Coseismic and pre-seismic subsidence associated with great earthquakes in Alaska.

Abstract

Alternating beds of peat and mud in sediment sequences on the south-central Alaskan coast record coseismic and inter-seismic relative land and sea-level movements caused by repeated great earthquakes on the Alaska–Aleutian subduction zone. During the AD 1964 Mw=9.2 earthquake, tidal marshes and wetlands around upper Cook Inlet experienced up to 2 m of subsidence, burying peat-forming communities with intertidal mud. Here we use quantitative analyses of fossil diatoms within peat–mud couplets to reconstruct land/sea-level changes for the 1964 and five earlier great earthquakes during the past 3300 years. In contrast to geodetic observations that are limited to the present post-seismic phase, we quantify varying spatial patterns of uplift and subsidence through complete earthquake cycles. Relative land uplift characterises most of the inter-seismic phase of each cycle at our sites, whereas each great earthquake was preceded by a short period of pre-seismic relative land subsidence.