Seismotectonics of the Mount Cameroon Volcanic Region, West Africa

Abstract

Mount Cameroon is an active volcano located on the continental margin midway along a 1600 km chain of volcanic centres called the Cameroon line. Detailed microearthquake surveys conducted in the Mount Cameroon region, during three consecutive field seasons from 1984 to 1987, reveal an overall seismicity pattern characterised by, the predominant clustering of earthquakes at a few distinct localities, with no apparent clear lineations of epicentres. The earthquakes occur both as discrete events and in swarms at a rate of about two events per 3 days. Duration magnitudes ranged from 0.5 to 4, although most of the events are smaller than M_D = 3. Hypocentre depths extend from near the surface to about 55 km, indicating the presence of both crustal and subcrustal activity. Calculated b values are low compared to those reported for most volcanic regions. Focal mechanisms are highly variable and show mostly normal and strike slip solutions. The subcrustal earthquakes have mechanisms which are consistent with one another and these show nearly pure strike slip faulting, with NE oriented P axes. The focal mechanisms of the subcrustal events, coupled with the general SW-NE alignment of the volcanic cones on the mountain, suggest an approximate SW-NE orientation of the maximum compressional axis of the regional stress field in the region. Apart from Kilauea volcano, Hawaii, Mount Cameroon is the only other intraplate volcanic centre having such deep seismic activity. The cause of such subcrustal activity is unknown, but probably relates to the presence of a zone of weakness, which both concentrates regional stress and fails more easily than its surroundings.