Preliminary Investigation of the Coulomb Stress Transfer of the 1939 6.2Mw Accra Earthquake, Ghana

The June 22nd, 1939 Accra earthquake (Mw = 6.2) of Ghana is one of the most devastating intra-plate earthquakes in the sub-Sahara West African region. The waveform inversion earlier carried out suggested that the earthquake was composed of two events. The smaller event (6.1 Mw) occurred 9.5 s before the onset of the larger event (6.4 Mw). The smaller event has a focal mechanism that suggests it occurred immediately north of the intersection of the Akwapim and Coastal Boundary fault. This study resolved the static Coulomb Failure Stress (CFS) change onto the finite fault models of the 6.4 Mw and 6.1 Mw earthquakes by USGS and its effect on associated receiver faults. Aftershocks were poorly spatially correlated with the enhanced CFS condition after the 6.4 Mw main shock and were explained to correlate with release of seismic energy from the associated secondarily stressed prominent strike-slip (Akwapim) fault and strike-slip (coastal boundary fault). Abrupt termination of the northeastward propagation of 6.1 Mw rupture surface was due to interaction with the strike-slip coastal boundary faults. The existing intersection between the Akwapim and Coastal boundary faults favored the enhanced CFS to generate the next major event of 6.4 Mw due to the deflection of motion transmitted from the seismically active fractured zones in the mid-Atlantic ridge (the boundary between the African plate and the South-American plate).