Constraints on the thickness, transitional boundaries, and composition of Earth's crust are pivotal in studying its formation and evolution. We use data from 132 seismic installations throughout the northeastern US to explore how tectonic events, such as orogenesis and rifting, have altered the crust of the northeastern US and southeastern Canada, and to distinguish between Laurentia and the Appalachian terranes. We include data from seismic installations from the NEST and SEISConn experiments, spanning the Laurentia–Appalachian boundary, and present estimates of crustal thickness, V p / V s , and thickness of the transition between crustal and mantle rocks using Ps receiver functions. We find some first-order differences between Laurentia and Appalachian terranes, with Laurentia exhibiting thicker crust ( c. 39 v. c. 33 km) and a broader crust–mantle transition thickness ( c. 3 v. <1.5 km). Average V p / V s values are similar between Laurentia ( c. 1.77) and Appalachian terranes ( c. 1.74); however, we identify anomalous V p / V s in a few regions, including high V p / V s around the Adirondack Mountains and low V p / V s in southern New England. The southern New England region is also anomalous in terms of its systematically thinner crust and sharper crust–mantle transition, which may be a consequence of the formation and collapse of the Acadian altiplano during the mid-to-late Paleozoic.

AbstractSeismic tomography observations show a low‐velocity feature in the upper mantle beneath eastern North America known as the Northern Appalachian Anomaly (NAA). Proposed models for the formation of the NAA include a remnant high‐temperature feature resulting from the passage of the Great Meteor Hotspot, edge‐driven convection, and ongoing asthenospheric upwelling. We investigate the structure of the lithosphere above the central portion of the NAA using data from the New England Seismic Transects (NEST) experiment. Ps receiver functions reveal two consistent interfaces beneath the dense northern line of NEST: the Moho (the base of the crust) and a deeper negative velocity gradient (NVG) feature located at depths between 60 and 110 km. We consider several potential explanations for this NVG feature; based on comparisons with previous results, we propose that it likely corresponds to the lithosphere‐asthenosphere boundary. Our results indicate that the lithosphere beneath New England is nonuniform and has likely been thinned.

On 5 April 2024, 10:23 a.m. local time, a moment magnitude 4.8 earthquake struck Tewksbury Township, New Jersey, about 65 km west of New York City. Millions of people from Virginia to Maine and beyond felt the ground shaking, resulting in the largest number (>180,000) of U.S. Geological Survey (USGS) “Did You Feel It?” reports of any earthquake. A team deployed by the Geotechnical Extreme Events Reconnaissance Association and the National Institute of Standards and Technology documented structural and nonstructural damage, including substantial damage to a historic masonry building in Lebanon, New Jersey. The USGS National Earthquake Information Center reported a focal depth of about 5 km, consistent with a lack of signal in Interferometric Synthetic Aperture Radar data. The focal mechanism solution is strike slip with a substantial thrust component. Neither mechanism’s nodal plane is parallel to the primary northeast trend of geologic discontinuities and mapped faults in the region, including the Ramapo fault. However, many of the relocated aftershocks, for which locations were augmented by temporary seismic deployments, form a cluster that parallels the general northeast trend of the faults. The aftershocks lie near the Tewksbury fault, north of the Ramapo fault.

AbstractPrevious geophysical studies in the New England Appalachians identified a ∼15 km offset in crustal thickness near the surface boundary between Laurentia and the accreted terranes. Here, we investigate crustal structure using data from a denser array: New England Seismic Transects experiment, which deployed stations spaced ∼10 km apart across the Laurentia‐Moretown terrane suture in northwestern Massachusetts. We used receiver function (RF) analysis to detect P to SV converted waves and identified multiple interfaces beneath the transect. We also implemented a harmonic decomposition analysis to identify features at or near the Moho with dipping and/or anisotropic character. Beneath the Laurentian margin, the Ps converted phase from the Moho arrives almost 5.5 s after the initial P wave, whereas beneath the Appalachian terranes, the pulse arrives at 3.5 s, corresponding to ∼48 and ∼31 km depth, respectively. The character of the RF traces beneath stations in the middle of our array suggests a complex transitional zone with dipping and/or anisotropic boundaries extending at least ∼30 km. This extension is measured in our profiles and perpendicular to the suture. We propose one possible crustal geometry model that is consistent with our observations and results from previous studies.

Nestled between the Cocos, Nazca, Caribbean, and South American plates, the Panama microplate represents an area of rapidly evolving tectonics throughout the past ~10 m.y. Past and current studies have observed a notable amount of seismicity throughout this region, in particular the Caribbean coast of Costa Rica, which experienced a Mw 7.7 earthquake in 1991 CE. We investigated the crust and upper mantle structure of this region using the receiver function methodology and report two results: (1) first-order lateral constraints on the position of the Panama microplate boundary near the intersection between the Central Costa Rica Deformed Belt (onshore) and North Panama Deformed Belt (offshore), and (2) an impedance contrast south and east of these belts, supporting that the Caribbean plate currently subducts beneath the Panama microplate. Observed local seismicity is a consequence of the recently (ca. 14 Ma) initiated Caribbean plate subduction beneath the overlying Panama microplate. Our results are also consistent with a doubly convergent subduction margin dominating southern Costa Rica tectonics, uplifting the Talamanca Cordillera, and causing the cessation of southern Costa Rica volcanism over the past ~10 m.y.

Data sources, details of data analysis methodology, and additional diagrams and maps of shear wave splitting measurements.

Past studies of southern Costa Rica have generated a multitude of tectonic scenarios to account for different data sets. Flat slabs, detached slabs, and slab windows have been proposed to address the uplift of the Cordillera de Talamanca (CT), cessation of volcanism, and absence of deep seismicity beneath southern Costa Rica. In this study, we investigate the crust and the upper mantle along the southwest flank of the CT using the receiver function methodology. We observe two regional positive P‐to‐S converted pulses at delay times of ∼2–4 s and ∼5–8 s. The first likely represents a gradational crust‐mantle boundary of the upper plate. The second represents a similar impedance increase ∼50–60 km deep that extends from central Costa Rica to Panama. Compared to well‐located seismicity, this boundary is offset to the NE from the Cocos plate Benioff zone beneath northern CT, and remains observable through a gap in seismicity farther to the southeast. This offset makes it difficult to interpret this feature as related to the presently subducting lithosphere. Instead, we propose that the 50–60 km deep boundary marks the Moho of a lithospheric fragment left behind under the CT in the course of Panama Triple Junction migration through Costa Rica over the last 10 Ma. Our interpretation accounts for the geophysical, geochemical, structural, and geomorphic observations in the literature explaining the complex geodynamic scenario observed in southern Costa Rica.

An apparent gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile (~28°–33°S) marks a section of flat slab subduction. In this tectonic environment, the fate of fluids released from the subducting Nazca slab remains uncertain and the degree of their interaction with the basal layer of the continental lithosphere is poorly understood. Results of a RF investigation and forward modeling effort at three long‐running stations of the Chilean National Seismic Network allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone in Central Chile. Our observations suggest a transition in seismic character, from a weakly anisotropic contact in the normally subducting section north of the flat slab region to a strongly anisotropic plate contact within the flat slab region. We attribute this change to a transition from sheared olivine to serpentinized peridotite generated as a result of fluid release across the flat slab. This interpretation is supported by forward modeling synthetic RFs at each of the stations. We propose that the identified layer extends across the flat slab region, acting as a mineral reservoir that captures and, possibly, transports fluids from the dehydrating Nazca Plate as it subducts below South America. We note that the Ps converted phase at the slab interface at southernmost station GO04 suffers a 2‐s discontinuity at 180° back azimuth, consistent with a 15–20‐km scarp or kink in the Nazca slab to the south of the station.