The Morocco High Atlas 6.8 magnitude 2023 event: New insights from geodetic and seismological modeling

Morocco's High Atlas is an intracontinental orogenic belt located at the northern edge of the West African Craton (WAC). This major belt extends from what is now the Atlantic margin of Morocco to the Mediterranean coast of Tunisia, spanning the Sahara Atlas in Algeria. Within the context of the convergence of Nubian and Eurasian, GPS measurements across the High Atlas in Morocco indicate a very low surface deformation rate (<1mm/year). This study examines co-seismic deformation through InSAR modeling, Coulomb stress change analysis, and aftershock distribution. Additionally, it explores the crust-uppermost mantle's structure beneath the High Atlas and adjacent regions using ambient noise seismic tomography and P-wave coda autocorrelation to analyze the earthquake sequence within the context of regional geodynamics. Our InSAR modeling supports a NE-SW trending reverse faulting mechanism with a dip of ~69◦ towards the Northwest. The maximum slip is about 2m at an average depth of ~24km. The fault extends over 20km length from 10km to 40km depth. The reactivated fault stands beneath a flower structure and its surface expression coincides with Tizi n’Test Fault mapped fault. Analysis of Coulomb failure stress changes from the mainshock on nearby major faults showed a stress increase of ~5 bars above the rupture, aligning with areas of aftershock activity. Ambient seismic wave tomography and teleseismic P-wave coda autocorrelation from a network of 21 broadband stations allowed us to constrain the depths of the Moho and Lithosphere-Asthenosphere Boundary (LAB) beneath the Western High-Atlas. Our results showed that notable increase in Moho depth, with measurements ranging from 45 to 50 km in the epicentral area, therefore compensating the High Atlas topography. Conversely, in the surrounding low topography areas, the Moho depth remains relatively shallower, averaging around 35 km. Interestingly, we found that LAB is shallower in the region where we observed a thicker Moho, and vice versa. The 2023 High Atlas blind earthquake clearly ruptured at least 2/3 of a 45-50 km thick crust sitting above a 30-35 km thin lithospheric mantle.