High-resolution 3-D regional P wave velocity tomography of the Puget

basin region from SHIPS first-arrival data

R. S. Crosson,

K. C. Creager (Geophysics Program, Box 351650, University of Washington,

Seattle, WA 98195)

N. P. Symons (Sandia National Laboratories, Box 58000, Albuquerque, NM

87111\)

T. Van Wagoner,

Y. Xu,

L. A. Preston (Geophysics Program, Box 351650, University of Washington,

Seattle, WA 98195)

T. M. Brocher,

T. Parsons,

M. A. Fisher (U.S. Geological Survey, 345 Middlefield Rd., Menlo Park,

CA 94025)

T. L. Pratt (U.S. Geological Survey, School of Oceanography, Univ. of

Washington, Seattle, WA 98195)

C. Weaver (U.S. Geological Survey, Box 351650, University of Washington,

Seattle, WA, 98195),

U. S. ten Brink (U.S. Geological Survey, Woods Hole, MA 02543)

K. Miller (Dept. of Geological Sciences, Univ. of Texas, El Paso, TX

79968\)

A. Trehu (College of Oceanic and Atmospheric Science, Oregon State

Univ., Corvallis, OR 97331)

R. Hyndman (Geological Survey of Canada, Pacific Geoscience Centre,

Sidney, BC, Canada V8L 4B2)

G. Spence (Earth and Ocean Sci., Univ. of Victoria, Victoria, BC Canada

V8W 2Y2)

SHIPS first-arrival travel time observations provide an unprecedented

opportunity to develop high-resolution tomographic images of the crust

in the Puget basin region. Using SHIPS data alone, we have produced

preliminary P wave velocity images using 3-D nonlinear tomography that

fit the data remarkably well and show excellent detail in the upper 15

km of the crust. Travel time variation at similar range of the order of

3 s or more is typical in the distance range 15-45 km. Most of this

scatter is explained by three-dimensional structure variation. Using

approximately 52,000 observations, overall, travel time data are fit

better than .12s RMS, with only .05 percent of residuals above 1 second.

Our images confirm a depth of the Seattle basin of 8-9 km, and suggest

that the Seattle fault dips southward at a average dip of approximately

45 degrees. Although the Seattle basin is generally well imaged, there

is some loss of structure resolution on the east margin of the basin.

The Hood Canal fault appears to represent a sharp NNE trending velocity

contrast juxtaposing Crescent/Siletz basement rocks on the west against

lower velocity basin rocks to the east. This fault is clearly imaged in

the upper 5 km and appears to bend northward along the west margin of

Dabob Bay after leaving Hood Canal proper. A strong linear velocity

gradient trending eastward from the north end of Dabob Bay may also

represent a shallow fault boundary. A \"plug\" of high velocity basement

rock about 15 km in diameter southeast of Bremerton appears to merge

with Crescent/Siletz basement at about 5 km depth. The Gold Mt. uplift

lies on the north side of this feature. An objective of our work is to

combine earthquake and SHIPS airgun/explosion data in a joint

tomographic inversion to produce a significantly improved regional 3-D

crustal velocity model for structure and earthquake location studies.

After the velocity model is completed, we will relocate significant

earthquakes in the Puget lowland region using this new model.