Seismicity induced by fluid migration in the Main Ethiopian Rift

<p>Faults can act as preferential degassing pathways for fluids of deep origin. Their migration and consequently variation of fluid pore pressure can cause a reduction of normal stress on the fault planes and trigger earthquakes. This can generate not only microseismicity but also events with significant magnitude. To understand this phenomenon, we studied the spatial, temporal and waveform characteristics of local seismicity from the northern sector of Main Ethiopian Rift (MER) of East Africa near Fentale and Dofen volcanoes. The seismic database contains events occurred in the MER from October 2001 to January 2003, and acquired by the Ethiopia Afar Geoscientific Experiment (EAGLE Project). The recorded events 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 border faults of the Ethiopian plateau and within the rift. On the border faults, 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 sub-vertical and dipping consistent with Andersonian normal faults. 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. To understand if the different clusters have been conditioned by fluid migration we have also analyzed the frequency content, release of seismic moment, and b-val is cut out. The link between earthquakes and fluid migration has also been explored by interpreting the distribution of seismicity using remote sensing mapping of faults, fumaroles and hydrothermal springs. Understanding where and how the fluid migration occurs will aid geothermal exploration efforts in the region, also improved knowledge of where geothermal activity is linked to seismicity has implications for seismic hazard estimation, which is very important for this densely and economically active areas.</p>