Crustal structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa): 2. Insights from the 23-year recording of seismicity since the last rifting event

Abstract : The Asal-Ghoubbet (AG) Rift has sustained a major volcano-tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys. It is therefore an ideal place to study the transient magmato-tectonic processes that operate after a rifting episode. We examine the space-time evolution of $2500 Md 2.8 earthquakes recorded in the rift from 1979 to 2001. We focus on the relationships between this seismic activity and both the three-dimensional structure of the rift and its postrifting behavior depicted from geodesy. The results highlight the major role of the central magmatic system (Fieale-Shark Bay) on the structure, seismic activity, and overall behavior of the rift. From 1978 to 1986, the rift opens at a fast rate, yet mainly aseismically; the opening is magmatically driven and accommodated. Since 1986, when the opening rate abruptly decreased, the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system. These pulses result from that magmatic zone undergoing alternating stretching and inflating episodes. Thus, while the plate-driven induced stresses have been rebuilding in the rift since 1987, the rift opening is still essentially accommodated in the axial magmatic zone. The AG Rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode. That transient opening has essentially occurred aseismically, and most tectonic faults remain relaxed and locked.