EARTHQUAKE HAZARD OF THE HAT CREEK FAULT, NORTHERN CALIFORNIA: THROW ANALYSIS IN YOUNG LAVA FLOWS TO DETERMINE SLIP RATES, RECURRENCE INTERVALS, AND EARTHQUAKE MAGNITUDE

Normal faults are prevalent in basalt lava flows in response to the common association between basaltic volcanism and rifting. Such faults have distinctive surface morphologies where they cut through near-surface lavas and remain active during volcanic periods such that variably aged lava flows cut by the fault can be used as temporal markers of slip rates and slip history. We use the case example of the Hat Creek fault in northeastern California to illustrate the efficacy of using offset lava flows to constrain slip histories and earthquake potential for this fault style. We address the throw characteristics, fault evolution, slip history, and earthquake potential along the fault based on surface morphologies and variable throws in differently aged lavas. The 47 km-long, NNW trending Hat Creek fault is located along the western margin of the Modoc plateau within the extending arc-to-backarc of the Cascadia subduction zone. In response to interaction with active volcanism in the area, the fault system has progressively migrated westward since the Late Pleistocene, causing older scarps to become successively abandoned. The most recent earthquake activity broke the surface through ~24 ka basaltic lava flows to form a prominent, vertical scarp with a maximum throw of 56 m in these lavas. Prior to surface rupture, the fault formed a fault trace monocline that attained a height of up to 33 m. Subsequent earthquake activity reduced the monocline to a rubble pile in many places. Dislodged small columns thrown up onto the lava surface indicate >1g surface acceleration. Previous work described the scarp as having seven left-stepping segments with a combined length of 23.5 km; however, our field observations and updated mapping suggest the active scarp is composed of 9 segments and is at least 30 km long. We estimate a surface-breaking rupture could produce an earthquake magnitude of at least M_w 6.8 with a recurrence interval of 558±140 years in response to a rapid slip rate in the range 2.2-3.6 mm/yr, providing a moderate near-future risk given a lack of historical earthquake events. The style of the Hat Creek fault is common in rift valleys, but also shallow subaqueous components of spreading centers and volcanic island rift zones, offering a potential tsunami hazard. Our analysis technique can be applied to these analogous faults.