Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. ...Between 2005 and 2009 the ∼70 km long Dabbahu‐Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started from a <5 km radius zone at the middle of the Dabbahu‐Manda Hararo segment, and traveled northward and southward along the rift axis. Small magnitude earthquakes associated with the margins of the propagating dike tips are followed by the largest magnitude, primarily low‐frequency earthquakes. The seismic moment distributions show >80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low‐frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment.
Key Points
Seismicity patterns of dike intrusions in Afar, Ethiopia
Location of magma feeder zones with in the active rift
Timing and dynamics of dike intrusion processes
The process of dike emplacement changes the stress field in the intruded region, causing swarms of migrating earthquakes. We determine source mechanisms of the largest earthquakes (M
L ≥ 3.5) induced ...by the emplacement of two large volume dikes along an incipient seafloor spreading segment in Afar, Ethiopia to determine their space-time relations. Given the possibility of complex source mechanisms during dike emplacement, we solved for four different source models: double couple (DC), DC + isotropic (DC + ISO), deviatoric (DVMT), and full moment tensor (FMT). The solutions obtained using the different models indicate that the earthquakes have non-double couple components. The best double-couple mechanisms, which are mainly normal faulting with small strike-slip components, have slip planes nearly perpendicular to the geodetically determined plate opening direction. Most of these earthquakes are low-frequency earthquakes with peak frequencies ≤2 Hz, and they occurred during the propagation phase of the dikes. The space-time distribution of the source mechanisms with respect to the migrating patterns of seismicity during dike emplacement, the shallow source depths estimated, the lack of mechanisms with ∼90º rotated P-axes from the regional maximum compressive stress and the non-double couple nature of the mechanisms indicate that the largest magnitude earthquakes are generated mainly by normal faulting above the dikes probably with some component of tensile opening under the influence of dike-related fluids. These observations suggest that normal faulting above dikes is the main process of seismic energy release during dike intrusions. Assuming that faults above dikes follow length-displacement relations found for tectonic earthquakes, total seismic slips of 0.8 m and 1.3 m are estimated for the November 2007 and October 2008 dikes, respectively. Similarity of the total slip estimates from the largest earthquakes and from elastic dislocation model estimates of normal fault slips and the larger total seismic versus geodetic moment deficits indicates that most of the plate opening by dike intrusion is accommodated aseismically. Our results show that during dike intrusions surface faulting above dikes is the main process that controls the development of rift zone geomorphology and topography.
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.
Key Points
Seismicity in Northern Afar is focused at the rift axis and western rift margin
Low‐frequency seismicity at the rift axis shows evidence for an ∼12 km deep magma complex
Seismicity at the western rift margin is caused by faulting associated with enhanced crustal thinning or crustal flexure
Using local body wave arrival‐time tomography methods to determine 3‐D seismic velocity structure, we imaged the plumbing system of Sierra Negra Volcano, Galápagos. This hot spot volcanic chain ...includes some of the fastest deforming volcanoes in the world, making this an ideal location to study shield volcano plumbing systems. We inverted P and S wave arrivals recorded on a 15‐station temporary array between July 2009 and June 2011 using an a priori 1‐D velocity model constrained by offshore refraction studies. With local seismicity from nearby volcanoes as well as the ring fault system, the model resolution is good between depths of 3 and 15.5 km. The propagation of S waves throughout this volume argues against any large high‐melt accumulations, although a shallow melt sill may exist above 5 km. We image a broad low‐velocity region (>25 km laterally) below Sierra Negra at depths ~8–15 km. No large, regional velocity increase is found within the limits of good resolution, suggesting that crust is thicker than 15 km beneath the western Galápagos archipelago. Our results are consistent with crustal accretion of mafic cumulates from a large‐volume magma chamber that may span the boundary between preplume and accreted crust. The similarity between our results and those of Hawaii leave open the possibility that the crust has also been thickened by under‐plating.
Key Points
Image crustal structure beneath Sierra Negra Volcano using body wave tomography
Broad low‐velocity zone below ~5 km depth interpreted as magma storage zone
Ocean island grows from settling of cumulates in lower crustal magma chamber
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