ANALYSIS OF GEOPHYSICAL DATA SUPPORT ONGOING SEISMIC-HAZARD EVALUATIONS IN SOUTHERN NEVADA

Two geophysical methodologies are key elements in understanding seismic hazards in southern Nevada: (1) gravity inversions provide basin thickness models for predicting ground shaking, and (2) gravity and magnetic interpretations help locate concealed faults characterized by abrupt variations in density and/or magnetic properties. Our areas of focus have included Las Vegas (LV), Pahrump, and Virgin Valleys. We model the basin configuration beneath these valleys with a 3-D inversion of gravity data constrained by geologic, drill-hole, and seismic-reflection data. In each case, the pre-Tertiary bedrock surface is complex, with transtensional sub-basins buried beneath the flat alluvial surfaces of the valleys. Beneath LV Valley, sub-basins 2-4 km deep and elongated N70W and N50W are interpreted as strike-slip basins formed by movement along the LV Valley shear zone. Pahrump Valley is underlain by two deep sub-basins (2 and 5 km deep) separated by a ridge oriented along the State Line fault. High-resolution aeromagnetic data contain subtle anomalies that coincide with the edges of the subsurface ridge. Alignments of springs along magnetic anomalies at these locales suggest that faults cut the alluvium and displace magnetic rocks at depth. The ridge between the basins may have formed because of slightly more westerly orientations of second-order faults within the State Line fault zone.

The lower Virgin River Valley is separated into two subsurface structural basins by an intervening subsurface ridge that strikes roughly N-S along the eastern margin of Mormon Mesa. The eastern basin is separated into two domains divided by the Virgin River (VR). The sub-basin N of the VR is rectangular in plan-view and 8-10 km deep. Quaternary N-striking faults are concentrated within the deeper portions of the basin. South of the river, the basin trends NE, parallel to mapped Quaternary faults. The ridge between the two sub-basins coincides with a right step in the course of the VR. The ridge may be caused by a compressional step in a left-lateral fault system.

All of the major basins in S. Nevada are comprised of structural sub-basins at depth; thus, inventories of potentially seismogenic faults must include buried ones, and amplification effects will be poorly characterized by considering only the surface expression of the basins.