Seismic hazard parameter estimation of the Mount Cameroon volcanic region (Cameroon) based on a combination of mixed catalogs

The Mount Cameroon volcanic region is one of the most seismically active areas of Cameroon, with earthquakes of duration magnitude $M_{\text{D}}$ ≥ 0.5 occurring at an average rate of one to three events every 3 days. The seismic hazard parameters of this region have been assessed on the basis of a procedure that combines historical events with complete instrumental data and takes into account uncertainty in the determination of the earthquake magnitude. The historical events include all felt and reported events that occurred prior to the establishment of a seismic network, whereas the complete instrumental data comprise of all instrumentally recorded events. The dataset of earthquake catalogs used in this study covers the period from 1909 to 2006. The overall seismicity pattern of the Mount Cameroon region during this period is characterized by the predominant clustering of earthquakes in four distinct regions denoted as: region A and B in the northern and southern part of the elongated axis, respectively, region C in the Douala region, and region D in the offshore area. For each region, the maximum regional magnitude $m_{\text{max}}$, the activity rate of seismic events $\lambda$, and the b-value of the magnitude–frequency Gutenberg–Richter relation were determined. The estimated b-value during the studied timeframe ranged from a minimum of 0.81 ± 0.07 to a maximum of 1.52 ± 0.03 across the four seismic regions. The highest b-value was observed in the NE–SW seismic zone B, and the lowest b-value was found to correlate with the northern part of the elongated axis of the volcano (region A). Focal depths of seismic events from zone B (between 35 and 55 km), coupled with the anomalous high b-values obtained, confirm the existence of a vertical magma conduit as inferred from previous studies. The mean activity rates for moment magnitude $M_W$ 3.5 range from about 2 to 14 events per month across seismic zones B and C, respectively. The maximum magnitude estimate has its highest value in seismic source zone B and the lowest value in the offshore region (seismic zone D). Differences in the estimated seismic hazard parameters from zone A to zone D reflect the seismogenic complexity of the Mount Cameroon region.