Probabilistic and Deterministic Seismic Hazard Assessments of the area comprised between west Gulf of Cádiz and east Alboran Sea

Abstract. The increased deployment of subsea infrastructures and the exploration of marine resources have heightened the need to assess seismic hazard on the seabed, especially in tectonically active offshore regions. The area between the Gulf of Cádiz and the Alboran Sea, which is rich in coastal and submarine assets, is located within the Ibero-Maghrebian Region (IMR) a seismically active region. While previous research has addressed seismic hazard in adjacent inland areas using deterministic and probabilistic approaches, few studies have focused on offshore zones. Moreover, existing models often overlook the amplification effects introduced by bathymetry and seafloor conditions, and they rely on ground motion prediction equations (GMPEs) derived from inland data. Consequently, seismic hazard in marine environments remains poorly constrained. This study aims to evaluate the feasibility and applicability of both probabilistic (PSHA) and deterministic (DSHA) seismic hazard assessments for submarine areas, using updated seismogenic zonation, a high-resolution bathymetric model, and GMPEs suitable for soft marine soils. The objective is to produce reliable peak ground acceleration (PGA) estimates at the seabed and to examine the convergence of deterministic and probabilistic approaches in a complex tectonic context. The analysis was conducted using the OpenQuake (OQ) engine for PSHA and a custom MATLAB© script for DSHA. The seismogenic sources and parameters were taken from the ESHM20 model, and four GMPEs—IDR91, HZ23, NT24, and DKK24—were selected for their relevance to offshore or soft soil conditions. These GMPEs were validated against six regional offshore earthquakes recorded in the IMR. Hazard estimates were computed over a bathymetrically refined grid and expressed as PGA maps for various return periods (100, 475, 2475, and 5000 years). The results show that significant PGA values occur over key submarine fault systems such as the Gorringe Ridge, Horseshoe Plain, and Arzew Fault. Offshore PGA estimates for return periods of 2475 years using NT24 and DKK24 are close to those derived from DSHA, settling convergence between methodologies. The use of seabed-adapted GMPEs resulted in higher and more realistic PGA values compared to inland models. This study demonstrates the applicability of seismic hazard assessment methods to offshore environments and highlights the importance of incorporating seafloor conditions into hazard modelling. The findings offer a methodological basis for improving the seismic design of subsea infrastructure in tectonically complex marine regions such as the IMR.