Tectonic synthesis and seismic risk along the Rio Grande de Santiago fault, in Jalisco, Mexico

Summary

Various kinds of geological evidence has recently been used to show the presence of a master fault along the course of the Rio Grande de Santiago (RGS). Triangulation data of small motions of survey monuments located in the neighbourhood of the Santa Rosa Dam for the period 1964–1981, show a pattern that closely matches the shape and orientation of the strain ellipsoid deduced previously from the regional structural framework. These and other observations on the mode of fracturing of both margins, and the instability of the wall rocks, indicate that the fault is active and therefore potentially dangerous to civil constructions. Petrologic parameters, sustained by major element chemical analysis and Strontium isotope data support the hypothesis of emplacement of nepheline bearing mildly alkaline basalts through fractures controlled by a complicated pattern of synthetic and antithetic faults, and a pull-apart basin development along the right lateral fault of the RGS. The area does not have a seismic history like the southern continental margin of Mexico (i.e. southern Jalisco, Guerrero, Michoacan, Oaxaca and Chiapas), yet in February 11, 1875, an earthquake of estimated magnitude 7.5, had its epicentre located in Zapopan, Jalisco, a village immediately adjacent to Guadalajara City, and only a few kilometres to the southeast of the study area. After that the region has remained relatively free of seismic activity.

An important conclusion is that if the motion of two large blocks sliding past each other along the surface of the fault has been aseismic, it implies the possible accumulation of stress at depth.

Seismic risk estimation should therefore be considered at a yellow alert, based on the following features:

• (a) Probability of occurrence is high, but at the moment the data is too ambiguous to permit a short term prediction.

• (b) Vulnerability of human settlements, production industries and civil infrastructures includes the villages of Santiago Ixcuintla, Villa Hidalgo, Villa Juarez, and small ranches along the course of the river, and access roads to the San Pedro Analco Mine, to the Dam of Santa Rosa, and of course the Dam itself.

• (c) Value of social and economic infrastructures is therefore very high.

It is recommended to establish a close survey of microseismic activity, a high precision triangulation and studies of paleoseismicity. A civil protection plan should therefore be carefully carried out to prevent a major disaster.