Geologic Traces of a Buried Rupture Earthquake Between the Southern Alaska and Bering Blocks

Abstract:

The nature and location of the transition between two large crustal blocks that comprise southern and western Alaska is unknown. Oblique subduction of the Pacific Plate beneath Alaska drives counterclockwise rotation of the Southern Alaska Block and clockwise rotation of the Bering Block, indicating crustal strain is occurring in a poorly delineated transition zone in Southcentral Alaska. Previously, no active faults have been identified in this transition zone, south of the Denali Fault and west of Cook Inlet. Near the north shore of Lake Iliamna, over 200 km from the nearest known active fault, we documented an 8 m drop followed by a 2 m rise in a 16,000 year-old abandoned lakeshore, interrupting 10 cm/km isostatic tilting. The deformation is spread over several kilometers. Evidence of dramatic liquefaction exposed in nearby bluffs dates to between 900 and 4200 years ago. These geologic traces tell a story of neotectonic deformation and strong shaking – consistent with a strong earthquake on a blind fault. Nearby geologic mapping and the graben-like depression in paleo-shoreline elevation we documented are consistent with extension accommodated along the Lake Clark Fault. Beyond relevance to regional tectonics, our results show previously undocumented seismic hazard in an area of a potential large-scale mining development (the proposed Pebble Mine), which would require perpetual waste storage.