The San Jacinto fault in Southern California is the site of frequent moderate earthquakes because it can adjust to applied stresses slowly and continuously at relatively shallow depths, suggests a paper published online this week in Nature Geoscience. This behaviour is in contrast to the nearby, and more quiescent, Southern San Andreas fault, in which the two sides of the fault stick together to a greater depth, and thus accumulates strain over longer periods.
Shimon Wdowinski compared seismic activity in the San Jacinto and Southern San Andreas fault systems. He found that only the top 10 km of crust at the San Jacinto fault accumulates strain, whereas the section deeper down in the crust adjusts through frequent small earthquakes and through continuous creep. Because of the limited amount of strain accumulation, the San Jacinto fault is unlikely to produce large earthquakes, instead experiencing frequent small or moderate events. In contrast, the neighbouring Southern San Andreas fault system accumulates strain throughout the entire 17 km of brittle crust, and releases its strain in larger seismic events.