Reassessment of focal depth estimates of small-to-moderate magnitude continental earthquakes in Western to Central Africa with the optimal period of Rayleigh wave amplitude spectra using sparse regional seismic data

Abstract

We investigate the focal depths of earthquakes in West and Central Africa using regional seismic data collected over the past two decades. Due to limited seismic network coverage, reliable source depth estimates are sparse in global earthquake catalogs for this region. To address this gap, we exploit the sensitivity of short-period (0.5–20 s) Rayleigh waves to source depths. Using the optimal period of Rayleigh wave amplitude spectra, we used an automated method to estimate earthquake focal depths. This method enabled us to resolve the depths of fifteen small-to-moderate earthquakes (mb 4.0–5.0) occurring in the region between 2000 and 2023. The focal depths ranged from approximately 3.3 km to 16 km, indicating that seismicity in the region is primarily confined to the upper crust. This finding is consistent with previous studies suggesting shallow crustal seismicity in the area. We conclude that the shallow stress regime in West and Central Africa reflects a localized weakness zone in the upper crust. Our study demonstrates the effectiveness of using the optimal Rayleigh wave period method to accurately determine focal depths of small-to-moderate earthquakes, particularly in regions with sparse seismic station coverage.