Ground motion characteristics and time histories in nearby urban centers from postulated large earthquakes (Mw>7) on the Santa Monica Mountain fault system

Chandan Saikia, & Paul G. Somerville

Published 2000, SCEC Contribution #499

The Santa Monica Mountain fault zone (SMMFZ), which is capable of generating large earthquakes involving multiple faults and slip directions, is located very close to 18 urban centers in Beverly Hills (bvh), downtown Los Angeles,Glendale, Hollywood (hsbg), Sherman Oaks (shro), Santa Monica (smn) andWestwood (Table 1). In this study, our objective is to investigate three possible earthquake scenarios on the Santa Monica fault system. These scenarios include a Mw7 earthquake on the HF-SMF-MF (Hollywood-Santa Monica-Malibu) fault, a Mw 7earthquake on the buried SMMTF (Santa Monica Mountain thrust) fault and a Mw 7.2earthquake generated by the co-seismic rupturing of both the blind thrust and strike-slip fault systems. Another objective of this study is to determine how site-specific crustal structure may influence the level and duration of ground motions. The technique used for generating ground-motion time histories is similar to the method used in our previous work. In this study, we modified this method to include the ability to simulate ground motions for large earthquakes due to a continuous rupture on a multi-segmented fault. This simulation was done using a one-dimensional crustal model. Our initial model consisted of 8 layers and was based on the crustal model for southern California. The influence of basin-like structure on ground motion levels was also investigated at each site. We conducted two separate simulations, one with the 1D base model for all sites and the other with site-specific 1D velocity models developed from the updated SCEC 3D velocity model of the Los Angeles basin. In these site-specific models we used a surface shear-wave velocity of 0.5 km/s and found three categories of site conditions that include all 18 sites. Category-I includes Santa Monica (smn), category-II includes three sites, namely lacc, la19 and bvh, andcategory-III includes shro, hsbg, ulg, ul7, la9, la12, la13, la15, la52, la54 and ulH. In the remaining three sites, nhol and glen, the surface shear-wave velocity is thesame as that of the base model which is expected to have the characteristics of rock-site ground motions. Both peak ground accelerations and velocities (PGA and PGV) from earthquakes on the HF-SF-MF fault are larger than those from earthquakes on the SMTF fault. The difference is about a factor of 2 to 5 at sites hsbg, shro, nhol,lacc, smn, bvh, ulg, la19, and ul7. We also found that the site-specific crustalmodels produced ground motions that differ from those of the base model at periods shorter than 5s. The peak velocity estimates in a 2s passband are similar to thepreliminary estimates simulated using a full 3D model of the Los Angeles basin (Pitarka et al., 1999). We also compared our simulation results with the empiricalrelation of Abrahamson and Silva (1997) and found that spectral accelerations for rock estimated in our studies match quite well with those predicted using the empirical attenuation relation. Soil effects are large in our simulation, partly because our simulations do not contain nonlinear soil effects, and because the slip models for the HF-SF-MF fault have strong slip near the top edge. Values simulated forearthquakes on the SMTF fault are consistent with those predicted from the empiricalrelation for both rock and site models.

Citation
Saikia, C., & Somerville, P. G. (2000). Ground motion characteristics and time histories in nearby urban centers from postulated large earthquakes (Mw>7) on the Santa Monica Mountain fault system. Bulletin of the Seismological Society of America, 88, 1-60.