Structure of the oceanic lithosphere and upper mantle north of the Gloria fault in the eastern mid-Atlantic by receiver function analysis

Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on-shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here, we use OBSs deployed as mid-aperture array in the deep ocean (4.5-5.5 km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beam forming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8 km) of oceanic crust. Observations at single stations with thin sediments (300-400 m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of ∼70-80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events

Une expérience d’enseignement par Internet sur le MOOC « Préparation à l'entrée dans l'enseignement supérieur »

Le but est de présenter un MOOC de préparation en mathématiques à l’enseignement supérieur et d’en analyser diverses utilisations. Après un exposé des motivations de sa création et la description du contenu, on décrit les différentes sessions qui ont déjà eu lieu. On examine ensuite les nombres d’inscrits aux différentes sessions ainsi que celui des certificats de réussite délivrés lord des sessions d’été, avant d’analyser les résultats aux QCM et aux devoirs en correction par les pairs. Après une brève description de deux expériences pédagogiques, on termine par un aperçu de la portée internationale du MOOC