Analysis of fluid induced earthquake swarms in Northern Main Ethiopian Rift

An increase of fluid pressure can induce fault slip and therefore lead to the occurrence of earthquakes. The aim of our works is to investigate this phenomenon from a seismic point of view. We analyzed the EAGLE seismic database, that includes the earthquakes that occurred in the Northern Main Ethiopian Rift (NMER) from October 2001 to February 2003, with the aim of achieving accurate earthquake locations that show subsurface fault structure and temporal behavior. The earthquakes in the database were relocated with a number different methods including double difference relative relocation following waveform cross correlation. We focus on the Fentale-Dofan magmatic segment, an area involved in the active rifting process with a widespread seismicity and with the presence of surface hydrothermal deposits that suggest ongoing hydrothermal activity. The earthquakes were first relocated with NonLinLoc using a non-linear method and the velocity model from controlled source seismology. The events relocated with NonLinLoc was divided in four distinct clusters, with three clusters in the rift and one cluster on the western border fault. Each cluster was then relocate separately with HypoDD double-difference location algorithm, including implementation of waveform cross correlation. From the earthquake magnitudes, b-values and seismic moment were also computed. Seismic data was interpreted with hydrothermal surface data obtained from automated remote mapping from Landasat 8 images. 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. These swarms are characterized by events with similar waveforms. There is direct correlation between the increase of seismic rate in the cluster and the presence of families of similar earthquakes. The values found for the seismic moment suggest that the events are originated from activation of rift related structures. This is supported by the N to NE elongation strike of seismic clusters highlighted by the HypoDD location, in accordance with the tectonic setting of the area. The events are mostly localized in the top 15 km of the crust. The b-values calculated for the clusters are smaller than 1, with the exception for the cluster localized near Dofan volcanic complex. The hydrothermal deposits mapped by us are mainly focused in two areas: on the western side of Dofan volcanic complex, in an area intense faulted by NNE-SSW faults; and around the Fentale volcano with a circular pattern on southern side of volcanic edifice. The no clear correlation between seismicity and mapped hydrothermal deposits suggesting that seismicity is not driven by shallow hydrothermal fluid flow. It is possible to conclude that these earthquakes have a component fluid induced, but the origin of these fluids are deeper than the fluids that feed the hydrothermal systems.