Relative Tectonic Activity Assessment of the Asir Province, Southwestern Saudi Arabia: Insights from Geospatial Analysis and Tectonic Geomorphology Applications

Abstract

Quantitative geomorphic analysis provides a valuable, cost-effective tool for evaluating relative tectonic activity across active rift zones and can support, but not by itself, seismic hazard assessment and infrastructure planning. This study evaluates the relative tectonic activity of the Asir Province, southwestern Saudi Arabia, a zone transitioning from the crystalline Arabian Shield to the Cenozoic Red Sea rift system. Using a 30 m Shuttle Radar Topography Mission (SRTM) digital elevation model, six geomorphic indices, a lithologically normalized stream length–gradient index, hypsometric integral, basin asymmetry, basin shape, the valley floor width-to-height ratio, and mountain front sinuosity, were combined into a composite Relative Active Tectonics Index (RTAI), an unweighted arithmetic mean classification of 63 sub-catchments into 4 relative activity classes. The findings reveal that 32% of the sub-catchments indicate high to very high relative tectonic activity, observed mainly along the high-terrain Sarawat mountain range and active rift cliffs. Raised stream length–gradient anomalies align with mapped NW-SE and NE-SW fault trends, and their spatial pattern correlates with the epicentral area of a May 2020 earthquake near Tandaha (Mw 3.45), presenting contextual support rather than independent validation, confirmation or support for the RTAI classification. The spatial distribution of the RTAI suggests modern morphotectonic rejuvenation and localized reactivation of Proterozoic structural features, including the Nabitah Fault Zone, which appeared due to Red Sea rifting events. These results indicate that the RTAI model is a valuable tool for assigning zones of relative tectonic activity, offering baseline geomorphic evidence that can effectively inform, though not substitute, for subsequent seismic hazard assessment, engineering infrastructure planning, and sustainable land-use management.