Bedrock Geology of the Ranges Bounding the Wells Earthquake of February 21 , 2008

The Wells 6.0 magnitude earthquake of February 21, 2008, occurred on a previously unrecognized northeast-striking southeast-dipping fault situated beneath Town Creek Flat, a few miles north of Wells, Nevada. Focal depth of the earthquake was 5 miles (8 km). Bedrock geology in the surrounding ranges did not indicate the existence of the fault prior to the earthquake, and no surface rupture has been observed related to the earthquake. Faults with a similar trend and attitude had not been observed in the southern Snake Mountains. Correlation of the earthquake fault to structures south of Wells that bound the East Humboldt Range and Clover Hill is plausible. High-angle northwest-trending faults in the southern Snake Mountains and Windermere Hills, adjacent to the earthquake foci, may have been responsible for partitioning aftershocks. This fault, referred to as the Wells earthquake fault (WEF), is an approximately 55° southeast-dipping normal fault (Smith and others, this volume). Its dip direction is opposite to that of most recognized Cenozoic (Neogene) high-angle faults in the immediate vicinity that are related to uplift of the Snake Mountains and Windermere Hills northwest and northeast of Wells, respectively. The geometry of the WEF was resolved by Smith and others (this volume) based on 3-D resolution of the main earthquake and subsequent aftershocks. The bedrock geology of the southern Snake Mountains, Windermere Hills, Clover Hill and northern East Humboldt, and Wood Hills, which bound the earthquake area, provided no suggestions that a fault, especially one capable of producing a large earthquake, was present at the surface location of the WEF. Attitudes of bedding-parallel faults and of stratigraphic units in the southern Snake Mountains are roughly parallel to the WEF, and thus, following the earthquake, it was initially postulated that there might be a relationship between the two. However, this does not appear to be the case. Rather, the earthquake fault may be related to a system of east-dipping normal faults south of Wells that bound the eastern flanks of the Ruby Mountains, East Humboldt Range, and Clover Hill (figure 1). Aftershocks from the main earthquake were distributed in a belt extending north from Wells and occurred in two major clusters/groups with an intervening area of lower occurrence (figure 2). Based on bedrock geology in the southern Snake Mountains and Windermere Hills, we interpret the frequency distribution of the aftershocks to be influenced/controlled by two or more high-angle west-northwest-striking faults, rather than a single northeast-striking fault. This interpretation suggests that the west-northwest-striking faults are deep-penetrating structures and of regional significance, as proposed by Thorman and Ketner (1979). It follows that future earthquakes may occur where oblique high-angle faults intersect rangebounding faults in the Basin and Range Province. This paper focuses on the nature of the west-northwest-trending faults, their ages, recurrent movements, and lateral and vertical continuity. In deciphering the geology of these ranges, comprehending the regional and local stratigraphy and structures is critical. As in much of the Basin and Range Province, it is impossible to do one without understanding the other.