Abstract

The Mw 8.0 earthquake of 9 October 1995 occurred within the Jalisco block that represents the northern part of the Mexican subduction zone where the Rivera plate subducts beneath the North America plate along the Middle American trench. The source rupture of the 1995 low-angle thrust-type event was complicated. The studies of rupture history had shown that the slip occurred within an area of about 180 × 90 km2 along the Middle American trench at the depth interval from 9 to 33 km. The rupture duration was estimated in interval from 55 to 62 sec. At least three main asperities ruptured along the fault plane.

The earthquake was felt along the 600-km coast of the Mexican states of Colima, Jalisco, and Michoacan and in the continental part of Mexico. The macroseismic study presented in this article was carried out in March through July 1997, more than one year after the earthquake. We had about 300 interviews with people who felt the earthquake in their houses located in 56 cities and towns of the states of Colima, Jalisco, and Michoacán. All our estimates of the earthquake intensity were referred to the intermediate type of masonry situated on the intermediate type of soils. For this purpose, corrections were introduced for type of soil and masonry.

The study of macroseismic effects related to the 1995 Jalisco earthquake allows description of some properties of the macroseismic field generated by the earthquake. (1) Three zones of intensity 4, 5, and 6 to 7 grades of MM (Modified Mercalli) scale were distinguished. (2) The zone of maximum intensity of 6 to 7 MM was heterogeneous. This heterogeneity was in accordance with the rupture asperities distribution. (3) The study of intensity attenuation along the coast had demonstrated the asymmetry in intensity distribution according to the epicenter. The maximum intensities were observed for the sites that were situated to the northwest of the epicenter. (4) A good correlation was observed between the observed intensities and the values of GPS displacements.

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