An update on the Laguna Salada Earthquake of April 04 (30 miles sse of Calexico, CA):
The shallow-depth quake has registered as a 7.2 on the Richter - larger than the recent Haiti quake, but far less destructive of human-built structures due to its isolation from population centers.
This area is known to be seismically active, with many nw-trending faults along the plate boundary of the nw moving Pacific Plate and the westerly moving North American Plate. Average motion to the nw of the Pacific plate is about 1.8 inches/year (at this latitude). This event was the largest quake here since 1892.
There were foreshocks, but difficult to isolate in this seismically active area. Initial motion was on a subsidiary fault, followed about 8 seconds later by the main quake on the Borregos fault. Surface movement indicates a plate motion of about 6-8 feet laterally (west side moving northwesterly) and about two feet down on the easterly side. Most of the motion was on the Borrego fault. This crack is distinct from but parallel to the vector and motion of the San Andreas fault.
There was about a 5% chance that this quake would be a foreshock to a larger quake somewhere along the plate boundary, but the chance of this decreases through time and is now reduced to about a 1% probability. There have been more than 500 aftershocks, many to the northwest and some on the US side of the border. These aftershocks appear to be increasing in intensity (today some in the 5 range on the Richter, increasing from 2's, 3's and 4's ).
More than a million people felt the main shock as strong shaking, with moderate structural damage and more than 3 million felt moderate shaking with little damage.
Since a major earthquake could occur at any time in the states bordering the Pacific (and elsewhere, as discussed on other earlier threads on this site) it is useful for those of us living in hazard areas to prepare for the possibility of a large quake - here is a good site for 'what to do':
http://earthquake.usgs.gov/prepare/
