SHALLOW CRUSTAL STRUCTURE FROM SHORT-PERIOD RAYLEIGH-WAVE DISPERSION DATA IN SOUTHWESTERN TAIWAN

Strong ground motions in the period range of 1 to 5 sec observed in Ta iwan in a dense network during the Tapu earthquake (M-L = 5.8) of 15 D ecember 1993 were dominated by fundamental-mode Rayleigh waves. The sh allow crustal structure beneath this network was determined from the g roup velocity dispersion data using standard inversion techniques. Res ults show that an alluvial layer with a thickness of only about 160 m exists over the sedimentary structure in the western coastal plain. A clear lateral variation in the shear-wave velocity along a cross secti on perpendicular to the western structural grain (He, 1982) was resolv ed. For the purpose of retrieving the ground motions to confirm the pr oposed model, forward modeling with a two-dimensional finite-element m ethod was used and agreed well with the observed seismograms when slig htly lower velocities than those of the inversions were used. It is co ncluded that the velocity model estimated using group velocity data ca nnot definitively correspond to the real one at such short distances a nd in such a complex structure. On the basis of quantitative simulatio ns, a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particl e motions. The results suggest that these surface waves may be generat ed by the conversion of body waves at the boundary of the western coas tal plain and foothills.