Abstract
In November 2010, intense seismic activity including 29 events with a magnitude above 5.0, started in the western part of the Gulf of Aden, where the structure of the oceanic spreading ridge is characterized by a series of N115°-trending slow-spreading segments set within an EW-trending rift. Using signals recorded by permanent and temporary networks in Djibouti and Yemen, we located 1122 earthquakes, with a magnitude ranging from 2.1 to 5.6 from 01 November 2010 to 31 March 2011. By looking in detail at the space-time distribution of the overall seismicity, and both the frequency and the moment tensor of large earthquakes, we reexamine the chronology of this episode. In addition we also interpret the origin of the activity using high-resolution bathymetric data, as well as from observations of sea-floor cable damage caused by high temperatures and lava flows. The analysis allows us to identify distinct active areas. Firstly, we interpret that this episode is mainly related to a diking event along a specific ridge segment, located at E044°. In light of previous diking episodes in nearby subaerial rift segments, for which field constraints and both seismic and geodetic data exist, we interpret the space-time evolution of the seismicity of the first few days. Migration of earthquakes suggests initial magma ascent below the segment center. This is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall. The cumulative seismic moment associated with this sequence reaches 9.1 × 1017 Nm, and taking into account a very low seismic versus geodetic moment, we estimate an horizontal opening of ∼0.58 to 2.9 m. The seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis, which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes. Over the whole period, the opening is estimated to ∼1.76-8.8 m across the segment. A striking feature of this episode is that the seismicity remained confined within one individual segment, whereas the adjacent en-echelon segments were totally quiescent, suggesting that the magma supply system of one segment is disconnected from those of the neighboring segments. Second, we identify activity induced by the first intrusion with epicenters aligned along a N035°E-trending, ∼30 km long at the northwestern end of the active opening segment. This group encompasses more than 7 earthquakes with magnitude larger than 5.0, and with strike-slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area. We propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation, at the locus of the trend change of the spreading ridge, which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust.
Highlights
1.1 Rifting episodes on the Aden/Red-Sea areaDuring the last decade, several important seismovolcanic events have occurred in Afar and its surrounding areas of the Red Sea and the Western Gulf of Aden (Fig. 1a), confirming the importance of volcanism in this extensional region above an abnormally warm mantle (Rooney et al 2012; Ferguson et al 2013; Hansen & Nyblade 2013)
We propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation, at the locus of the trend change of the spreading ridge, which corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust
14 dike intrusions have been identified along the ∼60 km long Dabbahu–Manda-Hararo (D-MH) rift segment between 2005 and 2010 from InSAR data (Wright et al 2006; Grandin et al 2009, 2010; Hamling et al 2009) and accompanied with hour to day long seismic swarms (Ebinger et al 2008; Ayele et al 2009; Keir et al 2009; Grandin et al 2010; Belachew et al 2011; Grandin et al 2011)
Summary
Several important seismovolcanic events have occurred in Afar and its surrounding areas of the Red Sea and the Western Gulf of Aden (Fig. 1a), confirming the importance of volcanism in this extensional region above an abnormally warm mantle (Rooney et al 2012; Ferguson et al 2013; Hansen & Nyblade 2013). In the Afar Depression, the so-called «Ardukoba» event in the Asal-Ghoubbet rift segment (Djibouti) in 1978, one of the first seismovolcanic events to have been monitored with geophysical methods, shed light on the important role of dike intrusions on the observed surface deformation (Abdallah et al 1979). The most recent documented magmatic event is the June 2011 eruption and subsidence of Nabro volcano (Ebinger et al 2013; Hamlyn et al 2014) Previous to these events, a dike intrusion and induced fault slip were observed near Dallol in October 2004 (Nobile et al 2012). Along submarine mid-oceanic ridges, the monitoring of active rifting process is usually achieved through seismic data from local or regional oceanic ocean bottom seismometer and autonomous underwater hydrophone networks (Fox et al 1995; Blackman et al 2000; Dziak et al 2009) or global land-based instruments We use the high-precision earthquake locations along with highresolution bathymetry data to discriminate between dike-induced faulting near the dike and later induced slip on a nearby transform fault
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