Seismic Hazard at Sites of the Campeche Bay for the Risk Assessment of PEMEX Offshore Facilities

摘要

This paper describes the seismic hazard study for four areas (represented by four typical sites) located at the Campeche Bay Zone (CBZ). The procedure utilized makes use of bayessian statistics to determine the rate of exceedance of the maximum ground intensity, at the mentioned sites. As the region might be potentially affected by three types of earthquakes, their source-site propagation characteristics are introduced through appropriate attenuation relations. The detailed features of the ground motions expected at the mentioned sites are obtained from site specific wave propagation analyses. The uncertainties on the values of the dynamic characteristics of the soils of the four sites are included in the study. The seismic design spectra for return periods of 200, 1,000 and 10,000 years (corresponding to long distance, large magnitude, surficial depth, subduction type I events; to intermediate distance, medium magnitude, intermediate depth, subduction type II earthquakes; and to close distance, small magnitude, cortical surficial type III events) were obtained for the four representative* areas of the region of interest. The expected peak acceleration and the design acceleration spectra for each of the four areas were computed at depths of 12m for the horizontal component. The main conclusions of the study are the following: The incorporation of the uncertainties on the soil dynamic properties of the CBZ soils, expressed in terms of the variation with the shear strain of the coefficient of variation of the ratio G/Gmax (where Gmax is the maximum shear modulus) and the soil damping ξ showed that it is not correct for the CBZ soils to use a constant coefficient of variation for the full range of shear strain(10{sup}-4 to 10), for those soil parameters. The seismic hazard at the four typical sites of the CBZ estimated in terms of the expected peak acceleration and the acceleration response spectra of the horizontal component at a depth of 12m depends on the return period (Tr) of interest (200, 1,000 and 10,000 years), on the type of earthquake considered (Type I, II, III), and on the soil uncertainties of the CBZ soils. For Tr=200 years the Type II earthquakes largely dominates the hazard, followed by Type III and Type I in that order. The soil sites included in the study behave basically in their lineal range. For Tr=1,OOO years the expected maximum accelerations for types II and III are similar; and the smaller values are again for Type I events. For the acceleration response spectra values, the Type II and Type III events dominates the hazard at the considered sites, followed by Type I earthquakes. In this cases a non lineal behaviour of the soil layers of the different sites was observed. For Tt=1O,OOO years the hazard is not longer dominated by Type II events if it is expressed in terms of the expected peak acceleration, instead Type III earthquakes provides the largest values, and the smaller ones are again those associated to Type I events. If the hazard is examined in terms of the response spectra, the observed hazard is similar for Types II and III events. An important non lineal behaviour of the soils was observed for this case. The seismic design acceleration spectra proposed here were obtained based on the IS0 guidelines (Craig, 1996) for the shape, and for their ordinates based on the site specific response studies performed in this work. The proposed design spectra consist on sets of spectra for the return periods of 200, 1,000 and 10,000 years, and each set includes the mean, mean + standard deviation (σ), and an envelope spectra (roughly equivalent to the mean + n σ, with n>= 2). The proposed design spectra can be divided in two groups, one which includes sites 2 and 7 and the second the sites 5 and 6 (Fig. 1). The former have a first descending branch at 1.0sec, while the latter at 0.7sec. The maximum ordinates of the envelope spectra are of 0.25, 0.35, and 0.50g, for 200, 1,000 and 10,OOO years of return period