Rupture models of the great 1700 Cascadia earthquake based on microfossil paleoseismic observations




Wang, Pei-Ling

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Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great AD 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here, we infer heterogeneous slip for the Cascadia margin in AD 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3-D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extents, the more precise subsidence estimates are best explained by a model with along-strike slip heterogeneity, with multiple patches of high moment release separated by areas of low moment release. For example, in AD 1700 there was very little slip near Alsea Bay, Oregon (~ 44.5°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for precise, high-quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.



great subduction earthquakes, paleoseismology, microfossils, coseismic deformation, dislocation model, Cascadia