Determinación de períodos fundamentales del suelo de la ciudad de Mérida, Venezuela, a partir de mediciones de ruido sísmico ambiental

Abstract

The Venezuelan Andes represent a high seismic hazard region due to the presence of the right-lateral strike-slip Bocono fault, with an extension of more than 400 Km., which affects the main cities of the mountain range. Thus, the Venezuelan Foundation for Seismological Research, with the collaboration of the different universities in the country, is undergoing several studies of seismic microzonation in different Venezuelan cities. As a part of this project, it was necessary to determine the fundamental periods for soft-sedimentary (soil) deposits of the city of Merida, based on measurements of ambient vibrations applying the Nakamura Technique. In order to reach this goal, the Nakamura Technique was applied to 164 measurements taken in different parts of the city with an approximate spacing of 500 meters between each station. The LabView and J–Sesame softwares were used to process and interpret the data. After the H/V spectral ratio was determined, a period’s distribution map of the city was created, where the areas of high seismic hazard are pointed out in correlation with the site’s geology. Proterozoic and Palaeozoic associations, Carboniferous-Permian, Jurassic and Tertiary formations and sediments belonging to the Pleistocene alluvial terrace of Merida were found. On the other hand, a proportional relationship between the period and the thickness of fan’s sediments was defined, where the highest ranges in periods correspond with the biggest thickness in the terrace, in agreement with the medium velocity and possible acoustics impedance contrasts. The values of thickness were obtained from the geophysical modeling project of the city´s metropolitan underlying basement area after processing and interpreting the gravimetric data. The ranges of periods that were determined for the city of Merida oscillate between 0,087 and 1,000 seconds.