HIGH RESOLUTION 3-D STRUCTURE
OF THE PUGET LOWLAND REGION USING EARTHQUAKES
AND EXPLOSIONS

USGS NEHRP GRANT 1434-HQ-97-GR-03084

Robert S. Crosson
University of Washington
GEOPHYSICS Box 351310
Seattle, WA 98195-1650
Phone: (206)543-6505, FAX: (206)543-0489
E-mail: crosson@u.washington.edu
URL: http://www.geophys.washington.edu/SEIS/PNSN
Program Element: I,II,III
Keywords: Seismotectonics, Wave Propagation

Investigations Undertaken

This report covers the year, from November 1, 1998 through October 31, 1999 of research under Grant 1434-HQ-97-GR-03084, "High Resolution 3-D Structure of the Puget Lowland Region using Earthquake and Explosions". This period corresponds approximately to the final extension year of this project. The project was originally proposed to develop state-of-the-art seismic tomography analysis methods, and to apply these methods to the central Puget basin region to obtain a high resolution image of the crust and upper mantle using available earthquake data and explosion data from several active source experiments that were conducted in western Washington. In the extension phase of this project, the SHIPS (Seismic Hazards in Puget Sound) project was being conducted, so we refocused our efforts on being able to analyze SHIPS data. The result of this effort has been a substantial improvement in our ability to image the crust and upper mantle by combining both SHIPS data and earthquake data recorded on the Pacific Northwest Seismograph Network (PNSN).

Results

Our primary efforts were focused on refining the methodology for tomographic inversion of large data sets of earthquake and active source data for high resolution structure. When this project was started, the SHIPS experiment was unknown to us and only in the very preliminary planning stages by investigators at the USGS. During this year, our primary focus was on ensuring that the technology that we had developed previously, primarily for earthquake data with sparse explosion data, was suitable for the very different data set forthcoming from SHIPS. This required restructuring and rewriting much of the software developed earlier, and developing the large amount of data manipulation software required to handle the extremely large amounts of data from SHIPS.

Our efforts have been successful, and we have now developed, tested and applied the non-linear tomographic inversion method described by Symons and Crosson (1997) to the SHIPS data set. Both earthquake and explosion data are simultaneously inverted for (P wave) structure volumes of data in the 100 thousand observations range using models that have approximately 1,000,000 velocity nodes. The addition of SHIPS data has permitted a very significant increase in the resolving power of our method, primarily by constraining the 3-D structure of the upper 20 km or so of the crust. The reports under projects 1434-HQ-98-GR-00017 and 99HQGR0036 describe the results of this analysis in more detail.

Our tomography method uses finite-difference travel time calculation, a dual node-based model description system, full locally linearized but non-linear coupling of the hypocenter and velocity model, ray-based computation of model sensitivities, generalized smoothing constraints, and conjugate-gradient least squares for solution of the linear system. We acquired a PC computer running Linux dedicated to the SHIPS project, and are able to store critical SHIPS data including seismograms on-line for rapid access. We have run our inversion computation on the PC system very successfully without nearing the limits of machine capacity so far.

Data Availability:
Data developed exclusively for this project consist of a high quality, repicked set of about 1000 earthquakes chosen for the structure inversion. These data can be shared for appropriate research purposes by contacting the Principal Investigator (R. S. Crosson, (206) 543-6505, crosson@u.washington.edu). SHIPS data are currently being shared among SHIPS investigators, and are submitted to the IRIS DMC for public distribution after the agreed upon 2 year period following the experiment. Contact Rick Benson, (206) 547-0393, rick@dmc.iris.washington.edu at the IRIS DMC in Seattle to obtain the waveform data.

Non-Technical Summary

We have developed state-of-the-art methods to image the crust of the earth using seismic signals from explosions detonated at the surface of the earth and earthquakes originating deep in the crust of the earth. These methods are analogous to medical tomography in using large numbers of sources and receivers of seismic waves at various places within the earth, just as medical tomography uses large numbers of x-ray sources and receivers placed around the body. The methodology is therefore called seismic tomography. The application of these techniques to the crust of the earth in the Puget Sound region of western Washington is producing new images of the crust of the earth that have major implications for understanding and predicting seismic hazards near the major population  centers of the Pacific Northwest.

Reports published:

Brocher, T.M., T. Parsons, M.A. Fisher, U.S. Ten Brink, P.C. Molzer, K.C. Creager, R.S. Crosson, A.M. Trehu, K.C. Miller, T.L. Pratt, and C.S. Weaver, 1999, Structure of the Seattle Basin: A tomographic model using data from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) Experiment, Washington State, Seismological Research Letters, V. 70, N. 2., p. 219.

Brocher, T M., T Parsons, M A Fisher, R J Blakely, U S ten Brink, K C Creager, R S Crosson, K C Miller, A M Trehu, G Spence, B C Zelt, R D Hyndman, D C Mosher Structure of the Puget Lowland: A Tomographic Model Using Data From the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) Experiment, Washington State, EOS, abstract, Fall AGU.

Crosson, R.S. and Symons, N.P., 1999, A model for localization of seismicity in the central Puget lowland, Washington, Seismological Research Letters, V. 70, N. 2., p. 255.

Crosson, R.S., 1999, Review of Instrumentally observed seismicity with tectonic implications for the central Cascadia subduction zone, Seismological Research Letters, V. 70, N. 2., p. 209.

Crosson, R.S., K.C. Creager, N.P. Symons, T. Van Wagoner, Y. Xu, L.A. Preston, T.M. Brocher, T. Parsons, M.A. Fisher, T.L. Pratt, C. Weaver, U.S. ten Brink, K. Miller, A. Trehu, R. Hyndman, G. Spence, 1999, High-resolution 3-D regional P wave velocity tomography of the Puget basin region from SHIPS first-arrival data, EOS, abstract, Fall AGU.

Fisher, M.A., T. Parsons, T.M. Brocher, R.D. Hyndman, A.M. Trehu, K.C. Creager, R.S. Crosson, N.P. Symons, T.L. Pratt, C.S. Weaver, and U.S. Ten Brink, 1999, Urban earthquake hazards of the Puget Sound Region, Initial findings from the 1998 SHIPS experiment, Seismological Research Letters, V. 70, N. 2., p. 210.

Fisher,M.A.,  T M Brocher, T Parsons, R S Crosson, K C Creager, A M Trehu, D L Mosher, R L Hyndman, R J Blakely, U S ten Brink, G Spence, T L Pratt, D Graindorge, B C Zelt, 1999, Results From the 1998 SHIPS Experiment, Regarding Urban Earthquake Hazards in Cascadia, EOS, abstract, Fall AGU.

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, 2593-2596, 1997.

Symons, N.P, S.C. Moran, R.S. Crosson, K.C. Creager, and M.A. Fisher, 1999, Seismic tomography in the Pacific Northwest and its interpretation; relationship between crustal structure and the distribution of crustal seismicity, Seismological Research Letters, V. 70, N. 2., p. 210.

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