Results From Seismic Data and Implications for Soil Dynamics Near Hospital Dr. Antonio Patricio De Alcalá, Cumaná, Sucre State, Venezuela

Abstract

The trace of the El Pilar fault crosses the city of Cumaná, and is located between the Caigüire hills and the main building of the Hospital Dr. Antonio Patricio de Alcalá, located only 100 m north of the hill foot. The existing geological interpretations are contradictory with respect to the exact location of the fault trace. Permanent deformations to the hospital structure might result in the case that a coseismic rupture would be located beneath the hospital. In order to better constrain the seismic hazard of the hospital, a 11-story building dating from the 1960's, seismic refraction measurements have been done around the hospital. The results indicate that the trace of the El Pilar fault should be located more than a 100 m south of the principal building, at the foot of the Caigüire hills. Through the analysis of the S-waves, the existence of soft soils, interpreted as Quaternary sediments from the coastal alluvial plain or sand barriers, was identified with seismic velocities of 150 and 280 m/s down to a depth of 10-20 m. Below, a layer with velocities between 325 and 520 m/s reaches down to 25-65 m in depth, followed by a layer with S-wave velocities of 460-700 m/s, interpreted as more consolidated Plio-Pleistocene sediments. The analysis of the models indicates, that the response spectra for very close, close and distant earthquakes generate maximum accelerations at the surface of the geotechnical soil profile that may double (in the most unfavorable case of very close earthquakes) the value of 0.40 g, established as the maximum acceleration (at rock surface) for the region in the seismic zoning map of the current seismic building code. Based on these results, additional geological and geophysical studies are recommended, aiming to better constrain the location of the active fault trace as well as the seismo-resistant behaviour of the hospital structure.