Three-Dimensional Simulations of Ground Motions in the Seattle Region for Earthquakes in the Seattle Fault Zone

We used the 3D finite-difference method to model observed seismograms of two earthquakes ( M L 4.9 and 3.5) in the Seattle region and to simulate ground motions for hypothetical M 6.5 and M 5.0 earthquakes on the Seattle fault, for periods greater than 2 sec. A 3D velocity model of the Seattle Basin was constructed from studies that analyzed seismic-reflection surveys, borehole logs, and gravity and aeromagnetic data. The observations and the simulations highlight the importance of the Seattle Basin on long-period ground motions. For earthquakes occurring just south of the basin, the edge of the basin and the variation of the thickness of the Quaternary deposits in the basin produce much larger surface waves than expected from flat-layered models. The data consist of seismograms recorded by instruments deployed in Seattle by the USGS and the University of Washington (UW). The 3D simulation reproduces the peak amplitude and duration of most of the seismograms of the June 1997 Bremerton event ( M L 4.9) recorded in Seattle. We found the focal mechanism for this event that best fits the observed seismograms in Seattle by combining Green9s functions determined from the 3D simulations for the six fundamental moment couples. The February 1997 event ( M L 3.5) to the south of the Seattle Basin exhibits a large surface-wave arrival at UW whose amplitude is matched by the synthetics in our 3D velocity model, for a source depth of 9 km. The M 6.5 simulations incorporated a fractal slip distribution on the fault plane. These simulations produced the largest ground motions in an area that includes downtown Seattle. This is mainly caused by rupture directed up dip toward downtown, radiation pattern of the source, and the turning of S waves by the velocity gradient in the Seattle basin. Another area of high ground motion is located about 13 km north of the fault and is caused by an increase in the amplitude of higher-mode Rayleigh waves caused by the thinning of the Quaternary deposits.

[1]  A. Frankel,et al.  Surface Seismic Measurements of Near-Surface P- and S-Wave Seismic Velocities at Earthquake Recording Stations, Seattle, Washington , 1999 .

[2]  K. Shedlock,et al.  Estimates of seismic source regions from considerations of the earthquake distribution and regional tectonics in the Pacific Northwest , 1991 .

[3]  Robert W. Graves,et al.  Ground-motion amplification in the Santa Monica area: Effects of shallow basin-edge structure , 1998, Bulletin of the Seismological Society of America.

[4]  C. Potter,et al.  Origin and evolution of the Seattle fault and Seattle basin, Washington , 1994 .

[5]  Pierre-Yves Bard,et al.  The seismic response of sediment-filled valleys. Part 2. The case of incident P and SV waves , 1980 .

[6]  Robert W. Graves,et al.  Simulating seismic wave propagation in 3D elastic media using staggered-grid finite differences , 1996, Bulletin of the Seismological Society of America.

[7]  A. Herrero,et al.  A kinematic self-similar rupture process for earthquakes , 1994, Bulletin of the Seismological Society of America.

[8]  D. Wells,et al.  New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement , 1994, Bulletin of the Seismological Society of America.

[9]  A. Frankel.  High-frequency spectral falloff of earthquakes, fractal dimension of complex rupture, b value, and the scaling of strength on faults , 1991 .

[10]  R. Bucknam,et al.  Abrupt Uplift Within the Past 1700 Years at Southern Puget Sound, Washington , 1992, Science.

[11]  Kim B. Olsen,et al.  Lens‐effect in Santa Monica? , 1998 .

[12]  T. L. Pratt,et al.  Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis , 1997 .

[13]  Arthur Frankel,et al.  Three-dimensional simulations of ground motions in the San Bernardino Valley, California, for hypothetical earthquakes on the San Andreas fault , 1993, Bulletin of the Seismological Society of America.

[14]  Arthur Frankel,et al.  A three-dimensional simulation of seismic waves in the Santa Clara Valley, California, from a Loma Prieta aftershock , 1992 .

[15]  D. Hadley,et al.  Prediction of strong ground motion in the Puget Sound region: The 1965 Seattle earthquake , 1986 .

[16]  Hiroshi Kawase,et al.  The Cause of the Damage Belt in Kobe: “The Basin-Edge Effect,” Constructive Interference of the Direct S-Wave with the Basin-Induced Diffracted/Rayleigh Waves , 1996 .

[17]  C. Langston,et al.  Effect of structure geometry on strong ground motions: The Duwamish River Valley, Seattle, Washington , 1983 .

[18]  David Carver,et al.  Site response for Seattle and source parameters of earthquakes in the Puget Sound Region , 1999, Bulletin of the Seismological Society of America.

[19]  N. Abrahamson,et al.  Characterizing Crustal Earthquake Slip Models for the Prediction of Strong Ground Motion , 1999 .

[20]  Stephen S. Gao,et al.  Localized amplification of seismic waves and correlation with damage due to the Northridge earthquake: Evidence for focusing in Santa Monica , 1996, Bulletin of the Seismological Society of America.

[21]  Kojiro Irikura,et al.  Three-dimensional simulation of the near-fault ground motion for the 1995 Hyogo-Ken Nanbu (Kobe), Japan, earthquake , 1998, Bulletin of the Seismological Society of America.

[22]  W. Stanley,et al.  Active tectonics of the Seattle fault and central Puget Sound , 1999 .

[23]  David Carver,et al.  Variability of site response in the Los Angeles urban area , 1997, Bulletin of the Seismological Society of America.

[24]  Arthur Frankel,et al.  Energy-flux model of seismic coda: Separation of scattering and intrinsic attenuation , 1987 .