Geophysical Setting of the February 21, 2008 M6 Wells Earthquake, Nevada, and Implications on Earthquake Hazards

The February 21, 2008 M6 Wells earthquake, centered about 10 km northeast of Wells, Nevada caused considerable damage to local buildings, especially in the historic downtown area. The earthquake occurred on a previously unmapped normal fault, and preliminary relocated events indicate a fault plane dipping about 55 degrees to the southeast (K. Smith, written commun., 2008). The epicenter lies near the intersection of north-northeast-trending Basin and Range bounding faults along the Ruby Mountains (Ruby Mountains fault system and Ruby Valley fault zone) and the northerly-trend of the Spruce Mountain Ridge and Independence fault zones. Regionally, the epicenter is approximately on trend with a geophysically defined crustal boundary on the east side of a V-shaped basement gravity high (e.g., Ponce and Glen, 2002; 2008). In addition, the Wells earthquake lies just east of a series of north-northeast-trending basins associated with Diamond, Huntington, and Lamoille Valleys along the western margin of the Ruby Mountains that extend from Elko to northwest of Wells. A regional depth to basement map, derived from the inversion of gravity data, indicates that a small oval basin north-northeast of Wells and just north of Wood Hills, may reach a depth of more than 2 km. Gravity and physical property data were collected in the vicinity of Wells during the summer of 2008 in order to improve the data coverage, which was relatively sparse throughout this part of northeast Nevada, especially on bedrock. These data will be used to better constrain the geophysical setting of the Wells earthquake and help assess the seismic hazard in this area. In particular, we will compute an updated depth to basement map in order to determine the geometry of the Wells basin which may yield information on whether or not the size and shape of the basin contributed to ground shaking. Modeling of magnetic and gravity anomaly data will help to locate subsurface structures or concealed faults that may be related to the Wells earthquake. Although large earthquakes in this part of Nevada are infrequent, the Wells earthquake indicates that normal faults are active in this region and are capable of generating large earthquakes.