Abstract

GARDUNO-MONROY, Víctor Hugo et al. Zonification of natural periods of surficial oscillation in Pátzcuaro city based on microtremors and paleoseismological studies. Rev. mex. cienc. geol [online]. 2009, vol.26, n.3, pp.623-637. ISSN 2007-2902.

The study of the site effects in south-central Mexico has a vital importance due to the large seismic events in its history. The 1985 earthquake revealed fundamental aspects of site effects in a lacustrine zone. Along the Mexican Volcanic Belt (CVM), and especially in areas of lacustrine basin formation, site effects have caused damages in urban zones and a major impact on the population. In this paper we study the site effects in the town of Pátzcuaro, which is located within a series of E-O and NE-SW lacustrine basins of the Morelia-Acambay fault system. In Pátzcuaro, and in other sectors of the CVM, urban areas have developed over lava flows spilled from monogenetic cones. These lavas consist of alternating block lavas and volcanic breccia, all covering lacustrine sequences developed under these volcanic and tectonic scenarios. In this work, we first present the results of a geological and paleoseismological study, conducted in the south margin of Lake Pátzcuaro, emphasizing structures with evidences of prehistoric and historical seismic activity. The results suggest that the area was subjected to various seismic events, and one in particular, generated during the Pleistocene (M = 7.3), caused the collapse of the northern flank of the El Estribo volcano and an associated rock avalanche deposit. In 1858, central Mexico and especially the towns of Pátzcuaro and Morelia suffered the greatest seismic event. In order to properly characterize the response to seismic events, we conducted an evaluation of site effects using ambient vibration caused by natural sources and human activities that lead to microtremors. We measured spectral ratios of horizontal and vertical components (H/V) of microtremors with the method of Nakamura (1989) to estimate dominant periods. The results show that, despite the rocky substratum of Pátzcuaro, there exist a distinctive response on the central and the eastern portion of the town, with dominant periods of 0.2 to 0.6 seconds. These periods represent a potential risk of resonance for existing buildings in this area. Finally we present a seismic hazard map, which takes into account the geological, historical and prehistorical seismic events and the results of the seismic zoning.

Keywords : seismic zoning; site effect; lacustrine basin; seismic hazard; Pátzcuaro; Michoacán; Mexico.

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