Normal faults in basalt have distinctive surface-trace morphologies and earthquake evidence that provide information about the slip behavior and earthquake potential. The 47-km-long Hat Creek fault in northern California (USA), a useful case example of this fault style, is a segmented fault system located along the western margin of the Modoc Plateau that is a regional earthquake hazard. In response to interaction with sporadically active volcanic systems, surface ruptures have progressively migrated westward since the late Pleistocene, with older scarps being successively abandoned. The most recent earthquake activity broke the surface through predominantly ca. 24 ka basaltic lavas, forming a scarp with a maximum throw of 56 m. Past work by others identified 7–8 left-stepping scarp segments with a combined length of 23.5 km, but did not explicitly address the throw characteristics, fault evolution, slip history, or earthquake potential. We address these deficiencies in our understanding of the fault system with new field observations and mapping that suggest the active scarp contains 2 additional segments and is at least 6.5 km longer than previously mapped, thus increasing the knowledge of the regional seismic hazard. Our work details scarp geomorphic styles and slip-analysis techniques that can be applied to any normal-faulted basalt environment. Applied to the Hat Creek fault, we estimate that a surface-breaking rupture could produce an earthquake of ∼Mw (moment magnitude) 6.7 and a recurrence interval of 667 ± 167 yr in response to a rapid slip rate in the range 2.2–3.6 mm/yr, creating a moderate risk given a lack of historical earthquake events.
- Received 11 February 2013.
- Revision received 10 July 2013.
- Accepted 13 August 2013.
- © Geological Society of America