ANALYSIS OF EARTHQUAKE SWARMS IN THE NORTHERN MAIN ETHIOPIAN RIFT

The Main Ethiopian Rift (MER) is a continental rift with northward increase in maturity in stages of continental break-up. The MER is characterized by ~60-km-long border faults with high vertical offset that delimit the rift. The majority of rift valley extension is focused to in-rift magmatic segments that have networks of small-offset faults, volcanic centres and associated cone fields. We have studied the spatial, temporal and waveform characteristics of local seismicity from the northern sector of MER. The seismic database contains events from October 2001 to January 2003, and acquired by the Ethiopia Afar Geoscientific Experiment (EAGLE Project). The earthquakes have been relocated with NLLoc using a new 3D velocity model derived from a wide-angle controlled source experiment. The relocated catalog contains a total of 1543 events with magnitudes between 0 and 4. The seismicity is mainly concentrated in two areas: near the Ankober border fault and within the rift near Fentale and Dofen volcanoes. On the border fault, events mostly occur down to 20 km depth, with an average depth of ~ 12 km. Within the rift, the events mostly happen down to 15 km depth, with an average depth of ~ 9 km. The seismicity is divided into several clusters aligned parallel to the rift direction, and in profile sections the clusters are mostly dipping steeply. The analysis of the temporal-spatial distribution of earthquakes shows that some of the clusters are strongly concentrated in time and in space, and therefore swarm-like. b-values were calculated for the identified clusters using the Maximum Likelihood method, with results showing values of b higher than 1. We have conducted a waveform cross correlation on waveform cut 10 seconds before and 60 seconds after P waves arrivals in order to individuate similar events and group similar earthquakes into families. Most of the earthquake clusters are composed of several swarms within which earthquakes are highly correlated, but with different swarms not correlating well with each other. Finally, the cross-correlated P arrivals were used in a new relocation with the HypoDD double-differencing software. Comparison between the different families of swarms, the mapped faults and the active geothermal sites suggest that some seismic swarms could be induced by geothermal fluids.