@article {lindner_bayesian_2023, title = {Bayesian regional moment tensor from ocean bottom seismograms recorded in the Lesser Antilles: implications for regional stress field}, journal = {Geophysical Journal International}, volume = {233}, number = {2}, year = {2023}, month = {12/2022}, pages = {1036{\textendash}1054}, abstract = {Seismic activity in the Lesser Antilles (LA) is characterized by strong regional variability along the arc reflecting the complex subduction setting and history. Although routine seismicity monitoring can rely on an increasing number of island stations, the island-arc setting means that high-resolution monitoring and detailed studies of fault structures require a network of ocean bottom seismometers (OBS). As part of the 2016{\textendash}2017 Volatile recycling at the Lesser Antilles arc (VoiLA) project, we deployed 34 OBS stations in the forearc and backarc. During the deployment time, 381 events were recorded within the subduction zone. In this paper, we perform full-waveform regional moment tensor (RMT) inversions, to gain insight into the stress distribution along the arc and at depth. We developed a novel inversion approach, AmΦB{\textemdash}{\textquoteleft}Amphibious Bayesian{\textquoteright}, taking into account uncertainties associated with OBS deployments. Particularly, the orientation of horizontal components (alignment uncertainty) and the high noise level on them due to ocean microseisms are accounted for using AmΦB. The inversion is conducted using a direct, uniform importance sampling of the fault parameters within a multidimensional tree structure: the uniXtree-sampling algorithm. We show that the alignment of the horizontal OBS components, particularly in high noise level marine environments, 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 compensated linear vector dipole (CLVD) contributions. Our Bayesian formulation in AmΦB reduces these CLVD components by nearly 60~per~cent and the aberration of the focal geometry as measured by the Kagan angle by around 40~per~cent relative to a standard L2 inversion. Subsequently, we use AmΦB-RMT to obtain 45 (Mw \> 3.8) regional MT solutions, out of which 39 are new to any existing database. Combining our new results with existing solutions, we subsequently analyse a total of 151 solutions in a focal mechanism classification (FMC) diagram and map them to the regional tectonic setting. We also use our newly compiled RMT database to perform stress tensor inversions along the LA subduction zone. On the plate interface, we observe the typical compressional stress regime of a subduction zone and find evidence for upper-plate strike slip and normal fault behaviour in the north that becomes a near arc-perpendicular extensional stress regime towards the south. A dominant slab perpendicular extensional stress regime is found in the slab at 100{\textendash}200~km beneath the central part of the arc. 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 pre-existing structures around the subducted Proto-Caribbean ridge.}, issn = {0956-540X}, doi = {10.1093/gji/ggac494}, url = {https://doi.org/10.1093/gji/ggac494}, author = {Lindner, Mike and Rietbrock, Andreas and Bie, Lidong and Goes, Saskia and Collier, Jenny and Rychert, Catherine and Harmon, Nicholas and Hicks, Stephen P and Henstock, Tim and the VoiLA working group} } @article {schlaphorst_local_2023, title = {Local seismicity around the Chain Transform Fault at the Mid-Atlantic Ridge from OBS observations}, journal = {Geophysical Journal International}, volume = {234}, number = {2}, year = {2023}, note = {Publisher: Oxford University Press (OUP)}, month = {08/2023}, pages = {1111{\textendash}1124}, abstract = {Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean{\textendash}bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterized by normal faulting and most of the transform events are characterized by strike-slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 {\textpm} 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600 {\textdegree}C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit.}, keywords = {3706 Geophysics}, issn = {0956-540X}, doi = {10.1093/gji/ggad124}, author = {Schlaphorst, David and Rychert, Catherine A. and Harmon, Nicholas and Hicks, Stephen P. and Bogiatzis, Petros and Kendall, J-Michael and Abercrombie, Rachel E.} } @article {dai_seismic_2023, title = {Seismic imaging beneath Cascadia shows shallow mantle flow patterns guide lower mantle upwellings}, journal = {Journal of Geophysical Research: Solid Earth}, year = {2023}, note = {_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2023JB026374}, pages = {e2023JB026374}, abstract = {The mantle transition zone plays an important role in modulating material transport between the upper mantle and the lower mantle. Constraining this transport is essential for understanding geochemical reservoirs, hydration cycles, and the evolution of the Earth. Slabs and hotspots are assumed to be the dominant locations of transport. However, the degree of material transport in other areas is debated. We apply P-to-S receiver functions to an amphibious dataset from Cascadia to image the mantle transition zone discontinuities beneath mid-ocean ridges, a hotspot, and a subduction zone. We find a mantle transition zone thinned by 10 {\textpm} 6 km beneath the ridges and by 8 {\textpm} 4 km behind the slab, closely resembling the 660 discontinuity topography. Depressions on the 410 discontinuity are smaller, 5 {\textpm} 2 km on average, focused in the north and the south and accompanied by supra-410 discontinuity melt phases. The depressions occur away from locations of uplifted 660 discontinuity, but near slow seismic velocity anomalies imaged in the upper mantle. This suggests lower mantle upwellings occur beneath ridges and slabs but stall in the transition zone, with upper mantle convection determining upward material transport from the transition zone. Therefore, upper mantle dynamics play a larger role in determining transfer than typically assumed.}, issn = {2169-9356}, doi = {10.1029/2023JB026374}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2023JB026374}, author = {Dai, Yuhang and Rycherrt, Catherine A. and Harmon, Nicholas} } @article {cooper_variable_2020, title = {Variable water input controls evolution of the Lesser Antilles volcanic arc}, journal = {Nature}, volume = {582}, number = {7813}, year = {2020}, note = {Number: 7813 Publisher: Nature Publishing Group}, month = {jun}, pages = {525{\textendash}529}, abstract = {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{\textemdash}that is, hydrated mantle rather than crust or sediments{\textemdash}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.}, keywords = {Geochemistry, Geodynamics, Geophysics, seismology, Volcanology}, issn = {1476-4687}, doi = {10.1038/s41586-020-2407-5}, url = {https://www.nature.com/articles/s41586-020-2407-5}, author = {Cooper, George F. and Macpherson, Colin G. and Blundy, Jon D. and Maunder, Benjamin and Allen, Robert W. and Goes, Saskia and Collier, Jenny S. and Bie, Lidong and Harmon, Nicholas and Hicks, Stephen P. and Iveson, Alexander A. and Prytulak, Julie and Rietbrock, Andreas and Rychert, Catherine A. and Davidson, Jon P.} } @article {Agius2017, title = {Mapping the mantle transition zone beneath Hawaii from Ps receiver functions: Evidence for a hot plume and cold mantle downwellings}, journal = {Earth and Planetary Science Letters}, volume = {474}, year = {2017}, month = {sep}, pages = {226{\textendash}236}, issn = {0012821X}, doi = {10.1016/j.epsl.2017.06.033}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0012821X17303497}, author = {Agius, Matthew R. and Rychert, Catherine A. and Harmon, Nicholas and Laske, Gabi} } @article {Neale2015, title = {Source regions and reflection of infragravity waves offshore of the USA{\textquoteright}s Pacific Northwest}, journal = {Journal of Geophysical Research: Oceans}, volume = {120}, number = {9}, year = {2015}, month = {sep}, pages = {6474{\textendash}6491}, issn = {2169-9275}, doi = {10.1002/2015JC010891}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/2015JC010891}, author = {Neale, Jennifer and Harmon, Nicholas and Srokosz, Meric} }