[1] The fore-arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. Joint analyses of the ocean bottom seismographs and land-based seismic data reveal that the swarms are located at depths of 50-150 km. Focal mechanism solutions, found by jointly fitting P wave first-motion polarities and S/P amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strike-and dip-slip mechanisms at depths of 50-100 km, while events at depths of 100-150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes: The slab segment east of 65 W longitude is dominated by trench-normal tensile stresses at shallower depths (50-100 km) and by trench-parallel tensile stresses at deeper depths (100-150 km), whereas the slab segment west of 65 W longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50-100 km). The simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench-normal tension and deeper trench-normal compression, is consistent with flexure of the slab due to rollback. These results support the hypothesis that the subducting North American plate is tearing at or near these swarms. The 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating.
Active plate boundaries in the Caribbean form a complex tectonic environment that includes transform and subduction zones. The Caribbean-North American plate boundary is one such active margin, where subduction transitions from arc- to oblique-type off the northeast coast of Puerto Rico. Understanding mantle flow in this region will not only help determine the nature of tectonic activity and mantle dynamics that control these margins, but will also aid our understanding of the fate of subducting lithosphere. The existence of tears, windows, and gaps in subducting slabs has been proposed at various locations around the world but few have been confirmed. Since mantle flow and crustal deformation are believed to produce seismic anisotropy in the asthenosphere and lithosphere, searching for changes in, for example, SKS splitting parameters can help identify locations at which subducting slabs have been disrupted. Several lines of evidence support the notion of a slab tear within the subducting North American plate at this transition zone, including the counter-clockwise rotation of the Puerto Rico microplate over the past ~10 Ma, clusters of small seismic events, and trench collapse initiating ~3.3 m.y. Here we present results from a detailed investigation of seismic anisotropy from 28 stations across six networks in the Northeast Caribbean that support the hypothesis of a significant slab gap in the vicinity of the U.S. and British Virgin islands. A regional synthesis of our results reveals fast shear wave polarizations that are generally oriented parallel to the plate boundary with intermediate to high SH-SV delay times. For example, polarization directions are oriented roughly NE-SW along the bulk of the Lesser Antilles, E-W along the Puerto Rico trench and the northern Lesser Antilles, and NW-SE beneath Hispaniola. Beneath the U.S. and British Virgin Islands, however, the fast polarization direction differs markedly from the regional pattern, becoming almost perpendicular to the plate boundary. Stations on Anegada, British Virgin islands and St. Croix, U.S. Virgin islands show a fast polarization direction that is oriented nearly NNE-SSW and smaller delay times than surrounding stations. These results suggest that mantle flow is redirected NE-SW at this location through a gap in the subducted lithosphere of the North American plate.
Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. Well‐documented slab tears that are associated with high rates of intermediate‐depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Seismicity at each of these locations occurs, at times, in the form of swarms or clusters, and various authors have proposed that each marks an active locus of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30–60 km vertically downward within the slab. We propose that these swarms and clusters are generated by fluid‐related embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be associated with the tearing process.
The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited.
Pioneer Natural Resources has recently made an intensive effort to acquire more than sixty microseismic jobs monitoring more than 1400 stages during the past two years in Texas unconventional shale plays. A majority of these data have been acquired over the Permian Wolfcamp Shale Formation of West Texas using one downhole acquisition system that allows for inter-comparisons of microseismic data. Pioneer has integrated microseismic data with 3D seismic attributes including elastic inversion products, well data including horizontal sonic scanners and formation micro imager (FMI) logs, along with completion data including tracers to better understand completions, geomechanical rock properties, and well spacing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.