Influence of forward modelling in locating earthquakes in three-dimensional media: Application to the Durance Network in the South of France

Anthony Lomax and Jean Virieux
UMR Geosciences Azur, 250 Rue A. Einstein, 06560 Valbonne, France

Philippe Volant and Catherine Berge
IPSN, BP 6, 92265 Fontenay-aux-Roses, France


The seismic activity in the region along the Durance river in the South of France has increased significantly the last two years. In this period there have been numerous felt earthquakes. As an aid in assesing the earthquake hazard in this tectonically active area, it is important to have both accurate earthquake locations and a good understanding of the relative and absolute uncertainties in these locations. In addition, because of the structural complexity and large velocity contrasts in this region, it may be important to use earthquake location procedures valid for 3D heterogeneous media.

Using the recordings from a recently installed network of 13 seismic stations in the Durance region and a velocity structure obtained from seismic profiles, we investigate different methods to obtain travel times and earthquake locations in 3D media.

We first compare various forward modelling methods to obtain traveltimes between each station of the network and all nodes of a regular, spatial grid. We examine an Eikonal Finite Difference scheme (EFG), a Ray Tracing code (RT) and other approaches. The EFG method produces only a single traveltime at each node for each phase but includes rays refracted at sharp interfaces. In contrast, the RT approach allows multiple traveltimes at each node but is only valid for smooth models and consequently only produces direct waves. Once traveltimes have been estimated for all stations and all nodes on the grid, we use a grid search over all nodes to obtain an optimal, preliminary location for each earthquake. Next, we compare linear and non-linear optimization approaches to further refine these preliminary locations and to obtain estimates of the uncertainties and trade-offs in the location parameters. Finally, we compare the 3D earthquake locations and uncertainties with those for 1D structures and we discuss the expected precision in earthquake location inside and outside the network.