Crustal structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa): 1. Insights from a 5-month seismological experiment

We seek to characterize how magmatic and tectonic activities combine and interact during the continental rifting process. We address this question in two companion papers. In both, we analyze the seismicity that occurs in an active magmato-tectonic rift, Asal-Ghoubbet ( East Africa), to identify the features and/or processes responsible for its activity. Here, we report results from a 5-month experiment that we conducted in the rift. Eleven seismometers were deployed to complement the eight-station permanent network. This allowed recording similar to 400 earthquakes in the rift; 200 events could be well located ( precision <40 m) and used in a tomographic inversion. Focal mechanisms were also determined for 71 events. The results show that current activity in the rift is mainly magmatically induced or accommodated. A similar to 2-km-wide pipe of hot rock is evidenced below the central Fieale-Shark Bay volcanic complex, likely above a deeper (>5-6 km) magma reservoir. Most events concentrate at the roof of the pipe ( at 3-4 km) and result from up and down slip ruptures on both the volcanic ( ring) and tectonic faults that enclose the pipe at depth. The up and down motions are likely driven by pressure changes in the magma reservoir. Hence, although a few rift faults were associated with seismicity, most remained seismically silent during the experiment. In the companion paper, we analyze the seismic activity in the rift over the 23 years that followed its last rifting episode. This confirms the importance of the Fieale-Shark Bay plumbing system in the overall rift behavior.