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: Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems1. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges1 and are thought to be present at about half the world’s rifted margins2, 3. Serpentinite is also inferred to exist in the downgoing plate at subduction zones4, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults3, 4. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.