Seismotectonic model of High-Middle Atlas Junction (Morocco) derived from earthquake focal mechanism and stress tensor analysis

In this study, we attempted to derive a seismotectonic model for the High-Middle Atlas Junction area, in the central part of Morocco, by determining new earthquake focal mechanisms and tectonic stress tensor parameters. For that, we have analyzed high-quality seismic waveform data of moderate earthquakes that were collected by local broadband seismological networks between the time period from 2008 to 2014. The focal mechanisms of well-relocated earthquakes were determined using the P-wave first-motion polarities that were retrieved from the earthquake seismograms. The tectonic stress parameters were then inferred by applying a stress tensor inversion to the calculated focal mechanisms. The results that we found show that most earthquakes are characterized by predominant pure reverse faulting to reverse faulting with a minor strike-slip component. The average trend of P- and T-axes is 223.94°±95.6° and 17.68°±13.02°, respectively, and the average plunge of P- and T-axes is 183.02°±95.22° and 56.32°±28.41°, respectively. The estimated stress tensor parameters, describing the principal stress directions show that the axis of σ1 has the trend of 178.0° and dipping southward of 2.0° and an axis of σ2 has the trend of 268.0° and dipping westward of 3.0°, indicating that both of σ1 and σ2 axes are sub-horizontal. The third stress axis, σ3, has the trend of 46.0° and the plunge of 87.0°, and tends to be sub-vertical. The stress shape ratio, R, is equal to 0.61 in the whole area. These results strongly suggest that the reverse-faulting regime is predominant over the study area, and the P-axes and the stress orientations are steeply consistent with the GPS velocities field pattern derived from recent geodetic models. The seismotectonic model proposed in this study is consistent with the NW–SE displacement vector of the Nubia plate with respect to Eurasia plate and currently accommodating the central High Atlas tectonic deformation system, enhancing the slight crustal shortening process at the northern active tectonic boundary of the study area.