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The October 6, 1974 Acapulco earthquake: An example of the importance of short-period surface waves in strong ground motion

UNIVERSITY OF CALIFORNIA, SAN DIEGO INSTITUTE OF GEOPHYSICS & PLANETARY PHYSICS SCRIPPS INSTITUTION OF OCEANOGRAPHY, LA JOLLA, CALIFORNIA 92093
INSTITUTE OF ENGINEERING UNIVERSIDAD NACIONAL AUTONOMA DE MEXICO, MEXICO20, DF Mexico

Abstract

The Acapulco earthquake of October 6, 1974 (m_b = 5.0, M_s = 4.75) resulted in 0.5 g accelerations in Acapulco at an epicentral distance of about 35 km. Extrapolation of the peak acceleration to the source region gives a near source acceleration of at least 1.0 g. If the teleseismically estimated source depth of 51 km is assumed, the Acapulco accelerogram must be interpreted as composed of primarily body waves. This assumption yields a moment estimate of 3.3 x 10²³ dyne-cm and a stress drop of 1.5 kbar. However, strong evidence indicates that the source depth is only about 1.0 km and that the record is composed mainly of high frequency (1.0 to 4.0 Hz) surface waves. The character of the record is that of a normally dispersed surface wave. The relatively simple form and high acceleration may be attributed to the high rigidity, crystalline rock types in the region. The three component record is fitted by summing the fundamental and first higher mode Rayleigh and Love waves using a model consisting of a single layer over a homogeneous half-space. The results are also checked using a direct wave-number integration program developed by Apsel and Luco. The moment estimate from the surface-wave synthetics is 2.0 x 10²³ dyne-cm.

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