The 2023 Mw 6.8 Al Haouz Lower Crustal Earthquake Buried in a Thick Crust Within a Thin Lithosphere Beneath the High Atlas Mountain Range (Morocco)

Abstract

Abstract The High Atlas (HA) forms a major intracontinental belt along the northern margin of the West African craton, stretching from the Atlantic coast of Morocco to the Tunisian Mediterranean across the Saharan Atlas. Global Positioning System data record surface deformation rates below 1 mm/yr, underscoring limited present-day tectonic activity in the framework of the convergence of the Nubian and Eurasian tectonic plates. This study investigates the coseismic deformation following the Mw 6.8 Al Haouz earthquake, combining Interferometric Synthetic Aperture Radar modeling, Coulomb stress change analysis, and the examination of aftershock distribution. We investigate the Earth’s structure to understand the earthquake sequence in relation to its regional geodynamics, using ambient noise seismic tomography and P-wave coda autocorrelation. Our findings show a west-southwest–east-northeast high-angle blind reverse fault dipping to the northwest that is located beneath a flower structure, with its surface manifestation directly correlating to the documented Tizi n’Test fault. Coulomb failure stress change calculations revealed that the mainshock loaded nearby major faults by ∼5 bars, a change that aligns spatially with subsequent aftershock clusters. The resolved Moho and lithosphere–asthenosphere boundary (LAB) depths beneath the western High Atlas revealed a significant increase in Moho depth, which reached 45–50 km in the epicentral region. In contrast, the Moho remains relatively shallow (averaging ∼35 km) in the surrounding low-topography areas. Notably, a clear inverse correlation was observed between Moho and LAB depth, with a shallower LAB corresponding to a thicker crust, and vice versa. The Al Haouz earthquake ruptured at least two-thirds of the thick crust that likely compensates the HA topography in a region characterized by a 30 km thin lithospheric mantle at the edge of the western African craton. This suggests that the slow deformation is likely governed by an oblique crustal convergence and a localized asthenospheric uplift caused by a lateral flow of the Canary Plume in interaction with the western African craton.