Destructive strong ground motion in Mexico city: Source, path, and site effects during great 1985 Michoacán earthquake

Abstract

Simultaneous consideration of source, path, and site effects on ground motion during the Michoacán earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in Mexico City by the rupture process, the mode of propagation of the waves between the epicentral zone and Mexico City, and the local amplification. In contrast to the horizontal component which showed dramatic amplification for the 2 to 3 sec motion at lake sediment sites, we observe almost identical vertical displacement seismograms containing ripples with 2 to 3 sec period throughout the Mexico City valley whether the recording site is on the lake sediments or on hard rock. We, therefore, conclude that the 2 to 3 sec motion responsible for the destruction of Mexico City was present in the incident field. After performing a phase analysis, we interpret the signal as the superposition of long-period Rayleigh waves and short-period Lg with a dominant period of about 3 sec. The analysis of the teleseismic records indicates that the radiation of this event is enhanced for waves around the 3 sec period. Except in the case of stations for which an anomalous path effect is suspected, the records present ripples appearing a few seconds after the beginning of the signal. The characteristics of near-fault records show that the rupture process consists of the growth of a smooth crack. The numerical simulation indicates that the 3 sec period ripples can be explained by a series of changes of the rupture front velocity. We examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in Mexico City. Our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in Mexico City. This interpretation leads to a very coherent scenario of what happened from the start of the failure on the fault up to the destruction in Mexico City. This example illustrates the need to consider simultaneously source, path, and site effects in order to understand strong ground motions.

Earthquakes in cities revisited

“…There remain many unanswered questions, the foremost of which concerns long-duration earthquakes in urban centers such as Mexico City. In the examples of the present paper, the durations of the shaking in the buildings or (connected or disconnected) blocks did not exceed 10 s whereas what has been recorded in Mexico City [7,9] and Sendai [12,11,36] are strong vibrations that last a few minutes. A clue to why this can occur has been provided in [70,71] concerning the factors that favor the coupling of seismic energy from its source to Love waves (which, like the Rayleigh waves, are manifestations of authentic resonant modes, in contrast to the interference waves of the type excited in horizontally-layered systems by plane waves that are only pseudo-resonant modes [21,25]), and therefore to ground (not loaded by added mass) responses of increased duration; this indication should give rise to a more global, although preliminary, study, employing the uniform city model in conjunction with the ICBA or boundary element methods of analysis.…”

“…In most studies of earthquakes in cities, it is assumed that the seismic disturbance takes the form of a vertically-incident (i.e., θ i = 0 • , the seismic source is laterally very near to, and vertically very deep under, the city) plane bulk wave. However, it is a well-documented fact [34], [51], [35], [11], [9], [7], [10], [45]) that many of the deadliest earthquakes in cities were due to laterally-far, and vertically-relatively-shallow, sources. To see how this affects the seismic response in the city, I simply chose incident angles θ i different from 0 • [63], [44], [15] (the authors of [41] even replace the plane wave by the wave radiated by a finite-fault source, but their site has no soft layer).…”

Resonant amplified seismic response within a hill or mountain

“…of, on account of the second VS condition µ [1] u [1] ,y (x, h 1 ) − µ [2] u [2] ,y (x, h 1 ) = 0 ; ∀N = 0, 1, 2, ... , (61) which shows that the continuity conditions on the segment of separation at y = h 1 are satisfied by the VS case solution for the diffraction coefficients.…”

“…In this section we show, via Figs. [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61], that this response can, in fact, be much stronger, even for a hill having a relatively-large aspect ratio w/h = 500 m/150 m = 3.333. Now, we assume that the hill is entirely-filled with a solid that is different from that of the underground.…”

“…Herein, we shall be particularly interested in the internal response of an isolated feature (looselytermed 'structure' [67,92,117,154,157,158,214,284,288,291]) located near or on the earth's nominally-flat boundary to an elastic wave (more precisely, seismic wave) whose sources are within the earth. The social and economic implications of this problem are considerable since they have to do with the disasters [141] provoked by earthquakes: destruction of the natural [103] and built environment [20,29,34,40,61,84,92,120,130,243,244,245,251,254,289] often resulting in considerable harm to the population.…”