Strong ground motions in the period range of 1 to 5 sec observed in Taiwan in a dense network during the Tapu earthquake (ML = 5.8) of 15 December 1993 were dominated by fundamental-mode Rayleigh waves. The shallow crustal structure beneath this network was determined from the group velocity dispersion data using standard inversion techniques. Results 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 section perpendicular to the western structural grain (Ho, 1982) was resolved.

For the purpose of retrieving the ground motions to confirm the proposed model, forward modeling with a two-dimensional finite-element method was used and agreed well with the observed seismograms when slightly lower velocities than those of the inversions were used. It is concluded that the velocity model estimated using group velocity data cannot definitively correspond to the real one at such short distances and in such a complex structure. On the basis of quantitative simulations, a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particle motions. The results suggest that these surface waves may be generated by the conversion of body waves at the boundary of the western coastal plain and foothills.

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