Abstract
Since 1890, the San Jacinto fault in Southern California has been the site of eleven earthquakes of moderate magnitude (6<M<7) and tens of thousands of small earthquakes, but none of large magnitude. This activity contrasts sharply with the seismic quiescence of the nearby southern San Andreas fault. Although this fault slips at a rate higher than that associated with the San Jacinto fault- 23-27 mm yr−1 versus 12-22 mm yr−1 (refs 3, 4)-it has produced very few earthquakes and no moderate or larger events in historical times. Here I use recent seismic and geodetic data to reveal that at depths of 10-17 km within the seismogenic (brittle) crust, the San Jacinto fault is creeping and releasing elastic strain by many small earthquakes. As a result, the accumulation of strain along this fault occurs mostly in its upper 10 km; moderate earthquakes are likely to be sufficient to release such strain. In contrast, the southern San Andreas fault accumulates elastic strain throughout its vertical extent in the seismogenic crust, which will most probably be released by stronger earthquakes.
