HR: 17:35h
AN: S24A-06    [Abstracts]
TI: Geophysical studies for the identification of basin effects in urban areas in Venezuela
AU: * Schmitz, M
EM: mschmitz@funvisis.gob.ve
AF: FUNVISIS, Venezuelan Foundation for Seismological Research, Caracas, Venezuela
AU: Rocabado, V
EM: vrocabado@funvisis.gob.ve
AF: FUNVISIS, Venezuelan Foundation for Seismological Research, Caracas, Venezuela
AU: Sánchez, J
EM: jsanchez@funvisis.gob.ve
AF: FUNVISIS, Venezuelan Foundation for Seismological Research, Caracas, Venezuela
AU: Reinoza, C
EM: creinoza@funvisis.gob.ve
AF: FUNVISIS, Venezuelan Foundation for Seismological Research, Caracas, Venezuela
AU: Amaris, E
EM: eamaris@funvisis.gob.ve
AF: FUNVISIS, Venezuelan Foundation for Seismological Research, Caracas, Venezuela
AU: Cornou, C
EM: Cecile.Cornou@obs.ujf-grenoble.fr
AF: LGIT, Grenoble, France
AB: Urban areas in northern Venezuela are subject to a moderate seismic hazard due to the interactions between the Caribbean and south American plates, which has been evidenced by historical damaging earthquakes as for example the 1812 and the 1967 earthquakes with a magnitude of 7.2 and 6.5, respectively. Strong damages in Caracas during the 1967 earthquake have been asociated to site effects produced by the sediment filled basin. This situation can be observed in most of the big cities in northern Venezuela, which initially developped on plain areas with quaternary basin fills of up to 500 m within mountainous areas, as for example Caracas, Maracay and Barquisimeto. In the mid- 1990ies FUNVISIS started to promote geophysical studies to investigate the shape and the properties of the basin fills in order to contribute to the earthquake disaster reduction. Methods applied for the investigations are gravimetry, microtremor measurements, seismic refraction, among others. In Caracas, a total of 350 m of cuaternary sediments with an average S-wave velocity of about 850 m/s have been derived by seismic investigations. The corresponding predominant periods from microtremor measurements amount up to 2.2 s. Integrating drilling information and 3D gravimetric modeling a detailed picture of the bedrock - sediment interface could be obtained. Results from numerical modelling as well as from experimental transfer function indicate amplifications point to amplifications of a factor of more than 10 related to the deep basin area. In Barquisimeto Metropolitan Area, sediment thickness reaches up to 500 m in the fast growing Cabudare area. Actually, modelling of a recent seismic refraction campaign is in progress, but predominant periods up to 3.0 s in the deepest part of the valley and gravity modelling point to the same order of cuaternary sediments. In other cities, as for example Carora and Mérida, geophysical studies are in progress, first with gravimetric and microtremor measurements which point to sediment with more than 150 m thickness. The subsoil information from geophysical studies will be used to define the distribution of microzones of equal seismic response in order to determine PSHA spectra. Contribution to projects FONACIT 200400738 and FONACIT-ECOS Nord 2004000347.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 9360 South America
SC: Seismology [S]
MN: 2007 Joint Assembly