CHARACTERIZING THE EARTHQUAKE POTENTIAL OF THE LAS VEGAS VALLEY FAULT SYSTEM

The Las Vegas Valley fault system (LVVFS) threatens a population of about 1.8 million people and includes the Decatur, Valley View, Cashman Field, Eglington, Whitney Mesa, and West Charleston faults. These faults were created by tectonic deformation, not hydrocompaction. They formed the mid-level basin (1-2 km deep), have basin-bottom bedrock offsets, and have shifted the depocenter of Las Vegas Valley from near Frenchman Mountain towards the center of the valley in late Cenozoic. The system is 30 km long, 11 km wide, has an overall trend of ~N20ºW, and has created fault scarps as-much-as 40 m high in mid to late Pleistocene deposits. These normal dip-slip faults are between 8 and 18 km west of the Frenchman Mountain fault and dip to the east presumably intersecting it. The north end of the LVVFS is likely truncated by the Las Vegas Valley shear zone, a buried, NW-striking, mid-Cenozoic right-lateral fault. Thus, the LVVFS intersects, or merges, with structures to the east and may or may not extend the full width of the seismogenic zone. Single-event, normal dip-slip offsets on secondary faults are 2 to 3 m. Considering fault length and displacement, maximum earthquake magnitude estimates range from Mw 6.3 to 6.9 for the LVVFS. These values have an uncertainty of about ±0.3 magnitude units and may be lower if faults are truncated at mid-seismogenic depths. The most recent earthquake known along the LVVFS occurred just before ~2,000 years ago along the Eglington fault, but the timing and recurrence of earthquakes is a major gap in knowledge. Composite vertical fault slip rate estimates range from 0.05 to 0.6 m/ky. Although many sites have been compromised for paleoseismic studies, an aggressive effort to directly characterize the earthquake potential, particularly the earthquake occurrence, of the LVVFS needs be immediately conducted.