ALBERT

Description: The Danakil region of northern Afar is an area of ongoing seismic and volcanic activity caused by the final stages of continental breakup. To improve the quantification of seismicity, we developed a calibrated local earthquake magnitude scale. The accurate calculation of earthquake magnitudes allows the estimation of b -values and maximum magnitudes, both of which are essential for seismic-hazard analysis. Earthquake data collected between February 2011 and February 2013 on 11 three-component broadband seismometers were analyzed. A total of 4275 earthquakes were recorded over hypocentral distances ranging from 0 to 400 km. A total of 32,904 zero-to-peak amplitude measurements ( A ) were measured on the seismometer’s horizontal components and were incorporated into a direct linear inversion that solved for all individual local earthquake magnitudes ( M L ), 22 station correction factors ( C ), and 2 distance-dependent factors ( n , K ) in the equation M L =log( A )–log( A 0 )+ C . The resultant distance correction term is given by –log( A 0 )=1.274336log( r /17)–0.000273( r –17)+2. This distance correction term suggests that attenuation in the upper and mid-crust of northern Afar is relatively high, consistent with the presence of magmatic intrusions and partial melt. In contrast, attenuation in the lower crust and uppermost mantle is anomalously low, interpreted to be caused by a high melt fraction causing attenuation to occur outside the seismic frequency band. The calculated station corrections serve to reduce the M L residuals significantly but do not show a correlation with regional geology. The cumulative seismicity rate produces a b -value of 0.9±0.06, which is higher than most regions of continental rifting yet lower than values recorded at midocean ridges, further supporting the hypothesis that northern Afar is transitioning to seafloor spreading. Electronic Supplement: List of all local earthquakes used in the study with calculated local magnitudes and associated magnitude error.

Description: Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.