Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high‐resolution seismic tomography

Abstract

The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new high‐resolution 3‐D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc‐parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high‐angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin.

Constraining fault interpretation through tomographic velocity gradients: application to northern Cascadia

“…There is no direct evidence for the presence of this fault from the tomographic velocity models constructed by Symons and Crosson (1997), Brocher et al, (2001) Van Wagoner et al, (2002), and Ramachandran et al, (2006). This fault could not be inferred previously from the tomographic velocity models due to the smooth nature of the velocity variations (see top panel of Figs.…”

“…1) forms the eastern boundary of the Eocene volcanic rocks (Johnson, 1984(Johnson, , 1985Johnson et al 1996). CRBF is inferred mainly from tectonic and sedimentologic evidence to lie beneath the eastern Puget Lowland where it strikes approximately N-S (Van Wagoner et al, 2002). Northward motion of the Cascadia forearc region during the early Tertiary may have been accommodated along this right-lateral strike-slip fault, which presumably separates rocks of the Coast Range terrane from the pre-Tertiary basement of the Cascades (Johnson, 1984(Johnson, , 1985Johnson et al, 1996).…”

High-Resolution Seismic Reflection Imaging of Growth Folding and Shallow Faults beneath the Southern Puget Lowland, Washington State

“…The black rectangles show the Tacoma and Olympia study light detection and ranging (LiDAR) maps shown in Figures 2a and 6a, respectively. Heavy dashed lines outline the Tacoma, Seattle, and Everett basins, as determined by the 4:25 km=sec contour on a tomography image at 2.5 km depth (Van Wagoner et al, 2002). SWIF: Southern Whidbey Island fault.…”

Earthquake recordings from the 2002 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS), Washington state

“…We thank Mary Templeton for reviewing this manuscript. shows the speed of sound at 2.5 km depth derived from a regional tomographic study (VanWagoner et al, 2002). The blue dots are the locations of the Seattle SHIPS seismometer sites.…”

“…The Seattle SHIPS experiment was designed to deploy and maintain a 3 -dimensional array of 87 similar seismographs distributed over the Seattle basin (Figures 1 and 2; Johnson et al, 1994;Pratt et al, 1997;Brocher et al, 2001a;VanWagoner et al, 2002). One set of 26 seismographs was deployed along an east-west line across the center of the basin and onto bedrock at both ends, approximately coincident with the 1999 SHIPS refraction profile (Brocher et al, 2000b;Snelson, 2001;Pratt et al, 2003).…”