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.
The magnitude 5 Epagny–Annecy earthquake of 1996 July 15 is the largest seismic event to have occurred in the Alps since the introduction of modern digital instrumentation. This strike‐slip event was located on the Vuache Fault, near the town of Annecy, in the northern French Alps.
The aim of our work was to retrieve the main parameters of the rupture process of this earthquake from seismograms recorded at local and regional distances (20–300 km). To eliminate path and site effects from the seismograms, we compared the main shock recordings at each station with those of the largest aftershocks nearby. We used a combination of techniques, including pulse‐width measurements and cross‐correlation of velocity traces, comparison of P‐wave displacement pulses, and empirical Green’s function deconvolution, to retrieve the apparent duration of the rupture process as seen at each station. Our results demonstrate that, in the absence of on‐scale data, P‐wave pulse‐width measurements on clipped signals can be misleading if the rupture process is complex. In the case of the Annecy earthquake, comparisons of on‐scale P‐wave displacement seismograms and the empirical Green’s function deconvolutions show that the rupture process consisted of at least two subevents separated by 0.2–0.3 s, and with a total duration of about 0.5 s. The systematic azimuthal dependence of both the shape and duration of the apparent source‐time function is consistent with a nearly unilateral propagation of the main rupture phase in a southeast direction along the fault plane and parallel to the direction of slip. An isochron analysis reveals that the first subevent occurred slightly to the northwest of the nucleation point but that the second subevent was located further to the southeast, thus confirming the overall rupture directivity towards the southeast. An interpretation of our results in light of the previously documented aftershock distribution and of observations of ground cracks in the epicentral area suggests that the main shock occurred on the Vuache Fault, and that rupture in a northwest direction was inhibited by a right‐lateral stepover in the fault. Accordingly, the vast majority of the subsequent aftershocks, which include several magnitude 3–4 events, occurred on a fault segment that is slightly offset from the inferred surface trace of the Vuache Fault and that was activated by the main shock.
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