Cutaway view into the three-dimensional seismic velocity structure of the Washington margin calculated with traveltime observations from earthquakes, explosions, and airguns. Velocity was found by minimizing residual times between calculated and observed. Lower velocities (warm colors) characterize the accretionary prism rocks offshore, whereas higher velocities (cool colors) are seen in the Siletz River volcanic rocks, partly outlined by the white dashed lines. Surface geology and coastlines are draped onto the topography.
This paper discusses three crustal-structure models developed because of the significant seismic hazard posed by Cascadia subduction margin to the highly populated Pacific Northwest region of the United States. The first is a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc recorded to study the major structures that contribute to seismogenic deformation. Observed were (1) an increase in the dip of the Juan de Fuca slab from 2º-7º to 12º where it encounters a 20-km-thick block of the Siletz terrane or other accreted oceanic crust, (2) a distinct transition from Siletz crust into Cascade arc crust that coincides with the Mount St. Helens seismic zone, supporting the idea that the mafic Siletz block focuses seismic deformation at its edges, and (3) a crustal root (35-45 km deep) beneath the Cascade Range, with thinner crust (30-35 km) east of the volcanic arc beneath the Columbia Plateau flood basalt province. From the measured crustal structure and subduction geometry, two zones were identified that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends westward from ~25 km depth beneath the coastline perhaps as far as the deformation front ~120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.
In addition to the two-dimensional cross-section model, two three-dimensional crustal models are discussed.
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This is one of a series of chapters in Earthquake Hazards of the Pacific Northwest Coastal and Marine Regions, USGS Professional Paper 1661, edited by Robert Kayen. The others consist of:
Crustal Deformation at the Leading Edge of the Oregon Coast Range Block, Offshore Washington (Columbia River to Hoh River), USGS Professional Paper 1661-A, by Patricia A. McCrory, David S. Foster, William W. Danforth, and Michael R. Hamer
Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper 1661-F, by Chris Goldfinger, C. Hans Nelson, Ann E. Morey, Joel E. Johnson, Jason R. Patton, Eugene Karabanov, Julia Gutiérrez-Pastor, Andrew T. Eriksson, Eulàlia Gràcia, Gita Dunhill, Randolph J. Enkin, Audrey Dallimore, and Tracy Vallier
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