SHIPS - Eos Article

Urban Seismic Experiments Investigate Seattle Fault and Basin

Eos, Transactions, American Geophysical Union, Nov. 14, 2000

Introduction

In the past decade seismologists have recognized the seismic hazards posed by crustal faults and sedimentary basins to Seattle, Washington . (Figure 1, enlargement with caption)

faults and shotpoint lines

In 1998, the U.S. Geological Survey and its collaborators initiated a series of urban seismic studies of the upper crust to better map seismogenic structures and sedimentary basins in the Puget Lowland. We call these studies the Seismic Hazard Investigations of Puget Sound (SHIPS).

In March 1998 we conducted our first SHIPS study, a regional investigation of the upper crustal structure of the Puget Lowland, nicknamed Wet SHIPS, using marine airgun sources and land recorders [Fisher et al., 1999]. In September 1999, we obtained a seismic refraction line to study the upper crustal structure in the Seattle area, in a land-based study nicknamed Dry SHIPS [Brocher et al., 2000] (Figure 1a).

In March 2000, we recorded the demolition of the Seattle Kingdome sports stadium using a dense array of seismic recorders for a detailed site response study, nicknamed Kingdome SHIPS (Figure 1b). Collectively, the three SHIPS experiments have provided new images of the subsurface geometry of the crustal faults and sedimentary basins in Puget Lowland, as well as evidence that these sedimentary basins significantly focus seismic waves from earthquakes at periods between 1 and 5 seconds.

Siting explosions and thousands of receivers in a densely-populated region, however, provides challenges and opportunities for seismologists. In the following, we describe the challenges and successes from our earthquake hazard studies nicknamed Dry and Kingdome SHIPS.

Challenges of urban seismology

Urban seismology poses different challenges than seismic work in remote areas, but urban seismology also offers great opportunities. Seismologists and residents are forced to interact in these studies - many landowners must be contacted in order for seismologists to gain access to property. A team of nearly 100 individuals is needed to perform the work at this level of detail, thus, urban work naturally attracts the attention of the citizenry and the media, leading to additional opportunities for public education concerning earthquake hazards.

That there are more constituencies to satisfy when working in densely-populated areas is also inescapable, including police and fire departments, Park and Recreation Departments, 911 departments, and in this case, demolition crews. The efforts of all these participants and stakeholders must be coordinated with the seismological field activities.

The stakeholders and residents of the Puget Sound region have been extremely interested in and supportive of our efforts despite our late-night disturbances during Dry SHIPS, and our interaction with the public has been an extremely rewarding part of SHIPS. In the following, we provide additional specific examples of these challenges and our solutions to these problems.

Dry SHIPS Experiment

Kingdome SHIPS Experiment

Seismic Wave Amplification in Seattle Basin

Length of Seattle fault

Continuing and future research

Acknowledgements

Tom Burdette, USGS, organized and arranged permits for both projects. K. Meagher and N. Sandoval coordinated field logistics. Many volunteers helped to deploy and retrieve seismographs. IRIS/PASSCAL provided the RefTeks. S. Azevedo, W. Zamora, and others of IRIS/PASSCAL, programmed and deployed the RefTeks. T. Burdette, E. Criley, D. Reneau, and others detonated the shot holes. L. Preston, University of Washington (UW), processed and plotted the RefTek data.

We thank NOAA-PMEL at Sand Point for use of their facilities during Kingdome SHIPS. We thank the many private and public landholders that allowed access to their land.

Dry and Kingdome SHIPS were supported by the USGS Urban Geological Hazards Initiative, and grants from the USGS National Earthquake Hazards Reduction Program to Oregon State University, the University of Texas El Paso, and the University of Washington.

References

Brocher, T.M., T. L. Pratt, K.C. Miller, A.M. Trehu, C.M. Snelson, C.S. Weaver, K. C. Creager, R.S. Crosson, U.S. ten Brink, M.G. Alvarez, S.H. Harder, and I. Asudeh, 2000, Report for explosion and earthquake data acquired in the 1999 Seismic Hazards Investigation in Puget Sound (SHIPS), Washington, U.S. Geol. Surv. Open-File Report 00-318, 85 p.

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, Trans. Amer. Geophys. Un., v. 80, no. 2, p. 13-17.

Frankel, A., D. Carver, E. Cranswick, M. Meremonte, T. Bice, and D. Overturf, 1999, Site response for Seattle and source parameters of earthquakes in the Puget Sound region, Bull. Seismol. Soc. Amer., v. 89, p. 468-483.

Johnson, S.Y., Potter, C.J., and Armentrout, J.M., 1994, Origin and evolution of the Seattle fault and Seattle basin, Washington, Geology, v. 22, p. 71-74.

Johnson, S.Y., S.V. Dadisman, J.R. Childs, W.D. Stanley, 1999, Active tectonics of the Seattle fault and central Puget Sound, Washington&Mac247;Implications for earthquake hazards, Bull. Seismol. Soc. Am., v. 111, p. 1042-1053.

Pratt, T.L., S. Johnson, C. Potter, W. Stephenson, and C. Finn, 1997, Seismic reflection images beneath Puget Sound, western Washington state: The Puget Lowland thrust sheet hypothesis, J. Geophys. Res., 102, 27,469-27,489.

Shin, T.C., K.W. Kuo, W.H.K. Lee, T.L. Teng, and Y.B. Tsai, 2000, A preliminary report on the 1999 Chi-Chi (Taiwan) earthquake, Seismol. Res. Lett., v. 71, p. 24-30.

Wells, D.L., and K.J. Coppersmith, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seismol. Soc. Am., 84, 974-1002, 1994.