STRONG-MOTION OBSERVATION DEMONSTRATION OF THE LONG-PATH EFFECTS FROM THE PACIFIC OCEAN TO MEXICO CITY

概要

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During the 1985 Michoacan earthquake (M=8.1), seismic waves were remarkably amplified in the lakebed zone of Mexico City, approximately 400 km from the epicenter, and the long duration of the lakebed seismograms was a great surprise. Many researchers thought that the origin of the long coda seen in the lakebed zone was surface waves caused by the deep Mexico basin and the soft surficial layers, but this explanation was not adequate. Another possible cause of the long coda is the long-distance path effects from the earthquake source to the Valley of Mexico. A recent earthquake of 14 September 1995 (M=7.3) has provided the best-quality strong-motion dataset ever obtained in Mexico. We investigated the long-path effects by basic analyses of the strong motions recorded during this earthquake. To verify the site effects, we measured microtremors along the long path. We show the existence of considerable long-path effects on the efficiency of wave propagation and the long duration of the records. In addition, we obtained a true image of the strong-motions in Mexico City.