GEOLOGIC SETTING OF LANDSLIDES TRIGUERED BY THE 5 JUIN, 1996 EARTHQUAKE AT THE ZIMAPAN DAM. CENTRAL MEXICO

GEOLOGIC SETTING OF LANDSLIDES TRIGGERED BY THE 5 JUIN, 1996 EARTHQUAKE AT THE ZIMAPAN DAM AREA. CENTRAL MEXICO.

This talk presents reconaissance field studies of the surficial geologic effects induced by the M 4.3 earthquake centered at Bellavista, Querétaro State. The purpose is to describe the tectonic setting, types and distribution of the earthquake induced landslides. Earthquake occurred on the Laramide el Doctor thrust close to the linked Aljibes half graben master fault within the boundary between the Sierra Madre Oriental fold and thrust belt and active normal faults crosscutting rocks of the trans-Mexican volcanic arc. The main event triggered numerous landslides throughout the region principally around the artificial lake. Types of landslides included rock falls, rock slides, disrupted soil slides, and reactivation of existing slides. The highest concentration of landslides, including two reactivations, were on steep slopes near the surface expression of the faults A-D of the Aljibes half-graben. These landslides were mainly falls and slides involving Cretaceous shales, and Miocene tuffs. The overwhemling majority of the slides occurred along the steeper slopes of the deeply incised drainages in the upland plateau. Some of the larger slump blocks and rotational slump observed contain some thousand cubic meters of material. The SMO structures were active mainly during Paleocene, but presently are considered inactive. Whereas normal faults are considered active because they offset Quaternary conglomerates. We interpret foci location as collapse of the SMO topographyc culmination causing extension inherited from the compresive phase.