Characterizing an Incipient Fault System: Insights From the Morphometric Analysis of the North-South Faults (Alboran Sea)

Abstract

Investigating the dynamic evolution of active fault systems is fundamental for deciphering regional tectonics and assessing seismic risk, particularly in offshore regions where direct observation is challenging. Our ultra-high-resolution bathymetric study of the North-South Faults (NSF) documents an incipient and dynamically evolving left-lateral transtensional system, comprising 41 right-stepping, highly segmented fault scarps and associated pockmarks. The NSF’s fault interactions and morphological scarp profiles analysis support a hybrid fault growth model, with coherent constant-length growth predominating, and a sharp northwestern boundary reflecting an actively propagating shear zone. This kinematic interpretation is reinforced by en echelon patterns and focal mechanisms displaying strike-slip and extensional components. The NSF appears to be the northward-propagating tip of the larger Al-Idrissi Fault, and its present-day activity may be supported by fault-aligned pockmarks. Empirical calculations show that faults can generate earthquakes up to Mw 6.5, and we propose the NSF as a plausible alternative source for the historical Mw 6.1, 1910 Adra earthquake, suggesting that a full system rupture could pose a greater seismic hazard to coastal populations than currently recognized. This research advances our understanding of early stage fault growth and provides valuable insights into the dynamics of evolving transtensional fault systems, applicable to other tectonically active offshore environments.