Constraining the Downdip Limit of the Lesser Antilles Seismogenic Zone With Seismicity Relocation

Abstract:

The Lesser Antilles subduction zone is characterized by a slow converging rate of about 19 mm/yr (DeMets et al., 2000). Several block models developed from GPS studies show a very low seismogenic coupling at the plate interface (e.g. Lopez et al., 2006; Symithe et al., 2015), resulting in a low seismicity rate and a low capacity to produce major earthquakes. In the Symithe et al. (2015) block model, the downdip limit of the seismogenic zone is fixed at 40 km, just below the tip of the mantle wedge. The mean downdip limit for world seismogenic zones is however deeper, at about 51 +/- 8 km (Heuret et al., 2011). Moreover, in the middle part of the Lesser Antilles, a felt thrust earthquake with a Mw of 5.8 occurred on February 3^rd, 2017 and was located at 51 km depth. Its aftershocks were all at depths around 50 km, suggesting a deeper downdip limit for the seismogenic zone (Bie et al., 2019). Here we study the interplate seismicity offshore Martinique. We use the catalogue of the Volcanological and Seismological Observatory of Martinique – Institut de Physique du Globe de Paris (OVSM-IPGP) from 2012 to 2019. We have relocated earthquakes of M > 2.8 using the NonLinLoc software (Lomax, 2000) associated with the recent velocity model from Bie et al. (2019). The new hypocenters of our dataset are distributed along the plate interface between 40 to 60 km, illuminating over the past decade a very deep part of the seismogenic zone, and confirming that the downdip limit of the Lesser Antilles seismogenic part can reach a depth to at least 60 km. This value is high in comparison with the mean values of world downdip limits. This surprisingly wide seismogenic zone may result in the propagation of a seismic rupture toward depth and thus the increase in earthquake magnitude and the seismic hazards. This could explain the occurrence of major historical events in the area (e.g. the Martinique M 8.0 earthquake in 1839 and the Guadeloupe M 8.5 in 1843) despite the low modeled seismogenic coupling on the plate interface.