A simple approach to constrain the position and the geometry of seismogenic structures: Application to the Karthala volcano (Grande Comores Island, Mozambique Channel)

A simple and quick approach is presented to constrain the position and the geometry of seismogenic structures using hypocentre locations and their associated uncertainties. It assumes that, for a given hypocentre, all other seismic events which are located inside its uncertainty ellipsoid are localised on the same structure. These can then be considered as independent measurements of the same variable which implies, from basic statistics, that the best estimate of the variable is the centroid of all these hypocentres. It is shown that the approach is robust and that it can successfully be applied even when location uncertainties are not well-constrained. When applied to the Karthala volcano (Grande Comores Island), the method, called the Best Estimate Method, constrains the direction of the main active structure. This is a plane oriented N165 degrees and dipping close to the vertical. This result, which is in agreement with recent studies of the Karthala volcano, demonstrates the potential of this technique. It is believed that such a process could advantageously be integrated in preliminary analysis of seismological monitoring.