A three-dimensional simulation of seismic waves in the Santa Clara Valley, California, from a Loma Prieta aftershock

Abstract The finite-difference method is used to propagate elastic waves through a 3-D model of the Santa Clara Valley, an alluvium-filled basin that underlies the city of San Jose, California. The model was based on depth to bedrock information from water wells in the area. The simulation corresponded to a region 30 (east-west) by 22 (north-south) by 6 (depth) km and contained about 4 million grid points. Synthetic seismograms from the simulation are accurate at frequencies up to 1 Hz. Motions from a magnitude 4.4 aftershock of the Loma Prieta earthquake were modeled. Snapshots of ground motion and synthetic seismograms from the simulation are presented. The simulation illustrates S -to-surface-wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock. Love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics. Because of the slow group velocity of the Love waves, sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges. Sites near the center of the basin also show larger peak amplitudes on the transverse component. Array analysis of the synthetic seismograms indicates that Love waves tend to propagate parallel to the eastern and western edges of the valley. Rayleigh waves are produced along the southern margin of the basin from incident S waves. Large radial motions occur where a Rayleigh wave impinges on the northeast margin of the valley. Some Rayleigh waves travel westward across the basin, after being scattered from the eastern edge of the valley. Synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the Sunnyvale dense array for this aftershock, although the duration of the tranverse component is not matched by the synthetic seismogram, using this basin model. The simulation indicates that the Love waves observed on the actual seismograms were produced by conversion of incident S waves at the southern margin of the Santa Clara Valley. 2-D simulations show how the S -to-Love-wave conversion is affected by the angle of incidence of the S wave and the sharpness of the velocity transition between the alluvium and bedrock. As more accurate basin models are developed, 3-D simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes.

[1]  Keiiti Aki,et al.  A study on the response of a soft basin for incident S, P, and Rayleigh waves with special reference to the long duration observed in Mexico City , 1989 .

[2]  Robert W. Clayton,et al.  A stable free-surface boundary condition for two-dimensional elastic finite-difference wave simulation , 1986 .

[3]  R. M. Alford,et al.  ACCURACY OF FINITE‐DIFFERENCE MODELING OF THE ACOUSTIC WAVE EQUATION , 1974 .

[4]  L. Pérez-Rocha,et al.  Diffraction of elastic waves by three-dimensional surface irregularities. Part II , 1989 .

[5]  E. E. Brabb,et al.  Maps showing aeromagnetic anomalies, faults, earthquake epicenters, and igneous and volcanic rocks in the southern San Francisco Bay region, California , 1979 .

[6]  N. A. Haskell,et al.  Crustal Reflection of Plane SH Waves , 1960 .

[7]  Comment on “A comparison of finite-difference and fourier method calculations of synthetic seismograms” , 1990, Bulletin of the Seismological Society of America.

[8]  P. Somerville,et al.  The influence of critical Moho Reflections on strong ground motions recorded in San Francisco and Oakland during the 1989 Loma Prieta Earthquake , 1990 .

[9]  A. Frankel,et al.  Observations of Loma Prieta aftershocks from a dense array in Sunnyvale, California , 1991, Bulletin of the Seismological Society of America.

[10]  M. Saito.  Excitation of free oscillations and surface waves by a point source in a vertically heterogeneous Earth , 1967 .

[11]  P. Bard,et al.  The two-dimensional resonance of sediment-filled valleys , 1985 .

[12]  M. Campillo,et al.  The Mexico Earthquake of September 19, 1985—A Theoretical Investigation of Large- and Small-scale Amplification Effects in the Mexico City Valley , 1988 .

[13]  D. Oppenheimer.  Aftershock slip behavior of the 1989 Loma Prieta, California earthquake , 1990 .

[14]  A. Rotenberg,et al.  Seismic waves in a quarter plane , 1969 .

[15]  W. B. Joyner,et al.  The effect of Quaternary alluvium on strong ground motion in the Coyote Lake, California, earthquake of 1979 , 1981 .

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

[17]  W. B. Joyner,et al.  Analysis of seismograms from a downhole array in sediments near San Francisco Bay , 1976 .

[18]  Roger D. Borcherdt,et al.  Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake , 1976, Bulletin of the Seismological Society of America.

[19]  Seismic wave resonances in 3-D sedimentary basins , 1989 .

[20]  金井 淸.  19. Relation between the Nature of Surface Layer and the Amplitude of Earthquake Motions. II. , 1952 .

[21]  R. M. Hazlewood.  Contour map and interpretive cross sections showing depth and configuration of bedrock surface, South San Francisco Bay region, California , 1976 .

[22]  W. Mooney,et al.  A seismic-refraction profile across the San Andreas, Sargent, and Calaveras faults, west-central California , 1985 .

[23]  Z. Alterman,et al.  Propagation of elastic waves in layered media by finite difference methods , 1968 .

[24]  Keiiti Aki,et al.  Surface motion of a layered medium having an irregular interface due to incident plane SH waves , 1970 .

[25]  J. Sochacki.  Absorbing boundary conditions for the elastic wave equations , 1988 .

[26]  K. R. Kelly,et al.  SYNTHETIC SEISMOGRAMS: A FINITE ‐DIFFERENCE APPROACH , 1976 .

[27]  William L. Ellsworth,et al.  The October 17, 1989, Loma Prieta, California, Earthquake and its aftershocks: Geometry of the sequence from high-resolution locations , 1990 .

[28]  David M. Boore,et al.  Average body-wave radiation coefficients , 1984 .

[29]  Donald V. Helmberger,et al.  Elastic finite-difference modeling of the 1971 San Fernando , 1988 .