EVIDENCE FOR A M7 EARTHQUAKE ON THE SHIVWITZ SECTION OF THE HURRICANE FAULT ZONE, NORTHWESTERN ARIZONA

The Hurricane fault is in close proximity to rapidly growing population centers in southwestern Utah and southern Nevada; major earthquakes on the Hurricane fault pose a seismic hazard to these areas. We are investigating the Quaternary behavior of the Arizona portion of the Hurricane fault, a 250-km normal fault in the transition zone between the Colorado Plateau and the Basin and Range physiographic provinces in southwestern Utah and northwestern Arizona. Geologic mapping along the southern Shivwitz section of the Hurricane fault identified surface rupture of late Quaternary landforms; no evidence of displaced Holocene geomorphic surfaces or landforms was observed. Paleoseismic investigations of a displaced alluvial fan, using fault trenching and surface age estimation from morphologic dating of the fault scarp and carbonate-rind chronosequence, indicate two surface-rupturing events in the last 15-78 ky. Total vertical surface displacement of the late Pleistocene fan surface is about 4.5 m, and the total throw at the fault is estimated to be 5.5 m. A radiocarbon age of sediment related to the MRE produced a 2-s age of 8900 to 10,400 calibrated years BP. Mapping and analysis of a faulted 850 ka basalt flow indicates a long-term slip rate of about 0.2 mm/yr for the Shivwitz section.

Interpretation of trench stratigraphy, retro-deformation analysis, and forward modeling considering different deformation scenarios, suggests that the most recent event had a far-field vertical displacement of ~2.5 m. Using empirical relationships between moment magnitude, surface displacement, and rupture length, this amount of displacement suggest that much or all of the 57-km-long Shivwitz section ruptured in this event, producing a M 6.8 – 7.1 earthquake. Our estimated displacement, potential fault rupture length, and the estimated moment magnitude are similar to the parameters observed for the 1983 Borah Peak earthquake.