To understand the nature of earthquake hazards in the greater Puget Sound region, it is necessary to understand the geological structure of the earth's crust. The goal of this project is to determine the detailed three-dimensional seismic velocity structure of the Puget Sound region (Fig. 1) using first-arrival travel-time tomography. The data used for this study are earthquakes and explosions recorded by the Pacific Northwest Seismograph Network (PNSN).

Our preliminary model indicates that the Olympic core rocks have low P-wave velocities, consistent with laboratory measurements of marine sedimentary rocks grading into metagraywacke at depths of 20-25 km. The low velocity of the Olympic core rocks clearly distinguishes them from the higher velocity basement rocks beneath the Puget Sound Basin. We interpret the basement beneath Puget Sound to be the Crescent terrane rocks that are exposed by uplift on the eastern margin of the Olympic Peninsula. The depth configuration of the Crescent formation suggests that it is warped upward along its western boundary by being underthrust by Olympic core rocks.

In the upper 10 km of our model, there are prominent regions of low velocity beneath the cities of Seattle and Tacoma. These correspond to sedimentary basins that may have an important impact on seismic hazard estimates for the urban areas of Seattle and Tacoma. In addition to the large low-velocity regions near Seattle and Tacoma, there is also some evidence for low-velocity basins near the cities of Everett and Chehalis, although these basins are not as well resolved as the Seattle and Tacoma basins.

In order to gain better understanding of earthquake hazards in the Puget Sound region of western Washington, it is necessary to have a clear picture of the structure of the earth's crust and upper mantle in this region. In this project, we use seismic waves from earthquakes and artificial explosions to construct a model of seismic P-wave velocity of the crust beneath the Puget Sound region. In the first phase of this research, we have developed a high-resolution structure modeling method and assembled a high-quality data set of seismic P-wave arrivals from network data. Preliminary results suggest that this analysis will result in improved understanding of the potentially seismogenic basement rocks beneath the Puget Sound region.

Articles

Symons, N. P., and R. S. Crosson, Seismic Velocity Structure of the Puget Sound Region from 3-D Non-linear Tomography, Geophys. Res. Lett., 24(21), 2593--2596, 1997.

Abstracts

Symons, N. P., and R. S. Crosson, P-wave Tomography in the Puget Sound Region, Washington: Preliminary Results (abstract), IASPEI 1997 Abstracts, The 29th General Assembly of the International Association of Seismology and Physics of the Earth's Interior, 49, 1997.

Symons, N. P., and R. S. Crosson, P-wave Tomography in the Puget Sound Region, Washington: Preliminary Results (abstract), EOS Trans. AGU, 77(4), F466, 1996. .ti -4 Symons, N. P., and R. S. Crosson, High Resolution arrival-time tomography in the Puget Sound Region, Washington, EOS Trans. AGU, 78, F469, 1997.