring on a regular basis. More information on the Brawley Seismic Zone and nearby faults can be found in the special report on the 2009 swarm.
The swarm includes more than 290 events so far (30 Sep 2016, 12:05PM PDT) in the magnitude range M0.7 to M4.3, there have been 17 events with magnitude greater than M3 and 97 events with magnitude greater than M2. The M4.3 exhibited strike-slip motion with one nodal plane at N47E, and the other sub-parallel to the strike of the San Andreas Fault. Relocations of these events show that the are located in the depth range 4 to 9 km.
The 2009 swarm had two large strike-slip events: M4.8 and M4.0.
The 2001 swarm had two large strike-slip events: M4.1 and M3.4.
There were 4 events during the 3 days prior to the earthquake (within a 1 km radius), the largest was M3.3 (2016/09/26).
Historical seismicity: since our records began in 1932 we’ve had 8 events of M4 or greater within 10km of today’s event, the largest was M4.8 (2009/03/24) and the most recent was M4.8 on 24 Mar 2009.
Nearby faults: Brawley Seismic Zone faults (orthogonal to San Andreas fault), San Andreas fault zone, Coachella section (5.8 km) and Hot Springs fault (10.3 km).