abstract

Along the Pacific Coast of Mexico and Central America, 26 local tsunamis have been reported during the period 1732 to 1973. Nine of these were caused by earthquakes with teleseismic hypocenters, all of which were located well inland. If these epicenters were correct, these earthquakes could not have generated tsunamis. Under the assumption that the true epicenters must have been located at the coast or off shore, it was estimated that teleseismic hypocenters in this area are mislocated by about 75 km toward the northeast, and 20 km toward greater depth. We propose that most teleseismic locations in this area are afflicted by this same error. The most likely cause for the mislocations are shorter than expected travel times for rays in the down-dip direction of the subducted lithospheric slab. These rays travel to North American stations which contribute strongly to hypocenter locations in Middle America.

The annual mean sea level of 13 tide gauge stations along the Pacific coast of Mexico and Central America were examined for evidence of vertical crustal deformation changes that could have been associated with earthquakes along this coast. Only one coseismic change could be identified in the annual mean sea level data. It occurred at Acapulco, Mexico, during the 11 May (Ms = 7.0) and 19 May (Ms = 7.2) 1962 earthquakes. The crustal uplift was about 22 cm, estimated from the difference of the 10-yr sea level means before and after the events. By comparing annual mean with daily mean sea level data, it appears that about 23 per cent of the permanent uplift observed at Acapulco was due to aseismic slip or aftershocks in this area. If tide gauge data in this area are kept current, long-term precursory crustal movements might be detectable if they exceed several centimeters.

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