Evidence of Fluid Induced Earthquake Swarms From High Resolution Earthquake Relocation in the Main Ethiopian Rift

Fluid overpressure and fluid migration are known to be able to trigger or induce fault slip. However, relatively little is known about the role of fluids on generating earthquakes in some of the major continental rifts. To address this, we investigate the interaction between fluids and faults in the Main Ethiopian Rift (MER) using a large seismicity catalog that covers both the rift axis and rift margin. We performed cross-correlation analysis on four major earthquake clusters (three within the rift and one on the rift margin) in order to significantly improve accuracy of the earthquake relative relocations and to quantify families of earthquakes in which waveforms are similar. We also analyzed variation of seismicity rate and seismic moment release through time for the four clusters. The major results are that for all four clusters the earthquake relocations are 5-15 km deep, aligned to clear N-NNE striking, steeply (>60 degrees) dipping planes. For the three clusters within the rift, the cross-correlation analysis identifies earthquake families that occur in short swarms during which seismic rate and moment release increases. Together, this space and time pattern of the seismicity strongly points toward them being fluid induced, with fluid likely sourced from depth such as mantle derived CO2. In contrast, the seismicity on the rift margin lacks earthquake families, with occurrence of earthquakes more continuous in nature, which we interpret as pointing toward tectonic stress-driven microseismic creep. Overall, our results suggest that deep sourced fluid migration within the rift is an important driver of earthquake activity. Plain Language Summary Fluids such as water and carbon dioxide that come from the deep Earth can move toward the surface by following fractures and faults. When this happens, these fluids make it easier for the faults to move, causing lots of small earthquakes to happen in short periods of time and in the same place. These earthquake swarms have typical characteristics such as waveforms that are incredibly similar to each other. In our study, we are interested in understanding how important the movement of fluids is for the generation of earthquakes during the breakup of continents. We investigated the presence of these characteristics for earthquakes in the Main Ethiopian Rift in East Africa. Major findings are that earthquake swarms within the rift have characteristics that indicate earthquakes are generated by fluid flow along faults. In contrast at the edges of the rift, the earthquakes are different in character, which indicates that they are caused by tectonic motion of the plates rather than fluid migration.