Sunday, April 4, 2010

7.2 Magnitude Earthquake in Baja California

A large earthquake (preliminary magnitude 6.9, depth 20 miles) has shaken the delta region of the Colorado River in Baja California. It was felt over a wide region of Southern California and western Arizona. The nearest towns are Guadalupe Victoria in Mexico (16 miles), and San Luis and Gadsen in Arizona. The magnitude currently being reported is equivalent to the Loma Prieta earthquake of 1989 that shook the Santa Cruz and San Francisco Bay regions.

A 5.1 quake has been reported farther north in the Imperial Valley of California moments ago as well. These earthquakes are not on the San Andreas fault, but can be considered related, as a series of northwest trending faults take up the stresses related to the spreading of the Gulf of California. Large quakes have occurred in the region in the past, most notably a 6.4 quake in 1979, and a 7.1 event in 1940 (with nine fatalities).

Update: The magnitude has been upgraded to 7.2 and the depth to 6.2 miles. Changes to the initial magnitude report are not unusual as the U.S. Geological Survey gets more information. The depth of the quake is also tricky, especially when the region is not covered by numerous seismometers. A series of aftershocks are now being reported, ranging as high as magnitude 5.4.

Update #2: There have been a large number (40 or so) of magnitude 3+ aftershocks in the region, most notably lined up to the northwest along the Laguna Salada fault, which lines up with the Elsinore fault in California, and produced a 7.2 quake in 1892. It will be a while before we hear of any ground rupture, but quakes of this size are capable of producing several feet of offset over several tens of miles.


charliewallace said...

Looking at the locations of the aftershocks - and their gradual northwestward shift - do you have concerns about this as a precursor to the big one?

Garry Hayes said...

I would speak cautiously, not being a seismologist with access to the data, and say probably not. The fault system this earthquake happened on is related to the San Andreas system but is not directly connected. The aftershock pattern is probably a "ripple" effect as other faults are triggered by the ground waves from the large initial quake. Unless the San Andreas is right at the tipping point of going off, it probably will remain quiet. But anything could happen. Events like this are good reminders that we should be ready for big events at all times.

That the aftershocks are not as obvious to the south may be in part because the seismometer coverage is not as extensive in that direction.