THE LAS VEGAS VALLEY FAULT SYSTEM: WHAT IF?

The Nevada Quaternary Fault Working Group has recently identified the Las Vegas Valley fault system (LVFS) as a high priority target for research because of the significant earthquake risk it could pose to Las Vegas and its ever-expanding population. The LVFS is composed of several subparallel sets of faults including the Eglington, Decatur, Valley View, Cashman Field, Whitney Mesa, and West Charleston faults. The origin and earthquake potential of late Quaternary scarps of the LVFS remains enigmatic. Previously proposed nontectonic mechanisms for the formation of the 10- to 40-m-high scarps include hydrocompaction, dissolution, and fluvial erosion (e.g., Mifflin, 1998). Although hydrocompaction was originally favored by many earth scientists, several studies have pointed out various weaknesses in the hydrocompaction hypothesis and more recently an earthquake origin for the scarps (at least in part) has been favored by many studies (e.g., Bell and dePolo, 1998). If the LVFS is seismogenic, the geometry and dimensions of potential rupture planes and scarp heights suggest that earthquakes of moment magnitude 6.5 and larger could be generated. Several major issues relevant to the ground shaking hazard in Las Vegas need to be addressed to quantify the probabilistic hazard: (1) the earthquake potential of each of the faults of the LVFS and collectively if they were to all rupture coseismically in a large earthquake; (2) fault slip rates since activity rates are the most significant source parameter impacting hazard. Slip rates for the LVFS based on limited data indicate maximum rates of a few tenths of a millimeter per year, except the Eglington fault, which has estimated late Quaternary rates ranging from 0.5 to over 1 mm/yr; (3) rupture models, which include both independent and coseismic rupture of various faults in the LVFS; and (4) the coseismic relationship of the LVFZ, if any, to the west-dipping Frenchman Mountain fault. In this study, we evaluate the sensitivity of these issues to the probabilistic hazard in Las Vegas in an effort to assist the Nevada Quaternary fault Working Group in making recommendations for future research.