TOWARDS A BETTER UNDERSTANDING OF LATE QUATERNARY FAULT RUPTURE, SOUTHERN ANDERSON JUNCTION SECTION, HURRICANE FAULT, NORTHWESTERN ARIZONA

An early Holocene or latest Pleistocene earthquake is known to have occurred along the southern Anderson Junction section of the Hurricane fault. Rupture from this event was identified in trenches at Cottonwood Canyon where approximately 60 cm of surface slip occurred. A late Pleistocene fan at Cottonwood Canyon records about 20 m of normal slip and may suggest that the most recent event was atypical for the fault in that location. Existing models partition the Hurricane fault in Arizona into individual rupture segments based primarily on the location of geometric changes, such as fault bends and steps; one such change is approximately 5 km south of the Cottonwood Canyon site. Was the most recent earthquake smaller than average for this section of the fault? Was it limited to the segment boundary or does it represent the end of a rupture where slip may be decreasing? We hope to improve our understanding of the extent and location of the most recent rupture and gain further insight into the segmented nature of the fault by trenching an additional site along the southern Anderson Junction section, at Rock Canyon.

An alluvial fan has been faulted at the mouth of Rock Canyon, resulting in an approximately 30-m-long fault scarp in unconsolidated alluvium. Two profiles across this scarp reveal that the surface has been displaced at least 3-4.5 m down-to-the-west. We interpret this displacement to be the result of more than one earthquake, primarily because there are smaller scarps (one meter or less) present within a kilometer of the fan, that appear to have formed during a single event. The fan is likely a latest Pleistocene surface, estimated by comparing the surface undulations, degree of dissection, and the relative elevation above the active stream channel to those investigated more thoroughly at Cottonwood Canyon, 4 km to the south. Trench exposures of the fault through the alluvial fan may reveal new clues about the most recent event along the southern Anderson Junction section and possibly information about earlier events.