Evidence for anomalous body-wave radiation between 0.3 and 0.7 Hz from the 1985 September 19 Michoacan, Mexico earthquake

Abstract

SUMMARY In order to better understand the causes of unprecedented damage to Mexico City during the 1985 September 19 Michoacan earthquake (Mw= 8.0) spectral ratios of teleseismic P-waves of this earthquake are studied with respect to those from five recent, large Mexican subduction zone earthquakes (7.0 ≤Mw≤ 7.7). The data are from vertical-component digital seismographs. It is found that the spectral ratios at stations in the NE quadrant are anomalously more energetic than those predicted by the ω−2 source model in the critical frequency range for Mexico City (0.3-0.7 Hz). The evidence is especially convincing for the spectral ratios with respect to the earthquakes of 1985 September 21 (Mw= 7.6) and 1986 (Mw= 7.0) since the data are available from several stations in the NE quadrant. The teleseismic P-wave spectral ratio in this quadrant with respect to the 1985 September 21 earthquake, in the critical frequency range, is close to the acceleration spectral ratio found in and near Mexico City (also in the NE quadrant). Velocity traces in the epicentral region of the Michoacan earthquake, obtained by integrating the accelerograms, also show oscillations with a frequency of about 0.4 Hz. Furthermore, a regression study of Fourier acceleration spectra at a hill-zone site in Mexico City demonstrates that the Michoacan earthquake was anomalously energetic in the city at the critical frequencies for an event of that magnitude and at that distance. If the data from 7.0 ≤Mw≤ 7.7 events can be extrapolated to estimate the ground motions from Mw≥ 8.0 earthquakes, then the evidence, supports an anomalously large body-wave radiation towards Mexico City between 0.3 and 0.7 Hz during the Michoacan earthquake. This anomalous radiation and the dramatic local amplification of seismic waves in the lake-bed zone of the city (∼ 10–50 times at frequencies between 0.3 and 0.7 Hz) appear to be the principal natural causes of the disaster. The anomalous teleseismic P-wave spectral ratios with respect to the earthquakes of 1985 September 21 and 1986 found in the NE quadrant are not observed in the data available from a small number of stations in the other quadrants. If this observation is true then it suggests a directional property to the anomalous radiation.