In this area we experienced a sharp 3.9 magnitude earthquake on Monday morning, April 23, at 10:37 AM. According to the LA Times this could have been on the San Joaquin Hills blind thrust fault, with epicenter around Dana Point. This fault was discovered in 1999 by Lisa Grant Ludwig and coworkers at UC Irvine. A detailed description with tentative location of the fault is given by Mueller et al. It may have the capability of a magnitude 7.0 or greater. The San Joaquin Hills are a set of hills a bit inland from the coast that were possibly built up by this thrust fault, running from Costa Mesa to San Juan Capistrano. The description of “blind” means that it is not visible on the surface.
Looking up the link in the LA Times article led to the 2003 scenario run by the US Geological Survey. The scenario was for a magnitude 6.6 near the coast, at a depth of 7.5 km. The Peak Ground Acceleration from the scenario is shown in the map below. The contours on it label the peak acceleration boundaries in percent of the acceleration of gravity g = 32 ft/sec/sec.
The scenario quake was located at the star in the center, and the maximum acceleration contour is at 42 percent of g. Tracing highway 74 on the map from Lake Elsinor (under the R), places the San Onofre plant below the left leg of the A, on the 24% of g contour. If the magnitude of the scenario is scaled up to 7.0, the extra 0.4 of magnitude should increase the shaking by 10^0.4 = 2.5, and the acceleration on that contour at the plant would be at 60% of g. We note that the reactor is built to withstand 0.67 g, so it could withstand a 7.0 on this fault.
In an earlier post I printed out the probability of a 7.0 in this area, and found that it was one in 300 years. I don’t know if the USGS now includes this fault or not. It was thought possible to be the fault of the quake that happened between the 17th and 18th century in this area, and that would put it about 300 years ago. One notices that the City of Irvine lies between the 36% and 42% contours in this scenario. Scaling them up by 2.5 to a magnitude 7.0 quake would have Irvine between the 90% to 105% g contours.