A Comprehensive Interpretation of Strong Motions in the Mexican Volcanic Belt

Large-amplitude seismograms with extremely long duration have been recorded in the Valley of Mexico. Recently, we came to recognize important effects of the Mexican Volcanic Belt (MVB) on the seismograms. We interpret strong motions observed in the MVB by analyzing the three components of surface and borehole accelerograms recorded during the 14 September 1995 earthquake (MS 7.3). The target period range is between 0.1 and 10.0 sec, with emphasis on a 2.0- to 3.0-sec period range. In the lake-bed zone inside the valley, wave types of seismic motions are identified by calculating theoretical amplification of various seismic waves and crosscorrelation functions between surface and borehole recordings. In the MVB, propagation velocities of dominant surface waves are estimated by analyses of recordings and are compared with theoretical velocities, and 2D/3D wave-propagation simulations for the MVB and the Mexico City Basin are performed. Our conclusions are the following. (1) We suggest that, while strong motions observed in the lake-bed zone are a mixture of P, S, Love, and Rayleigh waves, fundamental-mode Love waves are dominant. (2) Surface waves are found to be much more heavily amplified than S waves in the soft lake-bed deposit. (3) We confirm that the MVB is an important structure to amplify seismic motions. The lake-bed deposit, together with the basin structure, proves to remarkably amplify surface waves. (4) In the lake-bed deposit, quality factors are found to be about 3 in the long-period range of more than 1.5 sec.