ESTIMATED GROUND MOTIONS IN LOS-ANGELES DUE TO MW = 7 EARTHQUAKE ON THE ELYSIAN THRUST-FAULT

The Elysian thrust fault has been identified as a blind thrust fault ( Davis et al., 1989; Hauksson and Jones, 1989) representing a potential ly serious seismic hazard to the metropolitan Los Angeles and its neig hboring areas. We have simulated time histories, peak ground accelerat ions and their uncertainties using a semi-empirical method for a M(W) = 7.0 earthquake on the Elysian thrust fault using a flat-layered crus tal structure. The accelerograms from the 4 October 1987 Whittier - Na rrows aftershock (10h:50m; M(L) = 5.3) are used to represent the sourc e functions of each subfault on the fault surface. To account for the velocity variation in the surface sediments, we compared simulated gro und motions using two separate shearwave velocities, 0.6 km/sec and 0. 9 km/sec, respectively, in the top layer of the crustal model. The use of such a simple crustal model is validated by modeling the accelerog rams recorded during the 1 October 1987 Whittier-Narrows mainshock (M( L) = 6.0). The duration and the relative frequency content observed on these accelerograms are successfully modeled. At some stations, the s imulated peak accelerations agree well with the observed values; howev er, at some sites, the simulated and observed values differ by a facto r of 2 to 3. The variation is attributed to a laterally varying crusta l structure that is significant for small earthquakes. Additional site specific study is needed for an improved prediction of the observed p eak accelerations. For large magnitude earthquakes, the near-source pe ak ground acceleration appears to be controlled by the source-receiver geometry relative to the fault, as well as the location of asperities on the fault surface. This is validated by simulating the peak accele ration data of the 1989 Loma Prieta earthquake recorded within 30 km o f the source. For our simulations on the Elysian thrust fault, an addi tional component is added to the uncertainty by analyzing several aspe rity models. Finally, an analytical representation is given to the sim ulated average peak horizontal ground accelerations, Y(R), which is ex pressed by In(Y(R)) = (5,38 +/- 0.085) + (-2.09 +/- 0.0268)In(R + 8.0) +/- 0.343 in the range of 7.5 less-than-or-equal-to R less-than-or-eq ual-to 35 km (R is the closest distance to the seismogenic rupture zon e). This functional form predicts a rapid fall-off rate for the simula ted ground motions compared with the fall-off rate predicted by the ot her published empirical attenuation relations. We use the finite-diffe rence method to investigate the effects of irregular structure on grou nd motions resulting from point sources on the Elysian Park fault. The response computed at several depths beneath the basin suggests that t he response is dominated by the initial direct arrivals for sources lo cated interior to the basin. When the receivers are located at one end of the basin, and the seismic sources are located at the opposite edg e, the characteristic features on the seismograms are the long duratio ns caused by the trapped energy within the basin. The level of trapped energy decreases as the depth of the source increases. Thus, the faul t model becomes important in determining the level of peak ground moti ons and shaking duration.