High-precision earthquake locations in Switzerland using regional secondary arrivals in a 3-D velocity model

Abstract

We present a new approach to relocate earthquakes in the greater western Alpine region using main crustal phases (Pg, Pn, PmP) that takes advantage of recent developments in P-wave velocity models and modelling of the Moho topography in the region, as well as the ability to track reflected and refracted phases in three-dimensional (3-D) heterogeneous media. Our approach includes a new 3-D P-wave velocity model for Switzerland and surrounding regions that combines a first-order Moho discontinuity based on local earthquake tomography (LET) and controlled-source seismology (CSS) information and 3-D seismic velocity information based on LET. Traveltimes for the main crustal phases (Pg, Pn, PmP) are computed using a fast marching method. We use a non-linear, probabilistic approach to relocate earthquakes that has been extended to include the use of secondary phases. We validate our approach using synthetic data, which was computed for a real earthquake and different combinations of available phases (Pg, Pn, PmP). We also applied our approach to relocate four selected earthquakes, two shallow and two deep crustal events in the northern Alpine foreland, for which independent information (ground truth information) on their focal depths exist. Our results demonstrate that the precision and accuracy of focal depth estimates can be greatly improved if secondary phases are used. This gain is a combined effect of an improved range of take-off angles and the use of differential traveltimes between first and secondary arriving phases. Our results also show that reliable information on the Moho depth is crucial to obtain accurate focal depths, if Pn or PmP phases are used in the relocation process. Finally, our approach demonstrates that proper identification of the main crustal phases in combination with an appropriate model parametrization in the forward solver will significantly improve earthquake locations.