Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

Abstract

Continental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late‐stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ∼64% of the seismic moment release and exhibits a swarm‐like behavior. In contrast, seismicity at the marginal graben is characterized by high‐frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low‐frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ∼12 km beneath Alu‐Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E‐W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long‐term extension.

Observing Oblique Slip During Rift Linkage in Northern Afar

“…The structural data and our best fit model of the M L 5 earthquake show that the strain at AP is primarily accommodated by left‐lateral oblique slip along ~N‐S striking faults. The right‐lateral oblique slip along NW‐SE trending faults (Illsley‐Kemp, Keir, et al, ) likely represents a conjugate fault population, as commonly observed in shear zones (e.g., Davis et al, ; Dooley & Schreurs, ; Sibson, ). The occurrence of two conjugate fault systems can be explained by a model of plate boundary kinematics where two rift segments, TA and EA, are linked by an oblique right‐lateral transfer zone (Figure ).…”

Initiation of a Proto‐transform Fault Prior to Seafloor Spreading

“…These earthquakes have average location errors of ±1.9 km and ±4.1 km in the horizontal and vertical directions, respectively, and the catalogue is complete above magnitude 2.0 (Illsley‐Kemp et al, ). Generally, earthquakes are focused at the western rift margin, which separates the Ethiopian plateau from the Afar depression, or in the vicinity of volcanic centers (Illsley‐Kemp et al, ; Figure ). In addition, there is a cluster of 418 earthquakes focused within the Giulietti Plain, (Figure ).…”

“…The crust thins from ∼27 km in the central and southern Afar rift to <15 km beneath the Danakil region in the north (Hammond et al, 2011;Makris & Ginzburg, 1987). However, the crust in the Danakil region is (Illsley-Kemp et al, 2018), superimposed on topography taken from the Shuttle Radar Topography Mission. A cluster of events is located in the Giulietti Plain, between the offset Erta-Ale and Tat-Ale segments.…”

“…Main figure shows the location of the seismic network (blue inverted triangles) and all recorded earthquakes with horizontal errors <5 km (red circles) from 2011 to 2013 (Illsley‐Kemp et al, ), superimposed on topography taken from the Shuttle Radar Topography Mission. A cluster of events is located in the Giulietti Plain, between the offset Erta‐Ale and Tat‐Ale segments.…”

Using Ambient Noise to Image the Northern East African Rift

“…An exception to the fast Afar crust is a region of slower velocities broadly beneath the magmatic segments (Figures a–d “b”), down to 3.45 ± 0.04 km/s at a depth of 10 km and 3.85 ± 0.06 km/s at a depth of 40 km. This is most clearly evident at the Dabbahu and Hararo magmatic segments, the location of the 2005–2010 Dabbahu dyking event (Ebinger et al, ; Grandin et al, ; Illsley‐Kemp et al, ). Previous studies also find slow velocities (Korostelev et al, ; Stork et al, ) and high conductivities (Desissa et al, ; Johnson et al, ), which have been used to estimate a 500‐km 3 melt body with 13% melt beneath this region associated with the dyking event (Desissa et al, ).…”