From Geology to Realistic Large-Scale Spectral-Element Earthquake Simulations in the Pointe-à-Pitre Region
Résumé
The goal of this study is to perform realistic large-scale spectral-element simulations in the vicinity of Pointe-à-Pitre, valid up to rather high frequencies (around 5 Hz) with a minimal shear-wave velocity around 200 m/s and considering magnitude 5 to 6 earthquakes. For this purpose, a detailed modeling of both the seismic source and geology of the medium for wave propagation and of the local urban underground is necessary. The local geology (size: 7 x 7 x 0.1 km) has been constructed using brgm Geomodeler© software, by compiling the available underground data (e.g. boring data) in the vicinity of the city of Pointe-à-Pitre. Four main geological formations are considered for the regional model: a weathered limestone (which form the substratum) and clay, silt and ballast (which form soft sedimentary basins). The shear-wave velocities within each formation have been derived from the existing geophysical campaigns (e.g. SASW, cross-hole, H/V, etc.). The S-wave velocity value of the limestone is supposed to be 1000 m/s. For the clay, silt and ballast formations, S-wave velocity values are respectively equal to 350 m/s, 250 m/s and 200 m/s. In order to mimic energy dissipation, the constitutive law of these formations is supposed to be viscoelastic. The local geology of the urban zone is included into a larger geological model (Dorel, 1978) of size 40 x 30 x 30 km. Moreover, topography is included in the model by using a 40 meters DTM. In the present simulations, the Gosier fault, located to the Southeast of Pointe-à-Pitre, has been considered. The slip distributions of the finite-fault are generated according to the spatial random field model developed by Mai and Beroza, 2002. For magnitude 5 or 6 earthquakes, the kinematic fault is composed respectively by 899 or 7881 sub-faults with a heterogeneous slip distribution. The results of these simulations are shown as PGV map on the entire domain or time history recorded at high stake facilities of Pointe-à-Pitre (e.g. city council, hospital, schools, etc.).