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Study of the propagation and amplification of seismic waves in Caracas Valley with reference to the 29 July 1967 earthquake: SH waves

DEPARTMENT OF CIVIL ENGINEERING RENSSELAER POLYTECHNIC INSTITUTE, TROY, NEW YORK 12180-3590

Abstract

The antiplane response of a 2-D model of the valley of Caracas, Venezuela — a NS cross section through the Palos Grandes district — to seismic excitations such as those due to antiplane line (point) sources and incident plane SH waves is investigated. It is found that the steeply sloped northern edge of the valley is a more efficient generator and reflector of surface waves than the midly sloped southern edge. Also the spectral amplification and time response characteristics of this asymmetric wedge-shaped valley are a function of the direction of incidence of the excitation waves (e.g., incidence from the north versus incidence from the south versus vertical incidence).

The same 2-D model of the valley is used to simulate also the antiplane response of the valley to synthetic strong motions equivalent — in amplitude, frequency content, and duration — to those that most likely were generated by the 29 July 1967 Caracas earthquake. This event has been found to consist of four subevents, of which the second subevent — located at a distance of 25 km from the valley — caused the damage in Caracas. The source spectrum of each of the subevents used in the generation of the synthetic time histories was assumed to be a simple -square model. The locations, relative times of occurrence, seismic moments, and mechanisms of the subevents were available from an inversion study of teleseismic data. It is found that peak acceleration was fairly uniform across the valley, varying from 0.15 to 0.21 g. On the contrary, peak velocities and peak rotational (torsional) strains varied considerably (factor of 2 to 3) across the valley, with the maximum values being 25 cm / sec for velocity and 1.5 x 10^–4 for rotational strain. For sites in Palos Grandes, spectral accelerations S_a reached values as high as 0.4 g for periods in the range 0.4 T 1.4 sec and damping ratio 5%. Our estimates of the intensity of motion on rock and on the sediments are higher by a factor of 3 compared with estimates proposed by previous investigators, who assumed that all the seismic energy was released from a point located at a distance of 55 km from Caracas. Various pieces of evidence (e.g., seismoscope response recorded on rock) are presented that suggest that the intensity of motion was at least as high as estimated in this paper.

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