WP1-Current seismic sequence, deformation and kinematics
Leader: Nathalie Feuillet (IPGP) Participants & coll.: D. Bertil (BRGM), C. Doubre (IPGS/EOST), P. Briole (ENS), A. Lemoine (BRGM), J. Van der Woerd (IPGS/EOST), A. Maggi (IPGS/EOST), C. Satriano (IPGP), P. Bernard (IPGP), W. Crawford (IPGP). J. Battaglia (Clermont-Ferrand U.), A. Lemarchand (IPGP), M. Grunberg (IPGS/EOST), M. De Michele (BRGM), I. Thinon (BRGM), F. Paquet (BRGM), E. Jacques (IPGP).
The objectives of this workpackage are to understand the origin of the current seismo-volcanic sequence (magmatic or tectonic origin and possible volcano-tectonic interactions) and to propose a present-day kinematic model at the regional scale and at the scale of the Comoros axis.
* Task 1.1: Analysis of the current Mayotte Seismic Sequence (MSS) (Leader: D. Bertil)
The main objective of this task is to understand the origin and the dynamics of the MSS, in relation with the tectonic and volcanic structures of the area and with the geodetic observations. The seismic datasets acquired before (ex: RHUM-RUM project) and during the MSS will be analyzed. The study will be primarily conducted in the framework of a PhD thesis (PhD0, BRGM/IPGS co-funding, not included in this proposal).
The first step will be to construct an exhaustive catalog, with accurate focal parameters and magnitude estimates. It will benefit of the temporary deployment of both inland stations and OBS, since a part of the study will focus on the determination of the velocity model. Advanced detection and location techniques will be employed from OBS data to enrich the seismic catalogue. Seismic events will be relocated using well-located earthquakes from OBS as master events. This new catalogue will allow assessing the space-time statistics and dynamics of the seismicity to identify whether it is driven by cascade processes or by forcing transients. In order to characterize the intrusion/eruption dynamics and magma migrations, seismic signals will be classified into families, and characteristic repeating events will be looked for. The joint analysis of earthquake location and focal mechanism will allow to better understand the relationships between the MSS, structures and the stress regime at regional scale (linked to WP2).
A second step will be a systematic search for monochromatic low frequency tremors similar to the unusual event occurring November 11, 2018. The joint analysis of space-time dynamics of regular earthquakes and monochromatic tremor will help understand whether the latter is associated with shear or pressure sources. The precise timing of detected tremor events will be used to search for small transients in GNSS data, which may help modelling and interpreting the source mechanisms (linked to T1.2).
Finally, all the detection methods described above will be used on data preceding the start of the MSS (May 2018) to check whether MSS was anticipated by precursor activity.
- Task 1.2: Active Deformation (Leader: P. Briole)
The use of seismic data to quantify the co- and post-seismic and/or dyking deformation will be completed by the surface displacements on Mayotte, using spatial geodetic techniques (GNSS permanent and temporary network and InSAR observations). Offshore, the ground displacement field will be estimated from differential bathymetry method using BATHYMAY data (2004) versus the new data acquired during the MAYOBS cruise (INSU/Tellus). The location and the size of the active structures will be constrained by the observations of the seafloor during MAYOBS, such as submarine flank morphology, evidence of volcanic activity on the seafloor or within the sediment cover.
All these data and observations will be integrated into mechanical models to constrain the depth and size of the source(s) and its evolution over time, together with the estimate of the volume of magma involved over the whole MSS (e.g. Grandin et al., 2009, Lemoine et al., subm.).
- Task 1.3: Local and Regional Recent Kinematics (Leader: C. Doubre)
The main product of this study will be a compilation of active structures (faults, volcanism,…) at the regional scale and at the MSS scale. This compilation will come from those documented in the literature and from the interpretations based on recent bathymetric and seismic data (BATHYMAY, MAYOBS, SISMAORE, SISMAYOTTE). This new tectonic map will be used to compare the distribution and kinematics of active structures with the seismicity distribution and focal mechanisms. These works will provide a basis to evaluate whether the Comoros volcanism is spatially related to the distribution and development of the active faults in the same area. We will focus on the relationships between volcanism and major regional faults.
Together with the worldwide and Africa-Indian Ocean regional networks, the new GNSS data will allow us to build a regional present-day, plate tectonic, kinematic model in order to better understand kinematics of the southern termination of the East-African Rift, its relation with the Indian Ocean Ridge and other structures, such as the Davie Ridge etc…, and focusing on the kinematic nature (divergent vs. transform plate incipient boundary?) and role of the Comoros archipelago axis in the regional geodynamics.