PALEOSEISMIC INVESTIGATIONS OF THE 1999 M7.1 HECTOR MINE EARTHQUAKE SURFACE RUPTURE AND ADJACENT BULLION FAULT, TWENTYNINE PALMS MARINE CORPS BASE, CALIFORNIA

The 1999 M7.1 Hector Mine earthquake produced a maximum slip of 5.2 m on a ~48-km-long surface rupture involving the Lavic Lake fault and a portion of the southern Bullion fault. The Bullion fault exhibits multiple geomorphic features suggestive of a recent surface rupturing event, whereas the 1999 surface rupture on the Lavic Lake fault within the Bullion Mountains is devoid of youthful geomorphic expression suggesting a long period of quiescence prior to the Hector Mine earthquake. We designed a paleoseismic trenching program to test these initial geomorphic observations and to better constrain the timing of past earthquakes on the 1999 surface rupture within the Bullion Mountains (Drainage Divide Site) and the Bullion fault where no rupture occurred in 1999 (Bullion Fan Site). At the Drainage Divide Site, the rupture traversed dissected alluvial aprons whose smooth, stable surfaces exhibited a well-formed, varnished desert pavement indicative of a Pleistocene age. These surfaces appear to have been disrupted by only the 1999 event. Trenches at this site revealed evidence for prior events that are likely on the order of tens of thousands of years old. Pending cosmogenic dating of the alluvial surface will provide a minimum age of the pre-Holocene penultimate event. In contrast, trenches across the Bullion fault at the Bullion Fan Site revealed a latest Holocene (< 2 ka) rupture. Paleoseismic and geomorphic observations suggest that the Bullion fault slip rate is significantly greater than the Lavic Lake fault.