The United States Geological Survey has recently released an updated 3D velocity
model (USGS SF06) of the greater San Francisco Bay Area that will be used to
evaluate shaking hazard for possible future events in the region. In this study we
used the 3D model and the 3D finite-difference code E3D (*Larsen and Schultz*,
1995) to simulate ground motions
of the 1989 6.9 Loma Prieta event. Comparison of simulated ground motions with
observations served to validate the velocity model for scenario studies of
future large earthquakes in the region. This work was done as part of the
SF06 Simulation Project Group modeling of the 1906 7.8 San Francisco
earthquake (`http://www.sf06simulation.org`).

The modeling of the great 1906 San Francisco earthquake for the
event's 100th anniversary was one of the reasons for the USGS SF06
velocity model release. The model extends
to 45 km depth and consists of a detailed model with approximate
dimensions of
km. This detailed model is nested inside an
extended model which has approximate dimensions of
km.
The extended model was provided mainly to accommodate the 1906 San
Francisco earthquake simulations and as a buffer zone
for the simulations with either sources or stations close
to the edge of the detailed model. The model's main
improvement over the previous model versions is that more realistic
material properties were assigned
to individual geologic units (*Brocher*, 2005).
Also, the model now includes topography and
material properties of water for the Pacific Ocean and the San Francisco
Bay.

To use the USGS SF06 model with the finite-difference code, we queried
the model to extract values on a grid with a 125-m spacing.
For the Loma Prieta earthquake, we employed *Wald et al.* (1991)'s strong motion
and strong motion/teleseismic finite-source models. In addition, we used the
*Beroza* (1991) strong motion finite-source model as a comparison.
The dimensions of the simulation
were
km and the grid spacing was 125 m.
Because of computation limitations, the slowest velocities in the model were
increased to a minimum S-wave velocity of 500 m/s,
corresponding to a maximum modeled frequency of 0.8 Hz.
In our simulations we included water (S-wave velocity 0.0 m/s), but not topography
or attenuation.
The computations were performed on the BSL Linux cluster.
We compared results obtained with the three different finite-source models to
observations.
In addition, we compared the recent results to those obtained with the previous version
of the USGS (ver. 2) and UCB velocity models (*Stidham et al.*, 1999) that had 250-m grid
spacing and 1 km/s minimum S-wave velocity.

Maximum horizontal velocity maps for the Loma Prieta simulations with the USGS SF06 velocity model and three different finite-source models are shown in Figure 11.1 (top). Results show strong NW directivity of the Wald et al. SM/T finite-source model. Amplification of the ground motions over the basins is well observed for all three models.

Recorded and synthetic waveforms at some of the stations that
were included in the simulations are compared in Figure 11.1 (bottom).
Velocity waveforms were lowpass filtered at 0.8 Hz. The traces from top
to bottom are observations (gray), followed by the USGS SF06 velocity model synthetics
for the three different finite-source models (Beroza, Wald SM/T, Wald SM).
The results showed that the overall agreement between the USGS SF06 model
simulations and the observations is very good. Comparison to the results
obtained with the previous USGS (ver. 2) model and the UCB model showed that
the results obtained with the new USGS SF06 model better matched the
observations. Important differences between the results obtained with the three
different finite-source models can also be observed.
The results obtained for the Loma Prieta event as well as for other smaller events
in the region (*Dolenc et al.*, 2006; *Rodgers et al.*, 2006) indicate
that the remaining misfit between the simulations and
the observations at some stations requires further model refinement.

Beroza G. C. (1991). Near-source modeling of the Loma Prieta earthquake:
Evidence for heterogeneous slip and implications for earthquake hazard,
*Bull. Seism. Soc. Am. 81*, 1603-1621.

Brocher, T. M., Compressional and shear wave velocity versus depth in
the San Francisco Bay Area, California: Rules for USGS Bay Area Velocity
Model 05.0.0, *U.S. Geol. Surv. Open-File Report 2005-1317*, 2005.

Dolenc, D., D. Dreger, and S. Larsen, 3D simulations of ground motions
in Northern California using USGS SF06 velocity model,
*Seism. Res. Lett., 77*, 300, 2006.

Larsen S. and C. A. Schultz (1995). ELAS3D: 2D/3D elastic finite-difference
wave propagation code, *Technical Report No. UCRL-MA-121792*, 19 pp.

Rodgers, A., A. Petersson, S. Nilsson, B. Sjogreen, and K. McCandless,
Ground predictions using the USGS seismic velocity model of the
San Francisco Bay Area: Evaluating the model and scenario earthquake
predictions, *Seism. Res. Lett., 77*, 281, 2006.

Stidham, C., M. Antolik, D. Dreger, S. Larsen, and B. Romanowicz,
Three-dimensional structure influences on the strong-motion wavefield
of the 1989 Loma Prieta earthquake, *Bull. Seism. Soc. Am., 89*,
1184-1202, 1999.

Wald D. J., D. V. Helmberger, and T. H. Heaton (1991). Rupture model
of the 1989 Loma Prieta earthquake from the inversion of strong-motion
and broadband teleseismic data,
*Bull. Seism. Soc. Am. 81*, 1540-1572.

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