In order to fully understand earthquake hazards in the Puget Sound basin of western Washington, it is necessary to have a clear picture of the structure of the earth's crust in this region. In this project, we use seismic waves from earthquakes and artificial explosions to construct a seismic structure model for the crust beneath the Puget basin. Our initial model shows the relationships between several geologically important features of the crust, and provides the basis for further geological and geophysical investigations.

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 primary initial data source is a catalog of approximately 3000 well located digitally recorded earthquakes collected by the Pacific Northwest Seismography Network (PNSN) over the past 17 years. This data set is augmented by including approximately 20 explosions with known locations and origin times also recorded by the PNSN. In this investigation, we are able to include external constraints on the velocity where they are available.

• Figure 1 Map of our study area, including major geographic features and locations of previous seismic studies.

Although the final analysis portion of this project is not yet completed, we have published a preliminary seismic velocity model for the region (Fig. 2, from Symons and Crosson, 1997). Initial resolution analysis based on checkerboard tests indicate that we are able to accurately recover up to +/- 10% velocity variations throughout the target region of the central Puget basin. A final goal of this research is to define how accurately we can image very rapid velocity variations such as can be expected across major faults.

• Figure 2 Upper level depth slice and cross sections through velocity model.

Preliminary analysis of the model in figure 2 shows the following features: (1) the map view shows a clear low velocity region in the central Puget lowland, with local minima near Seattle and just south of Tacoma. These two local minima correspond to known sedimentary basins which extend to depths of about 3 to 6 km. (2) The westward transition from low to high velocity west of Seattle, at shallow depths, appears to correspond to the geologic transition from the sedimentary Seattle basin to the basalt of the Eocene Crescent formation which is exposed on the east side of the Olympic Peninsula. (3) Further west, cross-section A-A' suggests that the core rocks of the Olympic Peninsula, which are Eocene and younger may be thrust beneath the older Crescent formation rocks at depth. (4) Subduction geometry of the Juan de Fuca plate appears to also be reflected in the structural model.

Between the subducted slab and the overlying continental crust (and continental earthquake zone), there is a region of intermediate seismic velocity, from about 7.5 to 8.0 km/s, which we interpret to lie in a mantle wedge region. The transition from lower crust to upper mantle may be gradual. However both cross-sections A-A' and B-B' suggest that the lower crust-upper mantle transition is anomalously shallow beneath the central Puget lowland. The velocity reversal near the base of the mantle wedge region may reflect the subducted oceanic crust, or the modification of rock properties by fluids released by dehydration in the subducted slab.

Further analysis of the velocity model resulting from this study requires reexamination of all data to produce the highest quality data set for inversion, and tests of the reliability, accuracy, and resolving power of the inverted model. We are currently taking several different approaches to the accuracy problem including tests of the effect of different starting models on our results, and bootstrap and jackknife type statistical tests of the uncertainty of final model estimates.

Lees, J. M., and R. S. Crosson, Tomographic imaging of local earthquake delay times for three-dimensional velocity variation in western Washington, J. Geophys. Res., 95, 4763--4776, 1990.

Luetgert, J., T. Parsons, K. Miller, G. R. Keller, A. Trehu, S. Fleming, R. Clowes, and I. Asudeh, Crustal architecture of the Pacific Northwest: The 1995 seismic transect across southern Washington (abstract), EOS, 76, F399, 1995.

Moran, S.C., Three-dimensional P-wave velocity structure in southwestern Washington from local earthquake tomography, Ph.D. thesis, Univ. of Wash., Seattle, in press, 1997.

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.

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