Blakeslee, M.W., Kattenhorn, S.A. (2010)


Earthquake hazard and segmented fault evolution, Hat Creek fault, Northern California

Eos, Transactions of the American Geophysical Union 91, Fall Meeting Supplement, Abstract T31B-2164.

Precise insight into surface rupture and the evolution and mechanical interaction of segmented normal fault systems is critical for assessing the potential seismic hazard. The Hat Creek fault is a ~35 km long, NNW trending segmented normal fault system located on the western boundary of the Modoc Plateau and within the extending backarc basin of the Cascadia subduction zone in northern California. The Hat Creek fault has a prominent surface rupture showing evidence of multiple events in the past 15 ka, although there have been no historic earthquakes. In response to interactions with volcanic activity, the fault system has progressively migrated several km westward, causing older scarps to become seemingly inactive, and producing three distinct, semi-parallel scarps with different ages. The oldest scarp, designated the "Rim", is the farthest west and has up to 352 m of throw. The relatively younger "Pali" scarp has up to 174 m of throw. The young "Active" scarp has a maximum throw of 65 m in the 246 ka Hat Creek basalt, with 20 m of throw in ~15 ka glacial gravels (i.e., a Holocene slip rate of ~1.3 mm/yr). Changes in the geometry and kinematics of the separate scarps during the faulting history imply the orientation of the stress field has rotated clockwise, now inducing oblique right-lateral motion. Previous studies suggested that the Active scarp consists of 7 left-stepping segments with a cumulative length of 23.5 km. We advocate that the Active scarp is actually composed of 8 or 9 segments and extends 4 km longer than previous estimates. This addition to the active portion of the fault is based on detailed mapping of a young surface rupture in the northern portion of the fault system. This ~30 m high young scarp offsets lavas that erupted from Cinder Butte, a low shield volcano, but has a similar geometry and properties as the Active scarp in the Hat Creek basalt. At its northern end, the Active scarp terminates at Cinder Butte. Our mapping indicates that surface rupture undergoes a 4.5 km right-step from the Active scarp east to the Rim, perhaps due to the fault responding to localized volcanic activity at Cinder Butte. The newly mapped segment ruptured repeatedly along the pre-existing Rim, creating a cumulative throw of up to 150 m. The addition of this segment increases the seismic risk in this area. Previous studies suggest that a surface breaking rupture along the Hat Creek fault could produce an earthquake magnitude of at least M 6.0. However, with the increase in rupture length, the fault system has the potential to produce at least a M 6.5. Finally, we consider the importance of deformation within segment overlap zones (relay ramps), which contribute to the distribution of fault throw, and can create inaccurate anomalies in throw profiles if not properly accounted for. Relay ramps tilt either toward the front fault segment or toward the back fault segment. We measure the change in elevation across relay ramps perpendicular to the fault strike to quantify the ramp contribution to fault throw, then incorporate this data into throw profiles to eliminate anomalous peaks or lows where segments overlap. More accurate throw profiles lead to better slip rate estimates for the fault system.

External link: AGU database



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