Earthquake Relocation in the Middle Atlas: Apparently-Deep Events Resolved to be Shallow

Abstract:

The occurrence of intermediate depth seismicity in intracontinental settings is rare, but it has been postulated with various degrees of certainty in several regions. One such region is the Middle Atlas of Morocco. In the last twenty years, more than a dozen intermediate depth earthquakes were reported in this region by Spain’s National Geographic Institute (IGN). The apparent deep nature of these events is hard to reconcile with well-established geophysical evidence of a thin lithosphere under the Middle Atlas. We relocate 4 events with IGN-reported depths greater than 80 km that were recorded by a relatively dense temporary deployment; using a recent regional 3D velocity model obtained through teleseismic body and surface wave tomography. The relocation procedure uses a grid-search approach to minimize the mean normalized misfit, where each travel-time misfit is normalized by the estimated pick uncertainty. We find that our observed arrivals are much better fit by shallow (<5 km) depths than the reported depths of >80 km. We propose that these shallow foci earthquakes are the result of regional crustal deformation of this region caused by the present convergence between the African and Eurasian Plates. This study is an example of how local earthquake locations can be significantly improved by using a well-constrained 3-D velocity model and a dense local seismic array.