Geomorphology, Convergent Margins and Earthquakes

Underthrusting along convergent plate margins, is commonly accompanied by the occurrence of shallow focus, large magnitude thrust earthquakes ( greater than magnitude 7. 5). In the subduction zones that experience such earthquakes, the coastline adjacent to the trench is abrubtly deformed by coseismic uplift and/or subsidence of up to several meters per event. This deformation (particularly uplift) is recorded in the geomorphology as raised marine terraces. By comparing the geomorphic characteristics of coastlines along subduction zones where large magnitude shallow focus earthquakes are known to occur, the potential for such earthquakes to occur along subduction zones with no historic seismic activity (i. e. the Cascadia subduction zone off Washington-Oregon) can be evaluated. The results of such a comparison between the coastline of Washington-Oregon and the coastlines of other subduction zones indicate that the geomorphic characteristics of the Washington-Oregon coast are significantly different.