TIMING AND SLIP FOR PREHISTORIC EARTHQUAKES ON THE SUPERSTITION-MOUNTAIN FAULT, IMPERIAL-VALLEY, SOUTHERN CALIFORNIA

Trenches excavated across the Superstition Mountain fault in the Imper ial Valley, California, have exposed evidence for four prehistorical e arthquakes preserved in displaced lacustrine stratigraphy associated w ith ancient Lake Cahuilla. The presence of shoreline peat accumulation s along with abundant detrital charcoal allows for high-precision age determination of some stratigraphic units, thereby providing constrain ts on the timing of three of the paleoearthquakes. These three events occurred within a 480- to 820-year interval during the past 1200 years . The most recent earthquake (event 1) occurred during a fluvial phase of deposition between A.D. 1440-1637, immediately prior to the inunda tion of the Cahuilla basin at about A.D. 1480 and 1660. A channel marg in was offset 2.2 +0.4/-0.15 m in this rupture, suggesting an earthqua ke with a magnitude greater than or equal to 7. The penultimate event (event 2) also occurred during fluvial deposition after A.D. 1280 but before another lakestand at A.D. 1440-1640. Lateral slip could not be resolved for event 2. However, based on juxtaposition of dissimilar un its and the amount of deformation produced by this event, it is presum ed that this was also a large earthquake. The timing of event 3 is con strained to have occurred between about A.D. 820 and 1280. This event is represented by several fractures and small displacements that ruptu re up to a distinct stratigraphic level or event horizon. Slip was not resolved for this event. Finally, the timing of event 4 is very poorl y constrained to between A.D. 964 and 4670 B.C. Undoubtedly, many even ts may have occurred during this period. Notably, the past three earth quakes occurred within a period of less than 820 years, and it has bee n over 350 years since the last earthquake.