PRELIMINARY PALEOSEISMIC RESULTS FROM THE MESQUITE LAKE FAULT, TWENTYNINE PALMS, CALIFORNIA

We report preliminary paleoseismic results from two excavations across the Mesquite Lake fault, located on the Mesquite Lake playa near Twentynine Palms, California. The surface rupture of the October 1999, M_w 7.1 Hector Mine earthquake extends to within 15km of the playa site. The Mesquite Lake fault is part of the eastern California shear zone, a broad region of northwest oriented dextral shear along the Pacific/North American plate boundary.

The excavation is located between two sand dunes at the edge of the playa where the fault splays into two parallel strands. This is an ideal site for preserving event- stratigraphy; rapid aeolian sands regularly wash across the fault onto the playa surface, while lacustrine sediments favor preservation of detrital charcoal and offer an excellent depositional record. Two fault-perpendicular trenches expose lacustrine, alluvial and aeolian deposits. The lowermost exposed unit consists of locally tilted, well-bedded to massive, lacustrine strata. The lacustrine strata are overlain by well–bedded, alluvial sand, overlain by a weakly-bedded, bioturbated lacustrine silt and clay. These strata are overlain by a thin, massive to cross-bedded aeolian sand.

Trench wall exposures suggest evidence for a minimum of three prehistoric events. The thin uppermost aolian sand exposed in the trench wall is not cut by the fault and provides a minimum age estimate for the most recent event. The penultimate event occurs near the base of the lacustrine silt and clay and is marked by an abrupt upward termination of in-filled fissures. The pre-penultimate event is marked by a distinct deformation horizon in the well-bedded alluvial sands below the lacustrine silt and clay. Although radiocarbon dates are pending, we interpret the last two prehistoric earthquakes to be Holocene in age. This suggests that the Mesquite Lake fault experiences long recurrence intervals, similar to other faults of the eastern California shear zone.