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Today, resilience in the face of cyclone risks has become a crucial issue for our societies. With climate change, the risk of strong cyclones occurring is expected to intensify significantly and to impact the way of life in many countries. To meet some of the associated challenges, the interdisciplinary ReNovRisk programme aims to study tropical cyclones and their impacts on the South-West Indian Ocean basin. This article is a presentation of the ReNovRisk programme, which is divided into four areas: study of cyclonic hazards, study of erosion and solid transport processes, study of water transfer and swell impacts on the coast, and studies of socio-economic impacts. The first transdisciplinary results of the programme are presented together with the database, which will be open access from mid-2021.

SUMMARYWe investigate the upper mantle seismic structure beneath southern Madagascar and infer the imprint of geodynamic events since Madagascar’s break-up from Africa and India and earlier rifting episodes. Rayleigh and Love wave phase velocities along a profile across southern Madagascar were determined by application of the two-station method to teleseismic earthquake data. For shorter periods (

The Precambrian rocks of Madagascar were formed and/or modified during continental collision known as the Pan-African orogeny. Aborted Permo-Triassic Karoo rifting and the subsequent separation from Africa and India resulted in the formation of sedimentary basins in the west and volcanic activity predominantly along the margins. Many geological studies have documented the imprint of these processes, but little was known about the deeper structure. We therefore deployed seismic stations along an SE-NW trending profile spanning nearly all geological domains of southern Madagascar. Here, we focus on the crustal structure, which we determined based on joint analysis of receiver functions and surface waves derived from ambient noise measurements. For the sedimentary basin we document a thinning of the underlying crystalline basement by up to ∼60% to 13 km. The crustal velocity structure demonstrates that the thinning was accomplished by removal or exhumation of the lower crust. Both the Proterozoic and Archean crust have a 10 km thick upper crust and 10-12 km thick midcrust. However, in contrast to the typical structure of Proterozoic and Archean aged crust, the Archean lower crust is thicker and faster than the Proterozoic one, indicating possible magmatic intrusions; an underplated layer of 2-8 km thickness is present only below the Archean crust. The Proterozoic mafic lower crust might have been lost during continental collision by delamination or subduction, or thinned as a result of extensional collapse. Finally, the Cretaceous volcanics along the east coast are characterized by thin crust (30 km) and very large VP/VS ratios.

International audience; The crust and upper mantle of the Madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments. An island-wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations, derived from surface waves, across the entire island. Late Cenozoic alkaline intraplate volcanism has occurred in three separate regions of Madagascar (north, central and southwest), with the north and central volcanism active until

Madagascar occupies a key position in the assembly and break-up of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear-wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small-scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one-layer splitting parameters. We use finite-difference, full-waveform modelling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small-scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast-axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan-African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.

The seismicity and seismotectonics of Madagascar have been studied using an island-wide distribution of broadband seismic stations. The 28-station MAdagascar-COmoros-MOzambique (MACOMO) array was deployed for a 23-month period between 2011 and 2013. MACOMO data were supplemented by seven temporary stations from the Seismological Signatures in the Lithosphere/Asthenosphere system of Southern Madagascar (SELASOMA) project, ten temporary stations from the Reunion Hotspot and Upper Mantle - Reunions Unterer Mantel project (RHUM-RUM), and 11 permanent stations. A total of 695 earthquakes with magnitudes between ML1 and ML5.3 located within Madagascar were recorded, a new local magnitude scale was developed, and focal mechanisms were determined for 23 well-recorded events. Most of the seismicity is clustered within central Madagascar, forming NW-SE trends in the Ankaratra region and NE-SW trends in the Alaotra and Ankay regions that roughly align with extensional tectonic features. The northern and southern parts of Madagascar also show seismicity clusters that align parallel to existing tectonic features, primarily Precambrian shear zones. Focal mechanisms exhibit a wide orientation of nodal planes, show predominantly normal faulting throughout Madagascar, and provide no evidence for a sharp east-west striking plate boundary between the Lwandle and Somalian plates in the middle of the island. However, a diffuse plate boundary cannot be excluded. We suggest that topographically-generated extensional stresses is the cause of most seismicity, given that significantly fewer earthquakes are located in the lower elevation areas of the island compared to regions of higher elevations. The frequency-magnitude distribution has a b-value of ~1.2 and indicates that events with magnitude ML5 or greater should occur with an average repeat time of ~1.4 years. However, no earthquake exceeding ML6 has been recorded in the last century, suggesting that the frequency-magnitude distribution could be truncated.

We determined radial anisotropy in the crust of southern Madagascar from the differences between the speeds of vertically and horizontally polarized shear waves (VSV and VSH), which we derived from Rayleigh and Love wave dispersion determined from seismic ambient noise correlations. The amalgamated Precambrian units in the east and the Phanerozoic Morondava basin in the west of southern Madagascar were shaped by different geodynamic processes. The crystalline basement was strongly deformed and metamorphosed to varying degrees during the assembly of Gondwana in the Pan‐African Orogeny, whereas the Morondava basin was completed with the separation of Africa and Madagascar. The different developments are reflected in first‐order differences in the radial anisotropy patterns. In the Precambrian domains, positive anisotropy (VSVVSH) sandwiched in between. The upper crustal anisotropy may reflect shallowly dipping layering within the Archean and adjacent imbricated nappe stacks, whereas the lower crustal anisotropy likely represents fossilized crustal flow during the postorogenic or synorogenic collapse of the Pan‐African Orogen. The negative anisotropy layer may have preserved vertically oriented large shear zones of late Pan‐African age. Within the Morondava basin, negative anisotropy in the uppermost ∼5 km could have been generated by steep normal faults, jointing, and magmatic dike intrusions. The deeper sediments and underlying crustal basement are characterized by positive anisotropy. This is consistent with horizontal bedding in the sediments and with fabric alignment in the basement created by extension during the basin formation.

The aim of this study is to improve the knowledge of the seismotectonics of Madagascar. We first investigate the structure of the Earth beneath Madagascar through the joint inversion of receiver functions and Rayleigh wave group velocities. Then we use the obtained velocity models to relocate local earthquakes in order to analyse the distribution of seismicity. Finally, we use structural models and earthquake coordinates to compute focal mechanisms. Our retrieved Earth structure models confirm a thin lithosphere beneath Madagascar when compared to the nearby East African Rift. The High Plateau in the Central region coincides with the thinnest lithosphere over the slowest asthenosphere. Our results are in good agreement with the gravity anomalies and likely confirm a localised asthenospheric upwelling beneath the central part of Madagascar. The surface expression of the asthenospheric upwelling consists in a horst–graben structure. The moderate seismicity is localised along pre-existing structures reflecting an E-W extension that is mostly accommodated in the lower crust.