InSAR Analysis of the 2008 Wells, Nevada Earthquake

The February 21, 2008 Wells earthquake (M 6.0) was centered 10 km northeast of the Town of Wells, Nevada in Town Creek Flats, a small basin bounded on the west by the Snake Mountains and on the east by the Windermere Hills. The only mapped faults of Quaternary age in the basin are a series of north- to northeast-trending, west-dipping faults along the west flank of the Windermere Hills. Based on the geometry of these faults and the normal, northeast-oriented focal mechanism of the main event, it was initially assumed that the co-seismic slip had occurred on a northeast-striking, west-dipping fault. InSAR analysis of pre- and post-earthquake Envisat data in the WInSAR GeoEarthscope archive, however, indicates that the fault slip occurred on a northeast-striking, southeast-dipping fault bounding the eastern flank of the Snake Mountains. Both descending and ascending pairs delineate a similar oval-shaped deformation bowl about 15 km in width and 25 km in length, centered on the main event epicenter, and exhibiting more than 15 cm of subsidence. The similarity of the ascending and descending pairs indicates that there was little to no lateral displacement associated with the event. Displacement profiles through the zone of maximum surface deformation indicate that the subsidence bowl is asymmetric, with the greatest and steepest deformation occurring near the range front of the Snake Mountains and then gradually decreasing across the basin to the east. The profiles further indicate slight (1-2 cm) uplift on the footwall of the fault, an observation consistent with other normal faulting events in the Basin and Range. We applied inverse modeling to the results, in which fault parameters are selected to best estimate observed InSAR deformation patterns. The modeling shows that the deformation pattern is best fit by a normal fault striking N35E and dipping 45 degrees to the southeast. Fault slip is modeled to be 75 cm on a 4 km x 7.5 km fault patch, and the calculated moment magnitude is Mw 5.9. The InSAR results agree with independent seismologic data, including a well-defined aftershock pattern, and indicate that the earthquake occurred on a previously unknown Quaternary fault located along the eastern flank of the Snake Mountains. No surface rupture was associated with the earthquake, but field investigations found evidence for a small (15-30 cm) scarp that is likely of Holocene age. The seismogenic fault coincides with a previously mapped Paleozoic thrust fault and suggests that the earthquake may be an example of the re-activation of older, inactive bedrock structure in the present-day extension-dominated Basin and Range.