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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.

Like most other industrial activities that affect the subsurface, hydraulic fracturing carries a risk of reactivating pre-existing faults and thereby causing induced seismicity. In some regions, regulators have responded to this risk by imposing Traffic Light Scheme-type regulations, where fracture stimulation programs must be amended or shut down if events larger than a given magnitude are induced. Some sites may be monitored with downhole arrays and/or dense near-surface arrays, capable of detecting very small microseismic events. However, such monitoring arrangements will not be logistically or economically feasible at all sites. Instead, operators are using small, sparse arrays of surface seismometers to meet their monitoring obligations.The challenge we address in this paper is to maximise the detection thresholds of such small, sparse, surface arrays, so that they are capable of robustly identifying small-magnitude events, whose signal-to-noise ratios may be close to 1. To do so we develop a beam-forming-and-stacking approach, computing running short-term/long-term average functions for each component of each recorded trace (P, SH and SV), time-shifting these functions by the expected travel-times for a given location, and performing a stack.We assess the effectiveness of this approach with a case study, using data from a small surface array that recorded a multi-well, multi-stage hydraulic fracture stimulation in Oklahoma over a period of 8 days. As a comparison, we initially used a conventional event-detection algorithm to identify events, finding a total of 17 events. In contrast, the beam-forming-and-stacking approach identified a total of 155 events during this period (including the 17 events detected by the conventional method). The events that were not detected by the conventional algorithm had low signal-to-noise ratios, to the extent that in some cases they would be unlikely to be identified even by manual analysis of the seismograms. We conclude that this approach is capable of improving the detection thresholds of small, sparse arrays, and so can be used to maximise the information generated when deployed to monitor industrial sites.