Spectral Analysis of Strong Motion Seismograms due to the Mexico Earthquake (Sep. 19, 1985)

Abstract

The following spectral ratios of strong motion seismograms due to the Mexico earthquake of September 19, 1985 were investigated: (1) Spectral ratios of observation sites in Mexico city to UNAM (Universidad Nacional Autonoma de Mexico), and (2) Spectral ratios of sites in the Foothill zone of Mexico city and sites in Guerrero Network to Caleta de Campos.
UNAM is located on the Foothill zone according to microzonation of ground condition in and around Mexico city and so the strong motion of UNAM can be regarded as a seismic input on the base rock in the Transition and Lake zones. It was confirmed that the spectral ratios of sites in the Lake zone to UNAM were extremely high and the reason was due to large ground amplification of each site.
Caleta de Campos of Guerrero Network is located on the source region of main shock. The strong motion of Caleta de Campos, therefore, can be regarded as a standard one which is reflecting in source spectrum. The followings were observed: (1) Spectral ratios of sites along the Pacific coast to Caleta de Campos decreased with the epicentral distance and (2) Spectral ratios of sites in the land area and with the epicentral distance of around 400km, were higher than those of sites in the coastal area and with almost the same epicentral distance.
Mexico city is located at the epicentral distance of around 400km, but the spectral ratios of CU 01, TACY, SXVI in the Foothill zone and Transition zone, especially in the peried of 1.5 to 10.0sec, were very high and nearly equaled to those of the source region. This means that the seismic input between 1.5sec and 10.0sec on the base rock in and around Mexico city was very large in spite of the epicentral distance of 400km. This fact must be added to one of major causes of earthquake damages in Mexico city.