Validation of Teleseismic Inversion of the 2004 Mw 6.3 Les Saintes, Lesser Antilles, Earthquake by 3D Finite-Difference Forward Modeling

Abstract

We study the 21 November 2004 Mw 6.3 Les Saintes earthquake that occurred south of the island of Guadeloupe at a shallow depth, damaging some buildings on the island. The objective of this work is to assess the potential of a teleseismic source study to reproduce local ground motion. The velocity model of this area is not currently well known, and the near-field seismograms are affected by paths and site effects. We first analyze this earthquake as a point source and then as an extended fault using teleseismic broadband waveform inversion approach from the P and SH waveforms at stations well distributed in azimuth. We obtain two stable and reliable solutions of the spatiotemporal history of the earthquake rupture on each nodal plane. The models show a rupture duration of 6 sec and a maximum slip of about 1 m but have different locations of the main asperity. We then carry out the forward modeling of the seismic wave propagation at the regional scale, coupled with the obtained kinematic source models taking into account the topography, the bathymetry, and the sea layer. We investigate if one of the fault models could produce synthetics that match the local observed ground motions. The simulation results provide a better understanding of the uncertainty of source, path, and site effects even if our knowledge of the underground geological structure is limited to the 1D stratified model. It is inferred that the 1D model does not seem to be appropriate in the northern direction (middle of Guadeloupe Island) due to the unknown basin structure, while it is relatively sufficient in the eastern and the northwestern directions from the source. By careful comparison of the synthetic and observed displacement seismograms in terms of main phase forms and arrival times at low frequency, especially at the closest station GBGA, the fault model with northeast faulting is estimated to be the more relevant to the measured local ground motions.