Aseismic strike slip associated with the 2005-2010 Afar rifting episode

Abstract:

Himematsu and Furuya (2015) have reported that the 2007 dike intrusion episode in Tanzania accompanied significant aseismic strike slip at the subsiding zone above the tensile dislocation. No previous studies, however, have considered such strike slip components associated with dike intrusion events. Although seismic swarms during rifting events are often attributed to the magma intrusion, there is a hypothesis that aseismic slip may be responsible for the generation of seismic swarm (e.g., Lohman & McGuire, 2007). It is thus possible that similar aseismic strike slip could have generally occurred during any rifting events.

We re-examine the crustal deformation signals during the 2005-2010 Afar rifting episode using ALOS/PALSAR (L-band) images. The seismic swarm and the dike intrusion intermittently occurred from September 2005 to May 2010. Some previous studies have already reported the 3D displacements for the September 2005 rifting event (e.g., Wright et al., 2006). While two ~Mw5.5 right-lateral earthquakes were reported around the focal region, their results showed no remarkable horizontal displacement at the graben-like subsiding zone. However, we argue that the insignificant horizontal displacements along the intruded dike would be due to the lack of coherent signals near the subsiding zone.

We applied offset tracking technique to the ascending-track data set (12 June 2007 – 20 June 2010) to acquire the robust crustal deformation signals. Both the azimuth offset and the multiple aperture interferometry (MAI) data are sensitive to the displacement along the satellite track (~N350°E), and showed clearly the NNW horizontal displacement at the subsiding zone. At the subsiding zone the range offset and azimuth offset data depicts the maximum changes of about 225 cm and 110cm, respectively. Because focal mechanisms of the earthquakes reported by previous studies and Global CMT catalogue during the rifting indicate few strike components, we propose that the horizontal displacements may be caused by aseismic strike-slip. We will discuss the mechanism of the aseismic strike slip relating with the seismic swarm.