Geologic evidence for a Holocene earthquake near Lake Iliamna, Alaska

ABSTRACT

Liquefaction features, relief on a Pleistocene shoreline, and traces of ground failure give evidence for earthquake shaking and tectonic deformation on a buried fault near the north shore of Lake Iliamna, in a remote and little-studied region of southern Alaska. Along about 70 km of isostaticaly tilted late-Pleistocene shoreline above Lake Iliamna’s north shore, one 6 km section shows 8 m greater relief than expected from isostatic tilting alone. This relief suggests folding above a blind fault. A nearby lakeshore bluff exposes a sand intrusion, sourced in Pleistocene lake deposits, that erupted onto mid-Holocene peat. Along 2.24 km of another lakeshore bluff, younger lake sediments have been liquefied and homogenized to varying depths up to at least 8 m, destroying depositional structures and ingesting large chunks of overlying sediment. A 10 km by 3 km area of tundra vegetation lineaments, as well as a pair of lineaments elsewhere, may record lateral spreading and fissuring. The prospect of locally generated strong earthquakes implies greater seismic hazard for local communities and the Pebble Mine, proposed nearby.

The Lake Clark Fault, which points southwestward toward Lake Iliamna, is the nearest tectonic structure identified previously. Its nearest mapped trace is 50 km distant, and no part of the fault has documented Holocene displacement. However, Holocene displacement on a structure at Lake Iliamna would be consistent with a previous geodetic model that predicts active tectonic extension in this area.