In discussing the effects of the April 4 Earthquake with personnel associated with our major client the Imperial Irrigation District (IID), we understand as of today that there was a significant amount of sub-moderate to moderate damage to several canals, roads, slopes / embankments and otherwise. From preliminary discussions and review of photographs sent to us by IID, some areas of seiche (water thrown out of pools, ponds, and similar), areas of ground spreading (causing spreading / settlement cracks) and possible liquefaction (with minor water and possibly sand slurry ejecta), and ground lurching causing deformation and cracking along embankments were noted. Building damage in the United States area of the quake, was however, rather limited with respect to structural issues (ie. failure / collapse) however, we understand that damages due to shaking cause damages due to unsecured items being thrown off shelves, overturning, and similar. Another source of damage / effect was in the form of seiches in swimming pools and similar where reports of pools in Brawley and similar losing over a foot of water from sloshing.
The zone that these earthquakes are occurring in are related to the tectonic boundary of the Pacific Plate and the North American Plate of the Earth. The Pacific Plate is moving generally to the north in a complex system of spreading centers (a zone of crustal “opening”) and transform faults (which accommodate the spreading) – resulting in what may be termed an “un-zippering” of the upper northerly end of the Gulf of California opening into the Imperial and Coachella Valleys. These forces that have caused a northward “un-zippering” of the Gulf of California have divided Baja California from the mainland and continue the trend northerly. These forces are generated by a series of transform faults and spreading centers. In the current setting of the local Imperial area, the San Andreas, Imperial, and Brawley Faults (and the Laguna Salada Fault ZOne in Mexico) represent active transform faults, and the lower basin / Salton and East Mesa geothermal areas are zones of active crustal spreading. The interaction of these faults and transforms and the regional tectonics causes the Imperial / Coachella Valley to be deeply downdropped. The faults also cause displacements, including surface rupture, between fault blocks. The spreading centers also allow for concentrations of heat to occur where the mantle of the earth is exposed as windows through the crust.
It is interesting to note that this area has had significant activity in recent history along seismic zones associated with the major structural aspects of this tectonic un-zippering – including the southern Salton Sea swarms occurring on August 30, 2005 and similar swarms in 2007, 2008, and 2009; 5.9 M Baja Earthquake of 12-30-09; 6.9 M central Gulf of California Earthquake 08-03-09; 4.8 M Salton Sea area Earthquake 03-24-09; 6.6 M Gulf of California Earthquake 01-04-06; 4.2 M Brawley Earthquake of May 23, 2003, and numerous others including the large historic October 15, 1979 Imperial Valley 6.4 M event, and the 1987 Westmorland 6.5 M, and the June 23, 1915 Imperial 6.3M where several persons died.