PALEOSEISMOLOGY OF THE HURRICANE FAULT ZONE IN NORTHWESTERN ARIZONA

The Hurricane fault is a 250-km-long normal fault with substantial Quaternary displacement in the transition zone between the Colorado Plateau and the Great Basin in northern Arizona and southern Utah. Recent paleoseismic investigations of the Hurricane fault in Arizona provide estimates of slip rates and the ages of young paleoearthquakes on the fault, and provide some basis for speculating about fault segmentation.

Areas of substantial change in fault orientation, fault complexity, and total displacement may be segment boundaries. Using such reasoning, we divide the fault in Arizona into 4 sections: (1) Anderson Junction (~35 km, extending into Utah); (2) Shivwitz (~55 km); (3) Whitmore (~30 km); and (4) Southern (~70 km). No detailed investigations have been conducted on the Southern section. On the Whitmore section, measured displacements of late Quaternary basalt flows and alluvial surfaces combined with ³He cosmogenic exposure dating indicate that the fault has ruptured repeatedly in the past 150 ky with a slip rate of ~0.1 mm/yr. The youngest event probably occurred in the latest Pleistocene or early Holocene. Detailed studies including trenching on the southern part of the Shivwitz section indicate that at least two surface ruptures occurred in the past few tens of thousands of years, each involving 2-3 m of displacement. A ¹⁴C date from a fissure-fill deposit indicates that the youngest event occurred about 10 ka. Based on the large displacement, this rupture may have involved much or all of the Shivwitz section. Analysis of a faulted 850 ka basalt flow yields a long-term slip rate of ~0.2 mm/yr. Detailed investigations on the southern Anderson Junction section indicate that at least 15 km of the fault ruptured in the early Holocene, but displacement was less than 1 m. This rupture may have been the tail of a larger Anderson Junction or Shivwitz rupture, or it may have been a separate event. A ~100 ka alluvial fan has been displaced ~20 m across the fault yielding a slip rate of 0.2 mm/yr, which suggests the possibility that larger surface ruptures have occurred on this part of the fault. Much of the Hurricane fault in Arizona ruptured in the early Holocene-latest Pleistocene, but lack of tight age and rupture length constraints limits our understanding of the detailed spatial and temporal relationships between faulting events.