Crustal Structure of Washington State

This image through the crust of southern Wasington was developed by the USGS after seismic experiments conducted in 1995 and 1996. Thousands of sonic sources and receivers were deployed on land and in the ocean to create this image. It tells us the different rock types and shows boundaries (faults) between them. We have identified where great earthquakes are most likely to occur; a 60-km-wide zone located just offshore of Washington State. Additionally, a boundary between accreted rocks (brought to North America by the subducting oceanic plate) and the Cascade Arc is seismically active (as can be seen by the yellow dots that show where earthquakes greater than Magnitude=4.0 have happened).

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We apply 3-D techniques to image specific structures. In this case we are interested in the boundaries around a large volcanic crustal block (dark blue regions outlined by white lines) that occupies much of the crust in coastal Washington. Its boundaries are known sources of earthquakes because the block moves as a single unit.

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Source: Parsons, T., A. M. Trehu, J. H. Luetgert, K. Miller, F. Kilbride, R. E. Wells, M. A. Fisher, E. Flueh, U. S. ten Brink, and N. I. Christensen, 1998, A new view into the Cascadia subduction zone and volcanic arc: Implications for earthquake hazards along the Washington margin, Geology, v. 26, p. 199-202.