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In this paper, we perform full-waveform regional moment tensor (RMT) inversions, to gain insight into the stress distribution along the Lesser Antilles arc. We developed a novel inversion approach, AmPhiB - Amphibious Bayesian, taking into account uncertainties associated with OBS deployments like the orientation of horizontal components and the high noise level. The inversion is conducted using a direct, uniform importance sampling of the fault parameters within a tree structure. We show that the alignment of the horizontal OBS components influences the obtained source mechanism when using standard least-squares (L2) RMT inversion schemes, resulting in systematic errors in the recovered focal mechanisms including high artificial CLVD contributions. AmPhiB reduces these CLVD components by nearly 60% and the aberration of the focal geometry as measured by the Kagan angle by around 40% relative to a standard L2 inversion. Subsequently, we obtain 45 (Mw > 3.8) regional MT solutions. Combining our new results with existing solutions, we subsequently analyze a total of 151 solutions in a FMC diagram and map them to the regional tectonic setting. We perform stress tensor inversions along the arc. On the plate interface, we observe a compressional stress regime and find evidence for upper-plate strike slip and normal fault behaviour in the north and a near arc-perpendicular extensional stress regime towards the south. A dominant slab perpendicular extensional stress regime is found in the slab at 100-200 km at the arcs center. We interpret this stress condition to be a result of slab pull varying along the arc due to partial slab detachment along previously hypothesized lateral slab tear near Grenada, at the southern end of the LA arc, leading to reactivation of preexisting structures around the subducted Proto-Caribbean ridge., 27 pages of text, 13 figures, 2 tables; submitted for review to GJI

Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards. Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust.

The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp∕vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw = 8.7) and to the south in 2007 (Bengkulu earthquake, Mw = 8.5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ∼ 180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s−1 at 10 km depth and the vp∕vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp∕vs values of 1.85 at ∼ 150 km distance to the trench in the region of the Mentawai Fault. vp∕vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust.