Modeling crystal phases in southeast France for focal depth determination

Abstract

In numerous regions, the focal depths are not precisely determined from standard location methods because of the poor azimuthal coverage by the seismic network or because of the important geological complexity. In this study, we attempt to refine the determination of the focal depth of some events by analyzing the waveforms of the seismograms and the relative amplitude of the crustal phases. The discrete wave number representation method is used to compute synthetics corresponding to sources at various depths. We present three examples of events which occurred in southeast France in different geological units. We conclude that the waveforms are not very sensitive to slight changes of the crustal model or quality factors, but are very dependent on the focal depth. If a convenient crustal model is used in the computations, the numerical simulations constrain the depth in a better way than standard location methods using arrival times. In our examples, when the path from source to station crosses different structures, the best fit of the envelope of the seismograms by synthetics computed in a flat-layered model is reached for a crustal model corresponding to the source region.