Study of the aftershocks of the 7 December 1976, Mesa de Andrade, earthquake (ML = 5.3, MS = 5.7) on the Cerro Prieto fault in the Colorado River Delta, indicates a simple strike-slip transform fault mechanism. The fault plane defined by aftershocks is essentially vertical with a surface projection 1.5 km southwest of the surface trace of the Cerro Prieto fault. The main shock was complex and consisted of two large events. Aftershock hypocenters are distributed in two clusters separated by a gap suggestive of an unbroken barrier.

Station corrections of about 0.7 sec are indicated for arrival-time locations using stations both on the deltaic sediments and on the surrounding basement outcroppings (assuming a flat-layered velocity model). Differences of some 10 to 15 km are found between epicenters obtained using only United States stations and epicenters obtained with the addition of Mexican local stations.

Foreshock activity, recorded on a portable seismic network, was located in a zone coincident with the northern aftershock cluster. The abrupt onset of foreshock activity emphasizes the importance of continuous monitoring of the Colorado River Delta area in any scheme of earthquake risk studies.

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