Tidal triggering of microseismicity at the equatorial mid‐Atlantic ridge, inferred from the PI‐LAB experiment

The gravitational pulls from the moon and the sun result in tidal forces which influence both Earth's solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for fast- and intermediate-spreading mid-ocean ridges in the Pacific Ocean. Here, we present a microseismicity data set (4,719 events) recorded by an ocean bottom seismometer deployment at the equatorial Mid-Atlantic Ridge. We show that low, as well as decreasing ocean water level, result in relatively elevated seismicity rates at higher magnitudes (lower b-values), translated into increased probabilities of stronger event occurrence at or towards low tides. Moreover, seismic bursts (enhanced activity rate clusters), occurring at rates well above the reference seismicity, are exclusively present during values of either high tidally induced extensional stresses or high extensional stress rates. Although the b-value differences are not significant enough to be conclusive, the seismicity rate variations exhibit statistical significance, supporting the previous findings for tidal triggering at low tides within normal-faulting regimes and extending the range of observations to slow-spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modeling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to the presence of a shallow magma body beneath the ridge, as supported by previous seismic imaging in the region

The role of neutralizing antibodies in prevention of HIV-1 infection: what can we learn from the mother-to-child transmission context?

International audienceIn most viral infections, protection through existing vaccines is linked to the presence of vaccine-induced neutralizing antibodies (NAbs). However, more than 30 years after the identification of AIDS, the design of an immunogen able to induce antibodies that would neutralize the highly diverse HIV-1 variants remains one of the most puzzling challenges of the human microbiology. The role of antibodies in protection against HIV-1 can be studied in a natural situation that is the mother-to-child transmission (MTCT) context. Indeed, at least at the end of pregnancy, maternal antibodies of the IgG class are passively transferred to the fetus protecting the neonate from new infections during the first weeks or months of life. During the last few years, strong data, presented in this review, have suggested that some NAbs might confer protection toward neonatal HIV-1 infection. In cases of transmission, it has been shown that the viral population that is transmitted from the mother to the infant is usually homogeneous, genetically restricted and resistant to the maternal HIV-1-specific antibodies. Although the breath of neutralization was not associated with protection, it has not been excluded that NAbs toward specific HIV-1 strains might be associated with a lower rate of MTCT. A better identification of the antibody specificities that could mediate protection toward MTCT of HIV-1 would provide important insights into the antibody responses that would be useful for vaccine development. The most convincing data suggesting that NAbs migh confer protection against HIV-1 infection have been obtained by experiments of passive immunization of newborn macaques with the first generation of human monoclonal broadly neutralizing antibodies (HuMoNAbs). However, these studies, which included only a few selected subtype B challenge viruses, provide data limited to protection against a very restricted number of isolates and therefore have limitations in addressing the hypervariability of HIV-1. The recent identification of highly potent second-generation cross-clade HuMoNAbs provides a new opportunity to evaluate the efficacy of passive immunization to prevent MTCT of HIV-1

Earth science: The slippery base of a tectonic plate

In the theory of plate tectonics, the outer shell of the Earth, known as the lithosphere, consists of several rigid plates, which move relative to each other over the weaker, flowing asthenosphere. The bottom of the lithosphere, the lithosphere–asthenosphere boundary (LAB), is fundamental to our understanding of how plate tectonics works, although an exact understanding of the mechanism that gives the plates their rigidity and defines their thickness remains elusive and widely debated. On page 85 of this issue, Stern et al.1 describe how they have used reflected seismic waves generated by explosive sources in steel-cased boreholes to image the Pacific plate as it descends beneath New Zealand. They find a LAB that is less than 1 kilometre thick at the top of a 10-km-thick channel, in which slow seismic velocities may require the presence of water or melt (Fig. 1). The authors suggest that the thin channel decouples the lithosphere from the asthenosphere and allows plate tectonics to take place. The existence of such a localized channel probably has implications for the driving forces of plate tectonics and mantle dynamics

Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small‐Scale Convection From the PI‐LAB Experiment

The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies

Seismic imaging of melt in a displaced Hawaiian plume

The Hawaiian Islands are the classic example of hotspot volcanism: the island chain formed progressively as the Pacific plate moved across a fixed mantle plume1. However, some observations2 are inconsistent with simple, vertical upwelling beneath a thermally defined plate and the nature of plume-plate interaction is debated. Here we use S-to-P seismic receiver functions, measured using a network of land and seafloor seismometers, to image the base of a melt-rich zone located 110 to 155 km beneath Hawaii. We find that this melt-rich zone is deepest 100 km west of Hawaii, implying that the plume impinges on the plate here and causes melting at greater depths in the mantle, rather than directly beneath the island. We infer that the plume either naturally upwells vertically beneath western Hawaii, or that it is instead deflected westwards by a compositionally depleted root that was generated beneath the island as it formed. The offset of the Hawaiian plume adds complexity to the classical model of a fixed plume that ascends vertically to the surface, and suggests that mantle melts beneath intraplate volcanoes may be guided by pre-existing structures beneath the islands