Some notes on the ground acceleration of the Caracas earthquake of july 29, 1967

Autor: Guenther Fiedler B.

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Abstract:

The Caracas Earthquake was not recorded by Strong Motion Accelerographs, but by a Seismoscope and a mechanical Seismometer, which records the two horizontal components on smoked paper, with a peak magnification of 9,5 for ground periods of 0,8 sec. Due to the fact that there are no records for shorter periods, the evaluation of the ground oscillations and explanations of the very irregular distribution of damage are complicated. In this paper, by four different approaches it is tried to obtain the ground acceleration (for bedrock only) which was originated by the Caracas main Earthquake. From the record of the Seismoscope we obtain 12 gals for the resonance case, which corresponds to a ground period T. = 0,7 sec. For periods shorter then 0,4 ~ 0,3 sec., accelerations between 70,and 120 gals may be evaluated, applying recorded amplitude and direction of oscillation, but assuming periods in agreement with the period distribution in the records from aftershocks. With a measured predominant period T, = 0,25 sec. for the bedrock in the surroundings of the Seismic Institute Caracas, the equation of KANAIT, K. [1966] for maximum ground acceleration gives around 60 gals for a Richter-Magnitude of 6,3 and a hypocentral distance A’ = 90 kms. In a third approach, the line spectra of small local Caracas-aftershocks with known Magnitude and (S-P) = time are transformed by a Magnitude factor to higher values. Converting the Fourier coefficients with the corresponding periods into acceleration of the ground yields results which are in agreement with the other approaches. The last approach consists in the evaluation and proves of the seismogram of the strong motion seismometer CAR, for which the previous results are indispensable. Due to this, the seismogram was not included in my paper “Estudio Sismológico de la Región de Caracas con Relación al Terremoto del 29 de Julio de 1967’ Boletín N° 23-24, IMME, Caracas. The results show good correlation with that of the other approaches and are summarized in the last two figures.

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