Seismicity of Washington and Oregon

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Abstract
This chapter examines seismicity in Oregon and Washington, and combines historical accounts of damaging earthquakes with the detail of recent seismicity from modern instrumental locations. A revised seismic catalog has been compiled for Washington and Oregon from 1850 through 1987. This catalog provides a snapshot of the seismotectonic setting of the Pacific Northwest at a single point in geologic time. From the catalog we identify three primary elements of the seismicity distribution: (1) a dipping Benioff zone beneath western Washington and northwestern Oregon, (2) crustal earthquakes in the forearc that shallow eastward toward the volcanic arc, and (3) scattered earthquakes in the back-arc region east of the Cascade volcanic arc. The tectonics, geology, and seismicity of the Pacific Northwest are briefly reviewed in this introduction. Later, we describe the catalog, and include a review of data sources and data-selection criteria, a discussion of the largest earthquakes known in the Pacific Northwest, and a study of temporal variations by decades for earthquakes larger than magnitude 4 since 1960. Microearthquake data available from the University of Washington catalog reveal variations of seismicity in the crust of North America and in the Juan de Fuca plate. These more detailed seismity data allow us to describe variations that exist in the subduction zone framework, and to suggest that continental, as well as subduction tectonics have a profound influence on Pacific Northwest seismicity. In a section on earthquake hazards, we comment on two significant unresolved seismological issues: the probability of an earthquake of magnitude 8