A new view into the Cascadia subduction zone and volcanic arc: Implications for earthquake hazards along the Washington margin

Abstract

In light of suggestions that the Cascadia subduction margin may pose a significant seismic hazard for the highly populated Pacific Northwest region of the United States, the U.S. Geological Survey (USGS), the Research Center for Marine Geosciences (GEOMAR), and university collaborators collected and interpreted a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation. We observed (1) an increase in the dip of the Juan de Fuca slab from 2°-7°to 12°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, we identify two zones that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends from ~25 km depth beneath the coastline to perhaps as far west as the deformation front ~120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

“…Thus it is expected that Siletzia rocks are much higher velocity than the accreted sedimentary complex. To interpret the actual boundary between these rocks, we need to know their velocity-depth functions; while some data exist for the Siletz terrane [Parsons et al, 1998; N. I. Christensen, unpublished data, 1998], little is known about the accretionary complex. We can, however, gain insight from a global compilation of velocity measurements that shows a fairly narrow transition from metamorphosed sedimentary rocks into metamorphosed mafic igneous rocks [Christensen and Mooney, 1995].…”

“…Though this boundary is an important locus of strain accommodation, it is presently seismically quiet, and its earthquake potential is unknown. In the southwest Washington arc, however, the northwest trending Mount St. Helens and west Rainier seismic zones are thought to mark the eastern extent of Siletzia [e.g., Stanley et al, 1996;Parsons et al, 1998]. …”

“…A detailed 2-D cross-section model was developed across southern Washington from controlled source data [Parsons et al, 1998] (Plate 2). This profile, while providing a relatively high-resolution image of the velocity structure across the margin, is only a single cross section and could not constrain the dip of the boundary between Siletzia and the accretionary complex because of the wide range of possible velocities in the metamorphosed accreted rocks at shallow depths.…”

Probabilistic Analysis of Tsunami Hazards*

“…In addition, values for shear modulus are often taken from a standard reference earth model (Dziewonski and Anderson, 1981), though recent observations in subduction zones Lay, 1999, 2000) suggest that lower values near the surface may have a significant effect on tsunami generation (Geist and Bilek, 2001). Finally, other parameters that relate to fault zone geometry (dip, fault depth) have a significant effect on tsunami generation (Geist, 1999) -typically, these parameters are constrained by deep-crustal seismic imaging (e.g., Parsons et al, 1998) and precise hypocenter locations (e.g., DeShon et al, 2003).…”

Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop

“…pre-Tertiary North America [Wells et al, 1998]; here forearcseismicity generally outlines blocks of Siletz terrane [Trihu et al, 1994;Parsons et al, 1998]. In northern California, seismicity in both the upper and lower plates seems to be controlled in large part by the northward motion of the Pacific plate [Wang, 1996].…”