BROADBAND MODELING OF STRONG GROUND MOTIONS FORPREDICTION PURPOSES FOR SUBDUCTION EARTHQUAKESOCCURRING IN THE COLIMA-JALISCO REGION OF MEXICO

摘要

The high seismic potential of the Colima-Jalisco (CJ) region in northwestern Mexico has been shown over the last several centuries. For example, recent destructive large shallow thrust subduction earthquakes occurred the 3rd and 18th of June 1932 with Ms of 8.2 and 8, respectively, the 30th of January 1973 (Mw 7.6, Ms 7.3) and also on the 9th of October 1995 (Mw 8, Ms 7.4) and the 22nd of January 2003 (Mw 7.4, Ms 7.3). The frequent occurrence of large events calls for reliable seismic hazard estimates for the CJ region.rnIn the first part of this study, we present broadband synthetics generated by a hybrid model combining long period and high frequency simulations and compare them with the accelerograms recorded in Manzanillo (MZ), Ciudad Guzman (CG), and at the Colegio station of Guadalajara (G), which are located at 35, 140, and 240 km, from the epicenter of the 1995 earthquake, respectively. The low frequency (< 1Hz) synthetics were simulated by using a fourth order finite difference method (Olsen, 1994) including a finite-fault description of the source with four asperities in a 2.5D model constrained by gravity and seismological data. The high frequencies (> lHz) were modeled by the empirical Green function technique (Irikura, Procc. 7th JEES, 1986), the four-asperity source model, and the recordings of the fore-and aftershocks of the Mw 8 1995 event. The comparisons of the synthetics with the observed strong ground motions for the 1995 earthquake in the near field (MZ), intermediate field (CG), and the far field (G) are satisfactory for the entire modeled bandwidth (0.04 - 20Hz), including the local soil effects at CG and G.rnIn the second part of the study we present the results we obtained, by applying the proposed hybrid model, for the computation of the expected strong ground motions at G (free field soil site and at 35m rock depth) for a hypothetical Mw 8.5 (Ms 8.2) earthquake, similar to the 03/06/1932 event. Three rupture propagation scenarios were analyzed, the same 2.5D model was used, and the finite source was modeled with 12 asperities; for the high frequencies (> lHz) the same 12 asperity model and the recordings of the 1995 main shock were utilized.