The effects of upper plate deformation on records of prehistoric Geological Society, London, Special Publications

Geophysical data from the offshore Cascadia forearc reveal many Quaternary upper-plate faults and folds. Most active structures are within the accretionary wedge, but significant deformation is also found on the continental shelf. Several faults and synclines project into adjacent coastal bays where deformation of Pleistocene marine terraces is reported. Rapidly buried marsh deposits and drowned forests in these coastal lowlands are interpreted to record coseismic deformation by prehistoric subduction zone earthquakes. The extent and amount of such coastal subsidence has been used to infer characteristic magnitudes and recurrence intervals. However, the record may incorporate both elastic strain release on the subduction zone and localized permanent upper-plate deformation. Movement on upper-plate structures may be triggered by a subduction zone earthquake, as observed in the Nankai and Alaskan forearcs. Alternatively, they may deform independently of subduction zone earthquakes. Regardless of which style of deformation predominates, the record of coseismic subsidence is likely to be affected. Crustal deformation may also contribute to the preservation of subsided marshes. Modelling of subduction zone earthquake characteristics based on coastal marsh stratigraphy is likely to be inaccurate in terms of: (a) total apparent rupture length and earthquake magnitude; (b) amount of subsidence and hence the position of the locked zone; (c) recurrence interval. Most of these shelf and coastal structures respond to N-S compression, in contrast to convergence-related northeasterly compression in the accretionary prism, but in agreement with the regional stress field. Despite low historical coastal and continental shelf seismicity, upper-plate faults may also pose an independent seismic hazard. Geological and geophysical investigations of the Cascadia subduction zone during the last decade have increased public awareness of regional earthquake hazards from a subduction zone previously thought to be aseismic (Ando & Balazs 1979). Evidence for repeated abrupt subsidence in the last few thousand years is found in coastal bays along the active margin, in the form of buried marsh deposits and drowned forests (e.g. Atwater 1987, 1992; Darienzo & Peterson 1990; Atwater et al. 1995; Nelson et al. 1995; Yamaguchi et al. 1997). Previous workers have believed these deposits to be a result of coseismic subsidence and have attributed them to subduction zone earthquakes (e.g. Atwater 1987, 1992; Darienzo & Peterson 1990; Atwater et al. 1995; Nelson et al. 1995; Yamaguchi et al. 1997). However, the similarity of these deposits to the marsh stratigraphy of tectonically inactive coasts has led to the suggestion that many abrupt burials may be non-tectonic in origin and driven by local changes in intertidal environment (e.g. Long & Shennan 1994). A non-tectonic origin cannot be eliminated except in certain cases where the event is found to be regionally widespread, is associated with other coseismic phenomenon such as tsunami deposits or liquefaction, or where the magnitude of subsidence is too large to be explained by nontectonic mechanisms. The coseismic v. nonseismic origin of the subsidence events will not be addressed by this paper, but remains a topic