Regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern Washington

Abstract

The St. Helens seismic zone (SHZ) is a crustal earthquake zone in southwestern Washington that is defined by small‐ to moderate‐magnitude (2.5 to 5.5) earthquakes and focal mechanisms. This zone, seismically active along a distance of 90 km, is interpreted as a fault zone capable of generating a moderate‐ to large‐magnitude shallow earthquake. At the northern end of this seismic zone (about 15 km north of the Cowlitz River) a large area of seismic quiescence (for earthquakes above magnitude 3.0) has predominated since 1977. This quiescence leaves open the question whether the SHZ continues northward into southern Puget Sound. Earthquake focal mechanisms along the SHZ indicating nearly vertical strike‐slip faulting with the preferred fault planes striking north–south. From these focal mechanisms we infer that the direction of maximum compression is northeast, approximately parallel with the direction of plate convergence between the North American and Juan de Fuca plates. We interpret the focal mechanism data as evidence for locked subduction. This tectonic model raises the possibility of a large‐magnitude subduction earthquake. Neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for Washington.