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Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska
Journal article   Open access   Peer reviewed

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

Robert C Witter, Richard W Briggs, Simon E Engelhart, Guy Gelfenbaum, Richard D Koehler and William D Barnhart
Geophysical Research Letters, Vol.41(7), pp.2359-2367
04/16/2014
DOI: 10.1002/2014GL059393
url
https://doi.org/10.1002/2014GL059393View
Published (Version of record) Open Access

Abstract

Can a predominantly creeping segment of a subduction zone generate a great (M > 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 ka and imply slowly rising (<0.2 m/ka) relative sea level since ~3.4 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 (≤0.3 m) to perturb the onshore geologic record. Key Points Simeonof Island lacks evidence for great earthquakes and tsunamis Slow sea-level rise since ~3.4 ka was unperturbed by sudden tectonic jerks Creeping subduction in the Shumagin gap probably persisted in the late Holocene
 Aleutian megathrust paleoseismology seismic gap Shumagin Islands

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