Earthquakes in the Davie Ridge‐Madagascar region and the southern Nubian‐Somalian plate boundary

Abstract

The largest earthquake sequence ( m b up to 6.4 and M 0 up to 5 × 10 18 Nm) to have struck eastern Africa within the past 60 years occurred in May and June of 1985 off the coast of the Tanzania‐Mozambique border. The epicenters are located on the northern extension of the Davie Ridge, a prominent north‐south trending bathymétrie feature accompanied by distinct gravity anomalies. A joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation. Al teleseismic distances, P and SH waves generated by the two largest events of this sequence have nearly identical waveforms on the long‐period records of the WorldWide Standard Seismograph Network. A formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north. At least two point sources are necessary to model each event, one at a depth of 40 ± 10 km and the other at about 20 ± 5 km. On a regional scale, normal faulting seems to characterize the scattered seismicity throughout the region between Madagascar and the eastern arm of the East African rift zone, even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified. Since normal faulting events of up to 25 ± 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude, we propose that the southern termination of the Nubian‐Somalian plate boundary consists of a diffuse zone of east‐west extension up to 2000 km wide.