The Colima earthquake (magnitude 7.5) occurred just inland from the Middle America Trench, 110 km south of the Volcan de Colima and 160 km southeast of Manzanillo, Colima, Mexico. Damage at several cities and towns was severe, 30 people were killed, and hundreds were injured. Four days after the earthquake, a six-station portable seismograph array was set up in the epicentral area as part of a cooperative program between UCSD, the University of Mexico, and the Mexican Federal Power Commission. From about 330 aftershocks recorded in the following 212 weeks, accurate locations were obtained for 50. One large aftershock had a magnitude of 6.2, the others range in local magnitude from 1.5 to 4.5. The locations outline a region approximately 90 km long and 60 km wide, in nearly the same location as the aftershock zone inferred by Kelleher et al. (1973) for the 1941 earthquake. The focal depth of the aftershocks (ranging from 2 to 30 km) and the fault-plane solutions for the main event indicated a shallow dipping thrust plane (about 30°). The seismic moment estimated from mantle rayleigh waves is 3 × 1027 dyne-cm. The pattern of aftershocks was used to estimate the source dimensions. From the moment and source dimensions the average slip was estimated to be about 1.4 m, corresponding to a stress drop of about 8 bars.

The occurrence of this earthquake is discussed in terms of the general seismicity of the Middle America Trench, the convergence rate predicted by plate tectonics, and the use of seismic gap theory for earthquake prediction. The fact that this earthquake may have been in the zone of the 1941 earthquake rather than in the adjacent seismic gap, suggests that caution must be taken in using seismic gap theory to predict earthquakes in the region. It further suggests that in the adjacent seismic gap a large earthquake may be eminent, and thus the gap may be an important area for deploying seismic instruments.

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