THE DEATH VALLEY FAULT SYSTEM: AN UNAPPRECIATED SEISMIC HAZARD TO DEATH VALLEY NATIONAL PARK

Death Valley National Park (DVNP) is in one of the most highly extended tectonic regions of the Western U.S. During the past century, damaging earthquakes (M5+) have rarely occurred in the Park, even though it is bisected by the Death Valley fault system (DVFS), which has active (Holocene) geomorphic expression along much of its length. The DVFS extends from Fish Lake Valley, NV, 300 km south through the entire length of DVNP. The DVFS is comprised of four fault zones: Fish Lake Valley (on the north), Northern Death Valley, Black Mountains, and Southern Death Valley (on the south). The centrally located Furnace Creek fault zone, which is largely a pre-Quaternary part of the system, may no longer be a significant earthquake hazard.

Owing to environmental concerns about ground disturbance, paleoseismic research on the DVFS within DVNP has been mainly of geomorphology, not trenching. Slip rates have been estimated from measured offsets of deposits that are "dated" by regional correlations. However, in 1999 the USGS excavated a trench across the Black Mountains fault zone near Cow Creek, which indicated faulting about 400-700 years ago. Ongoing research using cosmogenic Cl-36 isotopes to date alluvial fans in Death Valley will help place better limits on the current fault slip rates within the Park. Conversely, north of the Park boundary, trenching studies by Reheis and Sawyer on the Fish Lake Valley fault zone have documented late Holocene faulting, high slip rates (ca. 2.5-7 mm/yr), and short recurrence intervals (500-1,500 years).

Although the paleoseismic history of the DVFS is still poorly understood, the USGS has calculated that there is a 10% probability of ground motions exceeding 0.5 g (rock sites) in the next 50 years as a result of large (M7) ground-rupturing earthquakes on the DVFS. Like their predecessors, future large earthquakes on the DVFS will cause ground ruptures, as well as strong ground motion, landslides, and liquefaction along the edges of the salt pan. As such, the DVFS threatens permanent NPS and commercial facilities at Furnace Creek (the Ranch, Inn, and Visitors Center), Cow Creek and Grapevine Junction. Although there are only a few hundred permanent residents in the Park, the population may swell to 20,000 during special events. In addition, several other Holocene faults in the Park are sources for large ground-rupturing earthquakes.