Annual Report 1997
Simulation of Ground Motion in the Los Angeles Basin

Yuehua Zeng and John Anderson
Seismological Laboratory and Department of Geological Sciences,
Mackay School of Mines, University of Nevada, Reno

Progress Report

This year we have studied the site-specific ground motion simulation and nonlinear site response of the Tarzana Hill station during the Northridge earthquake. We also investigated a simplified approach for basin response. Our method uses a ray-theoretical approach to trace the reflection, transmission and reverberation of rays inside the basin.

The January 17, 1994 Northridge earthquake (Mw=6.7) generated one of the highest strong motion accelerations ever recorded on a strong motion instrument at Tarzana, California, operated by the California Strong Motion Instrumentation Program (CSMIP). This site is located about 6 km south of the epicenter on a crest of Tarzana Hill. The record shows repeated acceleration over lg for 7 to 8 seconds, with a peak horizontal acceleration of about 1.8 g (Shakal et al., 1994). This site also recorded peak acceleration during the 1987 WhittierNarrows earthquake that was a factor of 10 times higher than that observed at other sites at similar distances (Shakal et al., 1988). This unusually high peak ground motion at Tarzana attracted a great amount of attention among seismologists and engineers. Using aftershock data from a temporary array on the hill, Spudich et al. (1996) studied the relative site amplifications, topographic effects, and possible polarization effects on the ground motion caused by the structure of the hill. They found Tarzana Hill, which is about 15 m high, 500 m long with a

strike of N78oE and 130 m wide, has dominant motions perpendicular to the strike of the hill. While the dominant motion is nearly north-south, surprisingly, the largest peak ground acceleration during the Northridge earthquake appeared on the east-west component. Using a composite source model, Su et al. (1995, 1996) have simulated the ground motion at Tarzana and found motions in the east-west direction are nearly nodal to the source radiation. By applying the weak motion site amplification to the synthetic simulation, We found the motions along the east-west direction are comparable to the observations. However, the synthetic northsouth component has a peak ground acceleration several times higher than the observation. There are clearly other processes that affect the ground motion at the Tarzana station, apparently on the north-south component. We resolve it to the nonlinear response of the site to the input ground motion. We assumed a nonlinear shear modulus reduction relation with strain, which represents nonlinear rock behavior (Stokoe et al., 1997), for the shallow surface column beneath the station, and computed the nonlinear response of the column to the input ground motion. This nonlinear response reduces the acceleration of the north-south component to a level comparable to the observation (Figure 1). The same nonlinear effect has a much smaller influence on the east-west component because of the relatively lower level of ground motion. Thus a model invoking nonlinear rock behavior in concert with otherwise high site effects provides a simple explanation of ground accelerations that is consistent with the well-documented source mechanism. This also explains the larger peak acceleration at Tarzana compared to other sites at similar epicentral distances observed during the Whittier-Narrows earthquake. A drilling
experiment at the Tarzana site found that rock below the surface is highly fractured (Nigbor et al., 1997), which is consistent with the strong nonlinear reduction in site effects for the Northridge earthquake ground motion at Tarzana Hill.

On modeling of basin effects, we have made sign)ficant progress in developing simplified approaches to simulate site response in basins. One characteristic of basin response is that the trapping of waves inside the basin produces long wave duration and excites resonant frequencies. As indicated by Olsen and Archuleta (1997) in their fmite difference simulation of the basin responses in southern California, after removing the impedance effect from the total site response, site amplifications averaged on basin depth and scenario events cause a maximum factor of two variability. These amplifications are results of this basin resonance response to the incident waves. Zeng (1997) showed that the effect on synthetics can be well described using a simple ray-theoretical method. In this approach, we use the Gaussian Beam method to sum up the contribution to the seismogram from multiple reverberations of rays within the basin layer assuming a constant beam width for each ray as it propagates. The results agree well with more precise numerical methods like the finite difference or analytical methods (Trifunac, 1971) both in wave amplitude and duration for stations inside the basin (Figure 2a). At the edge of the basin, the wave field is dominated by trapped Stonley type waves propagating along the basin boundary that are excited by incident waves to the basin as well as the reflected waves from the basin (Figure 2b). The attenuation of this trapped basin boundary wave strongly depends on its wave length and on the curvature of the basin boundary in addition to the intrinsic and scattering attenuation. In particular, it favors the high frequency trapped waves where the wave length is so short that it propagates along the basin boundary much like the Stoney wave propagates along a flat layered interface of the crust. This simple approach can provide a high frequency description of waves of engineering interests from a fraction of a Hertz up to 10 Hz.

References

Nigbor, R., R. Pyke, C Roblee, J. Shneider, W. Silva, R. Steller, and M. Vucetic (1997). Resolution of site response issues from the Northridge earthquake (ROSRINE), Seism. Res. Lett., 68, p303.

Olsen, K. B. and R. J. Archuleta (1997). Three-dimensional Los Angeles basin amplification effects, Seism. Res. Lett., 68, p304.

Shakal, A., M. Huang, and T. Cao (1988). The Whittier Narrows, California, earthquake of October 1, 1987: CSMIP strong motion data, Earthquake Spectra, 4, 75-100.

Shakal, A., M. Huang, R. Darragh, T. Cao R. Sherburne, P. Malhotra, C. Cramer, R. Sydnor, V. Graizer, G. Maldonado, C. Peterspm, and J. Wampole (1994). CSMIP strong motion records from the Northridge, California, earthquake of 17 January 1994, report OSMS 94-07, California Division of Mines and Geology, Sacrameto, California.

Spudich, P., M. Hellweg, and W. H. K. Lee (1996). Directional topographic site response at Tarzana observed in aftershocks of the 1994 Northridge, California, earthquake: implications for mainshock motions, Bull. Seis. Soc. Am., 86, S193-S208.

Stokoe, K. H., M. Darendeli, and S.-K. Hwang (1997). Evaluation of linear and nonlinear dynamic rock properties: Yucca Mountain samples, The University of Texas at Austin, Department of Civil Engineering.

Su, F., J. G. Anderson and Y. Zeng (1995). Integrated study of source, path and site effects on

o Northridge ground motion, EOS, Trans. A.G.U., 76, F352.

Su, F., J. G. Anderson and Y. Zeng (1996). Comparison of strong and weak motion site amplification from Northridge earthquake sequence, EOS, Trans. A.G. U., 77, p494.

Trifunac, M. D. (1971). Surface motion of a semi-cylindrical alluvial valley for incident plane SH waves, Bull. Seis. Soc. Am., 61, 1755-1770.

Zeng, Y. (1997a). A simple ray-theoretical approach to evaluate basin response, seism. Res. Lett., 68, p302.

Zeng, Y., F. Su, and J. G. Anderson (1997b). Site Effect at Tarzana Implied from the Northridge Earthquakes, submitted to EOS, Trans. A.G. U., 78.

Publicaffon List

Anderson, J. G. (1997). Expected shape of regressions for ground motion parameters on rock,, Bull. Seis. Soc. Am, in press.

Lee, Y. J., Y. Zeng and J. G. Anderson (1997). A simple strategy to examine the sources of errors in the attenuation relation,, Bull. Seis. Soc. Am., in press.

Lee, Y. J., Y. Zeng, J. G. Anderson, S.-D. Ni, and F. Su (1996). Evaluation of regression, draft of SCEC Phase m report.

M, S.-D., J. G. Anderson, Y. Zeng (1996). Expected signature of non-linearity on Regression for strong ground motion parameters, submitted to Bull. Seis. Soc. Am.

Zeng, Y. (1997). A simple ray-theoretical approach to evaluate basin response, seism. Res. Lett., 68, p302.

Zeng, Y., F. Su, and J. G. Anderson (1997). Site Effect at Tarzana Implied from the Northridge Earthquakes, submitted to EOS, Trans. A.G. U., 78.