BEHAVIOR OF A NEW NORMAL FAULT IN THE ROCKY MOUNTAINS: IMPLICATIONS FOR CONTINENTAL INTRAPLATE EARTHQUAKES

We offer a case study of a newly emergent normal fault as a potential analog for controls on the strain-release behavior of faults in the mid-continent. The Bear River fault (BRF) in Utah and Wyoming lies along the eastern margin of distributed extensional deformation, west of the relatively stable plate interior of the central and eastern U.S. The BRF is remarkable in that it ruptured to the Earth’s surface for the first time in the mid-late Holocene in a cluster of three large-magnitude earthquakes, yielding a short-term slip rate (~2 mm/yr) equal to central segments of the Wasatch fault, but notably higher than the present-day rate of extension (<1 mm/yr). The surface rupture pattern of the BRF in the context of preexisting structure illuminates the controlling influence of rock-strength heterogeneity on fault development and rupture behavior. In particular, the BRF developed along a ramp in an old thrust fault, a presumed zone of crustal weakness, and where the south end of the BRF intersects upturned strata along the south-dipping flank of the Uinta arch (a deep-seated basement-cored uplift), the main surface trace ends abruptly in a set of orthogonal splay faults that accommodate down-dropping of a large hanging-wall graben against the arch. We infer that until the recent period of activity, normal faulting was impeded by the presence of the mechanically strong Uinta arch. Eventually, enough elastic strain energy had accumulated to overcome the buttressing effect of the arch and destabilize the fault, resulting in a sequence of large, short-recurrence earthquakes. This points to the potential wider importance of strength heterogeneity, coupled with long-term accrual of tectonic stress, as a cause of earthquake clustering in tectonic environments where preexisting geologic structure and/or low strain rates allow for substantial inherited or fault-healing-related variability in strength. Although the BRF is seismotectonically distinct from faults in the central and eastern U.S, it shares with them low (although not as low) rates of tectonic loading, reactivation of inherited structures, deformation of strong cratonic crust, and spatiotemporal clustering of earthquakes, and therefore may have similar mechanisms of earthquake generation.