Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska

Abstract

Can a predominantly creeping segment of a subduction zone generate a great ( M &thinsp;>&thinsp;8) earthquake? Despite Russian accounts of strong shaking and high tsunamis in 1788, geodetic observations above the Aleutian megathrust indicate creeping subduction across the Shumagin Islands segment, a well-known seismic gap. Seeking evidence for prehistoric great earthquakes, we investigated Simeonof Island, the archipelago's easternmost island, and found no evidence for uplifted marine terraces or subsided shorelines. Instead, we found freshwater peat blanketing lowlands, and organic-rich silt and tephra draping higher glacially smoothed bedrock. Basal peat ages place glacier retreat prior to 10.4&thinsp;ka and imply slowly rising (<0.2&thinsp;m/ka) relative sea level since ~3.4&thinsp;ka. Storms rather than tsunamis probably deposited thin, discontinuous deposits in coastal sites. If rupture of the megathrust beneath Simeonof Island produced great earthquakes in the late Holocene, then coseismic uplift or subsidence was too small (&le;0.3&thinsp;m) to perturb the onshore geologic record.