Time and length scales of dike intrusions and associated faulting in the ongoing Afar rifting episode

Although dike intrusions achieve plate opening along mid-ocean ridges and during continental rupture, the time and length scales of the intrusion process are poorly understood. Likewise, the role of dike intrusion in the creation of rift zone morphology, including faulting above and ahead of propagating dikes, is only loosely constrained by laboratory models and a few rare rifting events. Our temporary broadband seismic array deployed in the highly evolved Afar rift, Ethiopia recorded tectonic and low-frequency earthquakes associated with the ongoing diking activity in the region. Seismicity data March -December 2007 is analyzed to get an insight into these processes. In addition to the high levels of background seismicity, two large dike intrusions occurred in 2007: 12-13 August with a fissural eruption, and 11-13 November. Earthquakes were relocated using the double-differencing technique. Earthquake epicenters are located in a ~45-km long, 3km- wide zone at the southern end of the 60 km-long rift segment that opened in September 2005. The previously active northern end of the segment around Dabbahu volcano and the 2005 eruption site is now aseismic. The epicentral distribution shows a 2 km westward shift relative to the 2006 activity. The 3D distribution also shows a similar southward decrease in depth with most of the deepest events in the central part of the segment. The low-frequency volcano-tectonic earthquakes analyzed separately show a clear northward dike propagation direction, which is away from the largely aseismic eruption site. The November event comprised at least two discrete dike intrusions, each of which was preceded by tectonic earthquakes, followed by low- frequency earthquakes that propagated southward. We use seismicity, remote sensing and geodetic data to refine the dimensions and rates of dike emplacement both vertically and laterally, to identify magma source regions, and to understand the interaction between dikes and brittle faulting within the regional tectonic context of an incipient spreading center.