Soft-sediment deformation structures induced by seismic activity in the San Pedro el Alto area, Acambay graben, Mexico

Abstract

Soft-sediment deformation structures identified in lacustrine sediments of the San Pedro el Alto area, within the Acambay graben, Mexico, can be attributed to Pleistocene-Holocene seismic events. The analysis of these deformation structures, seismites, is a key tool for the characterization of parameters such as the intensity and magnitude of the events, and the measurement of the affected area. Analysis of the shape, distribution, abundance and genesis of these secondary structures in lacustrine basins, also provide guidelines for assessing seismic hazard.
 The analysis of seismites enabled the identification of three M ≥ 5 earthquakes, minimum magnitude required for the formation of such structures, these on ESI 2007 scale (Environmental Seismic Intensity Scale) correspond to VII-X intensities. The structures were identified with coseismic stratigraphy analysis in three trenches and three sediment cores from the dam within the locality. The basin infill consists of lacustrine sediments with intercalation of volcanic material. The soft-sediment deformation structures appear in different sectors of the study area and consist of surficial ruptures, vertical displacements, slumps, pseudonodules, clastic dikes, cracks filled with recent material, and diapir-like and load-flame structures.