La secuencia de temblores del Valle de Toluca, México de agosto 1980

Abstract

Twenty nine small earthquakes in the magnitude range (ML) of 2.2 to 3.5 were located in the Valley 9fToluca, Mexico (19°12'N; 99°30'W) from August 18 to 31, 1980. This Pliocene plain part of the Neovolcanic Belt, is an area of characteristically low seismicity. The seismic activity is confmed to a zone 8 x 4.5 km. Focal depth of earthquakes vary from S to 23 km. The zone of seismic activity shows clearly two concentrations of earthquakes separated by a plane oriented E-W at 19°10'N. Events in the north clustering are deeper (18-23 km) as compared to events in the south (5-14 km). The distribution with depth of seismic foci suggest the existence of a number of blocks that step down toward the north. Probably the observed separation boundary at 19°10'N mark the contact zone between blocks. Even though it ~as not possible to obtain a unique composite fau1t plane solution the analized solutions are consistent with a normal faulting type.