The Ulakhan fault surface rupture and the seismicity of the Okhotsk-North America plate boundary

Abstract

New field work, combined with analysis of high-resolution aerial photographs, digital elevation models, and satellite imagery, has identified an active fault that is traceable for g1/490 km across the Seymchan Basin and is part of the Ulakhan fault system, which is believed to form the Okhotsk-North America plate boundary. Age dating of alluvial fan sediments in a channel system that is disturbed by fault activity suggests the current scarp is a result of a series of large earthquakes (M wg €...7.5) that have occurred since 11.6±2.7 ka. A possible channel feature offset by 62±4 m associated with these sediments yields a slip rate of 5.3±1.3mm yr g 1, in broad agreement with rates suggested from global plate tectonics. Our results clearly identify the Ulakhan fault as the Okhotsk-North America plate boundary and show that tectonic strain release is strongly concentrated on the boundaries of Okhotsk. In light of our results, the likelihood of recurrence of i wg €...7.5 earthquakes is high, suggesting a previously underestimated seismic hazard across the region.