Local Magnitude Scale for the Ethiopian Plateau

Abstract

A local magnitude ( M L) scale and seismicity catalog for the Ethiopian Plateau have been developed using data collected by the 2000–2002 Ethiopia Broadband Seismic Experiment. Locations for 253 local and regional events have been obtained using P -wave arrival times recorded on four or more stations. For constructing the M L scale, waveforms were corrected for instrument response and convolved with the nominal Wood–Anderson torsion seismograph response. Maximum S -wave amplitudes were then picked on the horizontal components of ground motion (3218 total observations) and used in an inversion for event magnitudes, two model parameters, and 54 horizontal component station corrections. The distance correction obtained from the inversion is -log A =0.726log( r /100)+0.000558( r -100)+3.0, where r is the hypocentral distance in kilometers. Seven of the 253 events can be considered ground truth (GT) events, with epicentral locations accurate to within 5 km according to the GT5 local criteria of Bondar et al. (2004). In contrast to previously reported ground-motion attenuation for the Main Ethiopian Rift, we find relatively low ground-motion attenuation for the Ethiopian Plateau, reflecting variations in crustal structure between the Ethiopian Plateau and Main Ethiopian Rift.