Quantifying Seismic Source, site and Path Parameters Using Body Wave Spectral Inversion: A Case Study from Southwestern Saudi Arabia

Dr. Saleh Qaysi^*

• King Saud University, Riyadh, Saudi Arabia

This study aims to investigate the seismic source characteristics, path attenuation , and site effects of earthquakes in southwestern Saudi Arabia, with the goal of improving understanding of the region's crustal structure and seismotectonic processes. The broadband seismic network in southwestern Saudi Arabia, provides an excellent framework for a comprehensive investigation of seismicity distribution, site response, seismic wave attenuation, and source spectra of earthquakes with local magnitudes (𝑀𝐿) ranging from 2.5 to 4.8 recorded between 2000 and 2023. A Spectral Inversion Method (SIM), constrained by a reference site, was applied to decouple site response, attenuation effect, and source characteristics from observed Pand S-wave spectra using iterative least squares analysis. The site responses derived from Pand S-wave spectral inversion reveal strong variability across stations located on contrasting geological settings of the Arabian Platform and Arabian Shield. P-wave amplification is generally higher and peaks at 8–15 Hz, while S-wave amplification and H/V spectral ratio peaks show close agreement, indicating that the H/V method reliably captures the fundamental site resonance controlled by shallow shear wave velocity structure. Frequency-dependent attenuation was quantified using empirical relationships, 𝑄𝑝= (112 ± 1.1)𝑓 (1.04±0.03)and 𝑄𝑠= (199 ± 2.9)𝑓 (0.98±0.01), indicating significant frequency dependence and suggesting a seismically active and structurally heterogeneous crust in the southwestern Arabian Shield. The displacement source spectra deviate from the classical ω⁻² source model, displaying a steeper high-frequency decay that more closely resembles an ω⁻³ spectral falloff. advanced source parameters, including seismic moment (𝑀𝑂), corner frequency (𝑓𝐶), moment magnitude (𝑀𝑊), source radius (𝑟), and stress drop (∆𝜎), were estimated from corrected displacement spectra of Pand S-waves over the frequency band 0.8–50 Hz. The seismic moment ranges from 2.12 × 1011 to 3.7 × 1014 N.m, source radii from 145.1 to 493.3 m, and corner frequencies from 3 to 10.2 Hz. Stress drop values vary from 0.002 to 1.06 MPa (P-wave) and 0.005 to 2.54 MPa (S-wave). The relatively low stress drops may reflect the influence of active Red Sea rifting and fluid upwelling beneath the Arabian Shield, contributing to the reactivation of pre-existing zones of weakness within the study area.