Variable short-term slip rate on the Imperial fault modulated by filling of the Salton Trough by Lake Cahuilla

Earthquake Science Center Seminars

Thomas Rockwell, San Diego State University

The Salton Basin was free of significant water between about 100 BCE and 950 CE but has filled to the sill elevation of +13 m six times between ca 950 and 1730 CE. Based on a dense array of cone penetrometer (CPT) soundings across a small sag pond, the Imperial fault is interpreted to have experienced an increase in earthquake rate and accelerated slip in the few hundred years after re-inundation, an observation that is also seen on the southern San Andreas and San Jacinto faults. This regional basin-wide signal of transient accelerated slip in interpreted to result from the effects of increased pore pressure on fault strength resulting from the ~100 m of water load during full lake inundations. If the relationship between co-seismic subsidence in the sag depression and horizontal slip through the sag is even close to constant, the slip rate on the Imperial fault may have exceeded the plate rate for a few hundred years due to excess stored elastic strain that accumulated during the extended dry period prior to ca 950 CE.