Sept 2 update:
Renewed seismic activity in the
Brawley Seismic Zone began
late on 28 August 2005 but reached a sustained higher level of activity,
announced by back-to-back M4.6 and M4.5 events at 3:47 and 3:50 p.m. (local
time; PDT [GMT-8]) on 31 August 2005. The swarm then intensified, with a M 5.1
event on 1 Sept. at 6:27 p.m. PDT. As of this writing, seismic swarm activity
persists at a slightly diminished level, yet activity can reasonably be
expected to continue (see STEP
map). We now have recorded a total of 8 events of M 4.0+, 29 additional events
of magnitude between 3.0 and 4.0, and 175 events between magnitude 2.0 and 3.0.
In all, 554 events of M 1.0+ have occurred in the swarm so far.
The activity is reminiscent of similar swarms that occurred
in the Brawley Seismic Zone throughout the 1970Õs and 1980Õs, during which time
the zone was among the most active areas in all of California. The Brawley
Seismic Zone is a north-striking zone of northwest and northeast-striking
faults that extends from the southern end of the San Andreas fault to the
northern end of the Imperial fault. Across this zone, tectonic spreading that
is oriented NW-SE occurs within the transition from the Gulf of California
mid-ocean ridge to the San Andreas transform fault. Historically, activity includes both northeast-oriented
cross-faults that typically involve left-lateral faulting, such as is currently
occurring, and activity on northerly to northwesterly-oriented strands of the
zone. The set of nearly orthogonal strike-slip faults accommodates the overall
tectonic NW-SE opening, partly through the rotation of crustal blocks.
The largest earthquakes to have occurred on the cross-faults
were the magnitude 6.2
Elmore Ranch event in 1987, which is considered to have
triggered the magnitude 6.6 Superstition Hills event less than 12 hours later,
and the magnitude 5.7 Westmorland event in 1981. Although Brawley Seismic Zone
activity is clearly proximal to the southern San Andreas fault, the San Jacinto
fault zone, and the Imperial fault, the current activity is remaining more than
10 km distant from any of these major faults. Obsidian Butte is part of the Salton
Buttes Lava Domes, which are primarily formed by their namesake volcanic
rock type, with nearby geothermal activity and energy production.
The pattern of Brawley swarms of the 1970Õs was a large
number of very small earthquakes (sometimes exceeding 10,000 events) with up to
a dozen moderate events of magnitude 4 or so, but no clear mainshock larger
than the other events. The 1970Õs
swarms would be highly active for a few days and then taper off over the next
week or two.
In 1999 and 2001, two other smaller swarms occurred on northeast-striking faults in the
Salton Sea. Compared to those two sequences, this weekÕs swarm has
significantly more events, but also is located significantly farther away from
the San Andreas fault.