The network of strong motion accelerographs in Mexico includes instruments that were installed, under an international cooperative research program, in sites selected for the high potenial of a large earthquake. The 19 September 1985 earthquake (magnitude 8.1) occurred in a seismic gap where an earthquake was expected. As a result, there is an excellent descripton of the ground motions that caused the disaster.
Mexico City has repeatedly suffered from the long-distance effects of the earthquakes that originate as far away as the subduction trenches near the Mexican Pacific Coast. The Michoacan, Mexico earthquake of 19 September 1985 was no exception and caused extensive damage to property and numerous loss of lives. The unique subsurface condition resulting from the historical lakebed has distinct resonant low frequencies around 0.5 Hz. The strong earthquake motions from long distances as well as the locally originating weak motions cause large amplifications at resonant low frequencies in the subsurface environment of Mexico City lakebed. In this paper, the resonant frequencies and associated amplification of motions in Mexico City are quantified in terms of spectral ratios using 19 September 1985 strong-motion data and weak motions recorded in January, 1986. These ratios confirm that the amplification of motions at resonant frequencies due to the subsurface conditions is indeed the culprit.
A network of 12 digital strong motion accelerographs has been operating in the Mexicali Valley, Baja California, Mexico, since 1978. The instruments have triggered reliably, except for external problems, such as power failures and site harassment. Data from two important earthquakes and several smaller shocks have been recovered. These data have a noise level which appears to be no worse than that on digitized records from analog strong motion accelerographs, in spite of design errors which resulted in increased instrumental noise levels in several of the accelerographs (these design errors have recently been corrected). Digital recording of strong motion promises great advantages over analog recording through recovery of the initial motions, greater dynamic range, rapid playback, and lower equivalent instrumental noise level because manual or semiautomatic digitization of an analog record is not required.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.