UPDATE ON PUGET SOUND CRUSTAL STRUCTURE

Rocks deep beneath Puget Sound were investigated in a recently published study using seismic tomography to probe rock velocities as much as 30 km below the earth's surface. The raw data for the study came from "travel-times" - the time it takes a seismic wave to travel from its source to the seismometer that records it. Tens of thousands of travel-times from earthquakes or active experiments (airguns or explosions) contribute to the construction of a mathematical model of Puget Sound crustal velocity variations.

The tomographic model treats the crust beneath Puget Sound as layers of discrete blocks. Each travel-time represents a wave that followed a known path through the subsurface, speeding up in fast rocks and lagging as in slow formations. Tomography finds the best velocity for each block by comparing travel-times computed from the model to real data, adjusting the block velocities, and repeating the procedure until the observed and calculated travel times agree. Because earthquake locations and origin times are also determined from the travel-times using the velocity model, the earthquakes must be relocated and the wave travel-paths and travel-times corrected in each iteration.

The tomographic model shows that Puget Sound is warped, faulted, and underlain by deep sediment-filled basins. The rocks beneath the basins have velocities comparable to basalts seen in the Olympics. The paper "Crustal Structure and relocated earthquakes in the Puget Lowland, Washington, from high-resolution seismic tomography" appeared in the Journal of Geophysical Research (Vol. 107, No. B12, 2381, doi:10.1029/2001JB000710, 2002), and was authored by T.M. Van Wagoner, R.S. Crosson, K.C. Creager, G. Medema, L. Preston, N. P. Symons, and T.M. Brocher.

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Summary by Ruth Ludwin