TIDAL MARSH STRATIGRAPHY, SEA-LEVEL CHANGE AND LARGE EARTHQUAKES .1. A 5000 YEAR RECORD IN WASHINGTON, USA

Many of the estuaries of the Pacific Northwest of the U.S.A. and Canad a contain stratigraphic sequences typified by alternating peat-mud cou plers. Recent studies in this region interpret such couplets as the pr oduct of repeated large (magnitude 8 or 9) earthquakes on the Cascadia subduction zone. The resultant pattern of land-level movements is des cribed by a model, the 'earthquake deformation cycle', of coseismic la nd subsidence followed by land uplift during interseismic strain accum ulation. However, peat-mud couplets similar to those recorded in the P acific Northwest are found on other less tectonically active temperate -latitude coasts, such as northwest Europe and the Atlantic coast of t he U.S.A., where they have been interpreted as the product of non-seis mic coastal processes. In this paper we apply the methods and scientif ic framework common to sea-level investigations in northwest Europe to a sequence of peat-mud couplets recorded in the lower Johns River, an estuary in southern Washington, to provide a test of the 'earthquake deformation cycle'. Stratigraphic investigations of the intertidal sed iments along the lower Johns River, using lithological, pollen, diatom and foraminiferal data, show evidence for eight coastal submergence e vents during the last 5000 years. To evaluate the 'earthquake deformat ion cycle' we assess the lateral extent of peat-mud couplets, the sync hroneity of submergence, the presence of tsunami deposits accompanying submergence, and the suddenness and amount of submergence. Each subme rgence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the 'earthquake deformati on cycle', demonstrating the continued intermittent seismic activity o f the Cascadia subduction zone throughout the mid and late-Holocene. Q uantitative analyses of contemporary and fossil biostratigraphic data, using TWINSPAN and Detrended Correspondence Analysis, enable us to es timate the magnitude of submergence accompanying each peat-mud couplet . One event was accompanied by submergence of about 1.5 m or more, fou r events by intermediate submergence of about 1+/-0.5 m, and a further three events by submergence of <0.5 m. There is evidence for non seis mic relative sea-level rise prior to two of the eight submergence even ts, but for at least the last 3500 years the magnitude of relative sea -level rise has been less than the combined influence of sediment accr etion following submergence and interseismic land uplift. Copyright (C ) 1996 Elsevier Science Ltd