LATE HOLOCENE TIMING OF A PENULTIMATE EARTHQUAKE ON THE LAVIC LAKE FAULT, MOJAVE DESERT, SOUTHERN CALIFORNIA

Four trenches were dug across the Lavic Lake fault on Lavic Lake playa. Three of the four trenches, trenches A–C, were dug across the 1999 Hector Mine rupture; the fourth trench, D, was dug across a vegetation lineament that had only minor slip at its southern end in 1999. Trenches A–C exposed strata with only 1999 rupture; trench D exposed horizontal bedding along with local warping and offset by faults. Faults revealed in trench D line up with the vegetation lineament at the ground surface. A depositional contact about 80 cm below the ground surface acts as the upward termination of fault breaks in trench D. Thus, this contact may be the event horizon for a surface-rupturing event prior to 1999. Carbon 14 dates of three detrital charcoal samples, one from above (1940 +/- 40 radiocarbon years B.P.) and two below the event horizon (1730 +/- 40, and 3040 +/- 40 radiocarbon years B.P.) indicates that the earthquake associated with the faulting occurred a maximum of 1700 to 2000 years ago. The 1300-year age difference between the two samples below the event horizon suggests the potential for long residence time of detrital charcoal in the area. Coupled with a lack of bioturbation, the charcoal dates provide only maximum bounding ages and the recognized penultimate event may be considerably younger. A late Holocene age of the probable penultimate event at trench D is significantly younger than for the penultimate event as indicated at a more southerly trench site across the 1999 rupture. However, our inferred timing of an event is in agreement with an event on the Pisgah fault less than 2000 years ago. Thus, an event on the Pisgah fault may have interacted with the northern part of the Lavic Lake fault, or, at a minimum, there was a close timing of events on the Pisgah and northern Lavic Lake faults.