EVIDENCE FOR 8 GREAT EARTHQUAKE-SUBSIDENCE EVENTS DETECTED WITH GROUND-PENETRATING RADAR, WILLAPA BARRIER, WASHINGTON

A new approach to detect Holocene subduction-zone earthquakes combines the results from ground penetrating radar (GPR), Vibracores, and acce lerator mass spectrometry (AMS) dates from a barrier spit located west of Willapa Bay, southwest Washington. GPR data show a 10-m-thick faci es of beach sand within which we identify, and Vibracores confirm, bea ch-parallel, wave-eroded, buried scarps mantled with multiple beds of magnetite. The eight GPR-detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subside nce during earthquakes associated with the Juan de Fuca plate subducti ng under the North American plate. Of these scarps, four have been AMS dated at 300, 1110, 2540, and 4250 (radiocarbon) yr B.P. No datable m aterial has yet been found for the other four radar-detected scarps, b ut we interpolate and extrapolate dates of 1800, 3400, 5000, and 5800 yr B.P.