This report covers our first year of progress, from November 1, 1998 through October 31, 1999, under Grant 99HQGR0036. This project is closely linked, with similar project title and goals, to USGS Grant 1434-HQ-98-GR-00017, which for administrative reasons is a separate grant. Since the results and goals of the two projects are similar, but the funding is being administered under different grants, our reports are similar except for grant identification. The first year of this project includes the "dry" SHIPS field operation in September 1999, and interpretation of "wet" SHIPS data.

Our primary efforts during this year were focused on interpreting the data recorded during the "wet" SHIPS ships phase of the experiment in March 1998. Our objective is to carry out two distinct but closely related studies. The first is to use the SHIPS data combined with earthquake data for high resolution first-arrival-time tomography. The second part is to use SHIPS recorded data for reflection interpretation.

To avoid excessive duplication of effort, it was agreed that one group of participants in the experiment would pick and assemble the preliminary first arrival time data for the entire data set. This task was undertaken by T. Brocher's group at the USGS in Menlo Park. They have picked approximately one million first arrival times from the SHIPS data and in July 1999 these data were distributed for interpretation among the participants of the experiment.

We have performed two distinct but overlapping tomographic inversion computations - one using SHIPS data alone in a smaller region of the earth's crust to depth of 30 km, and one using both SHIPS data and earthquakes to a depth of 60 km. The smaller region in imaged at higher resolution and reveals a number of interesting characteristics of the Puget basin structure at depths shallower than 20 km. Our preliminary interpretation of this high resolution image suggests that the youngest part of the Seattle basin, estimated to be of age less than 16MY, is underlain by a transitional layer of older sedimentary rocks that extends to a depth of perhaps 15 km. Therefore there is no sharp reflecting boundary at the base of what is regarded as the normal Seattle basin, explaining the lack of reflections observed in the SHIPS dry land station data. The Seattle, Everett and Tacoma basins are part of a larger arcuate basin structure that curves around the NE side of the Olympic Peninsula. This "super-basin" seems likely to have a genetic relationship to the Olympic structure. The Seattle fault, which bounds the south side of the Seattle basin, produces a high resolution image that is consistent with, but does not necessarily require thrust faulting with northward vergence.

Our second, intermediate resolution but larger volume, tomographic inversion reveals many new aspects of the regional structure of the Cascadia forearc. This inversion combines earthquake and SHIPS data to obtain structure information to nearly 60 km depth. The subducting Juan de Fuca slab is clearly imaged over a portion of the model at a depth greater than 40 km. The Olympic core complex rocks are of dramatically lower velocity than surrounding Crescent terrane rocks and are clearly imaged on their eastern extent. A dramatically new result, in preliminary interpretation, is that Olympic core rocks are being subducted beneath the Crescent complex in the central Puget basin. This may have very significant implications for understanding the tectonics and earthquake hazards of the basin. Our model is consistent with uplift and erosional removal of Crescent terrane rocks from the Olympic core complex. This process has reduced the cross section of Crescent basement in the central Puget basin and increased stress, providing a feasible physical explanation for the increase in earthquake occurrence rate in the central Puget region.

Using the Omega 3-D package we examined a few hundred shot lines (averaging 1000 shots per line) for near vertical reflections off deep structures in the Puget Lowlands. There was disappointingly little to see in these data. However, at offsets larger than about 80 km, wide-angle reflections off the subducting slab are very clear on several of the lines. These reflected arrivals are may be larger in amplitude than the direct arrival. In order to determine a precise location of this reflecting surface, we trace rays through the 3-D tomography model from each receiver and from each shot to all points in the 3-D volume. We invert observed times for the smoothest surface that simultaneously provides the best fit to the observed travel times and that has the smallest deviation from satisfying Snell's law at the reflection points.

The resulting surface is parallel to and generally slightly below the zone of Benioff zone earthquakes in the region. Our preliminary interpretation is that this surface represents the Moho of the subducting oceanic plate. However, prior to a final interpretation, we plan to further refine the velocity model used for travel time calculation, and to relocate deep earthquakes using the 3-D model to obtain improved statistical control of the relationship between earthquakes and the subducting slab.

Data Availability: Waveform data are already being shared fully among the P.I.s who collaborated in this experiment. Waveform data have been cut into 90 second windows after each shot, and written to SEGY format. We have also cut out 90 s time windows after local and regional earthquakes. Copies of these data from the 60 stations we operated have been sent to the IRIS DMC and will be freely copied and distributed by the IRIS DMC (//http:dmc.iris.washington.edu) 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.

There are no current standard formats for distribution of the complex data required to represent 3-D tomographic images at the regional scale. Therefore the comparison of results from different research groups is very difficult. We feel that the SHIPS group should take the lead in establishing appropriate standards for exchange of 3-D regional velocity models. Subject to additional refinement and adoption of appropriate data exchange standards, we are preparing to make our 3-D tomographic models available in a standardized format in the near future. This will be of great value for ground motion modeling, earthquake location, and for tectonic and structure interpretation. In addition, we are in the process of establishing a web site that will allow easy access to images (such as cross sections and maps) from tomographic images.

We are interpreting data collected during the 1998 Seismic Hazards in Puget Sound (SHIPS) experiment using two complimentary techniques. One technique obtains 3-D images of the wave speeds in the crust and uppermost mantle from the Cascade to Olympic Mountains and from Olympia to Everett. The second utilizes wide angle reflections to image the geometry of the subducting slab. Major goals are to detect previously unknown faults that may be active, improve resolution of known faults, and map the 3-D geometry and wave speeds of the Seattle Basin. The Seattle Basin is filled with as much as 10 km of layered sedimentary rock sitting on top of basaltic basement rocks. Knowledge of the basin structure will make it possible to predict how waves from large earthquakes will be amplified and resonate in the basin.

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, 1999, 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., 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.

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.

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.

Fisher, M.A., T.M. Brocher, R.D. Hyndman, A.M. Trehu, C.S. Weaver, K.C. Creager, R.S. Crosson, T. Parsons, A.K. Cooper, D. Mosher, G. Spence, B.C. Zelt, P.T. Hammer, U. ten Brink, T.L. Pratt, K.C. Miller, J.R. Childs, G.R. Cochrane, S. Chopra, and R. Walia, 1999, Seismic survey probes urban earthquake hazards in Pacific Northwest, (EOS), 80 (2), 13-17.

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.

Molzer, P.C., U.S. ten Brink, M.A. Fisher, T.M. Brocher, K.C. Creager, R.S. Crosson, 1999, Seismic Structure of Seattle Fault, Seattle Basin, and Kingston Arch, Washington, EOS, abstract, Fall AGU.

Pratt, T., T. Brocher, T. Parsons, M. Fisher, K. Creager, R. Crosson, C. Weaver, R. Hyndman, A. Trehu, K. Miller, U.S. Ten Brink, 1999, Ground Shaking in the Puget Lowland, western Washington state, from earthquakes recorded on the SHIPS land geophone array, Seismological Research Letters, V. 70, N. 2., p. 219.

Pratt, T.L., T.M. Brocher, T. Parsons, M.A. Fisher, K.C. Creager, R.S. Crosson, C.Weaver, R.L. Hyndman, A.M. Trehu, K. Miller, 1999, Site Response in the Puget Lowland, Western Washington State, Determined From the SHIPS 3-Dimensional Geophone Array, EOS, abstract, Fall AGU.

Preston, L.A., K.C. Creager, R.S. Crosson, T.L. Pratt, C. Weaver, M.A. Fisher, T. Parsons, and T.M. Brocher, 1999, 3D reflection imaging of the Puget lowlands, Cascadia, Seismological Research Letters, V. 70, N. 2., p. 255.

Preston, L.A., K.C. Creager, R.S., T. Van Wagoner, Y. Xu, N.P. Symons, T.L .Pratt, C. Weaver, M.A. Fisher, T.M. Brocher, T. Parsons, U.S. ten Brink, K. Miller, A. Trehu, R. Hyndman, 1999, 3D Reflection Imaging of the Subducting Juan de Fuca Slab Under the Olympic Peninsula, EOS, abstract, Fall AGU.

Symons, N.P., S.C. Morean, 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.

Ten Brink, U.S., P.C. Molzer, M.A. Fisher, T.M. Brocher, T. Parsons, R.S. Crosson and K.C. Creager, 1999, Seismological Re search Letters, V. 70, N. 2., p. 254.

Trehu, A.M., T.M.Brocher, M.A. Fisher, T. Parsons, K.C. Creager, R.S. Crosson, T.L. Pratt, C.S. Weaver, R. Hyndman, G. Spence, U.S. ten Brink, K. Miller, 1999, Structure and Reflectivity of the Subducting Juan de Fuca Plate Beneath the Straits of Juan de Fuca and Northern Olympic Peninsula, EOS, abstract, Fall AGU.