Off Shore Geodetic Measurements Simulations in the Context of Seismic and Tsunami Hazard Evaluation in the Lesser Antilles

Abstract:

Based on the current state of knowledge, the megathrust/tsunami hazard estimation in the Lesser Antilles forearc remains uncertain. Some major events have been reported (e.g. the 1843 earthquake estimated with a IX intensity), however no associated mega-tsunami has been recorded, maybe because of the nature of the event (slab locked up to the trench or not) or the too short observation period.

GNSS monitoring networks are deployed on all Caribbean Islands (Guadeloupe and Martinique in particular). However, land areas are far from the trench, and their configuration is not optimal for the strain measurement related to a possible locking between the two plates up to the seafloor.

The GPS/Acoustics (GPS/A) technique aims to overcome this limitation. It consists of a surface platform used as a relay between aerial and underwater media. The position is obtained in a global reference frame by GNSS kinematic processing and is transferred to the seafloor by acoustic ranging to a set of transponders permanently installed on the seabed. Repeated measurements over the years will allow to compute the velocity of the study area in a global reference frame.

We present a case study for a future deployment of this kind of submarine network off the French Caribbean Islands. Numerical simulations of GPS/A are performed in order to evaluate the accuracy achievable in the Antilles context, using water variability information from past oceanographic campaigns and MOVE buoys. The kinematic GNSS treatments are carried out on test cruises data by different methods (real-time differential, differential post treatment and Precise Point Positioning) to assess the performances in different conditions.
In order to characterize the geophysical context, we also present a reprocessing of the GNSS stations of the Guadeloupe and Martinique Islands using a PPP approach with the CNES GINS software, along with a finite element model of the subduction zone.