SEISMIC HAZARDS IN SOUTHERN NEVADA: WHAT WE HAVE LEARNED FROM CONTROLLED-SOURCE SEISMIC DATA

Assessing the seismic hazard for a region previously thought to have low seismic risk can be a challenge in today’s world such as in southern Nevada, where the population has boomed over the last 10 to 15 years. In the past 6 years, we acquired controlled-source data across several faults to provide sub-surface geometries of these active faults, which will assist in assessing earthquake hazard and subsequent risk. This region has several basins (e.g., Las Vegas Valley, Eldorado Valley, Pahrump Valley) that are cut by active normal and/or strike-slip faults. These basins can be deep (up to several km) and amplify energy from strong ground motions. The region is dominated by strike-slip faults to the west and normal faults to the east, which has the potential of producing M6+ within the highly populated region. A series of seismic reflection and refraction experiments (2002 to 2008) was undertaken to image the geometry of the Las Vegas Valley fault system and the State Line fault to better understand potential focusing effects as well as determine the depth, lithologies, location, and in some cases estimated slip-rate of these faults. These projects are part of a larger collaborative study called the Earthquakes in Southern Nevada (ESN - http://earthquakes.unlv.edu) project and the Las Vegas Valley Seismic Response Project (LVVSRP). In the Las Vegas Valley, we have developed a 3-D P-wave velocity model showing velocities ranging from 2.5 to 4.5 km/s within the basin, where the 4.5 km/s contour indicates the base of the crystalline basin. Several of the faults that were known at the surface can be mapped well into the basement rocks in the sub-surface. Reflection profiling across several of the active faults show colluvial wedges, indicating multiple episodes of recent faulting as well as providing the geometry of these faults. In addition, we have calculated a Quaternary slip-rate across the State Line fault. These seismic results have been used to guide future studies to obtain more detailed geologic information such as dates for these active faults. Combining these seismic results with known geology led us to assess the seismic hazard potential with more accuracy. As a result, we found a higher hazard within the Las Vegas Valley fault system and a lower hazard along the State Line fault.