Rupture of the southern section of the San Andreas fault, from the Imperial Valley to the Mojave Desert, is believed to be the greatest natural hazard that California will face in the near future. This event will have an estimated magnitude ranging from M 7 to M 8.2 and will result in violent shaking and disruption of lifelines (freeways, aqueducts, and power, petroleum, and communication lines) that may bring southern California and much of our nation, which depends on southern California, to a standstill.

As part of the nation's efforts to avert a catastrophe of this magnitude, a number of projects are underway to fully understand and mitigate the effects of such an event. One project, funded jointly by the National Science Foundation (NSF) and the U.S. Geological Survey (USGS), is to understand earth structure surrounding the San Andreas fault and also its geometry and interconnections with other faults along the boundary between the North American and Pacific plates. One adjoining plate-boundary fault is the Imperial fault, which extends from the Imperial Valley into Mexico. Earth structure surrounding the San Andreas and Imperial faults will determine how violent the local shaking will be in the Salton Trough, including Mexico, and fault geometry will determine how the earthquake rupture will progress along one (or possibly even both) of these faults and how much energy will be directed into urban regions of both the U.S. and Mexico. This project is a "seismic-imaging" survey. Seismic imaging is a collection of techniques, similar to ultrasound and CAT scan techniques in the medical industry, that enable scientists to obtain a picture of the Earth's subsurface. The petroleum industry routinely uses these same techniques for subsurface exploration for oil and gas, but NSF and USGS scientists need to be able to see deeper into the Earth's crust, namely to the depths of the bottoms of sedimentary basins along the faults (several miles) and also to the depths where earthquakes nucleate (5-10 miles). This project relies on generating and recording sound waves that move downward into the Earth and are bent (refracted) or echoed (reflected) back to the surface. The sources of sound waves are buried explosions (shots) that function like the abdomen vibrators in medical ultrasound investigations. Temporary deployments of receivers for these returned sound waves (seismographs) will take place in late 2009 and early 2010, to allow scientists to construct a picture of the Earth's subsurface.

In order to understand fully the earth structure of the Salton Trough, collaboration with Mexican scientists through cross-border seismic imaging is planned.

A second project is designed to raise public awaredness and preparedness. In November 2008, many government, public, and private agencies and entities will participate in the largest earthquake drill to date, featuring rupture on the southern section of the San Andreas fault in an assumed M 7.8 earthquake. A description of the "ShakeOut Scenario" can be found at:

Back to the Salton Trough Seismic Project Workshop page (2008 Annual Meeting)...