Broadband moment magnitude determination and strong ground motion simulation of moderate earthquakes southern Red Sea, Saudi Arabia

Abstract

This study examines the seismic source and ground motion characteristics of moderate magnitude earthquakes along the southern Red Sea rift in southwestern Saudi Arabia, a tectonically active region shaped by ongoing extensional processes. Broadband moment magnitudes ( M _wp ) were estimated for the 25 May 2001 ( M _wp  = 5.3) and 29 January 2004 ( M _wp  = 4.6) events using a spectral analysis of broadband P -wave data. M _wp provides an effective approach for rapid magnitude estimation from broadband records at regional distances, offering a useful complement to local magnitude scales. Strong ground motion parameters, including peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), and pseudo-spectral acceleration (PSA), were simulated for representative sites across the Jizan region. The largest PGA, PGV, and PGD values were obtained at Alamaridah district, primarily reflecting the combined influence of source to site distance, regional attenuation properties, and local site amplification. Simulated PSA values range from 2.3 to 27.4 cm/s ² . Although these levels remain below the design response spectra prescribed by the Saudi Building Code (SBC 301), their spectral shapes show close similarity, indicating pronounced frequency dependent site resonance and the relevance of local soil conditions in shaping the ground motion response. Comparisons between the simulated ground motions and regional Ground Motion Prediction Equations (GMPEs) demonstrate good overall agreement, supporting the reliability of the adopted source, path, and site modeling. These findings contribute to a more realistic representation of seismic hazard in the Jizan area and provide a scientific basis for refining regional risk assessment, engineering design considerations, and future seismic mitigation strategies.