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To evaluate the plate flexure model for the formation of the Appalachian Basin, we investigate the extent to which crustal structure beneath and surrounding the basin was modified by the Palaeozoic orogenic events that created the basin. We jointly invert receiver functions and surface wave dispersion measurements to obtain 1-D crustal Vs profiles for 261 seismic stations located within and around the basin. The average crustal thickness for the region is 44 km, and the crust gradually thins to the east, consistent with previous studies. Four areas of anomalous crust are identified with respect to the eastward thinning of the crust. An area of thick crust is found along the Grenville Front on the western side of the Appalachian Basin where the crust thickens by ∼5–10 km. Moho depths of up to 54 km in this region likely result from suture-thickening. The crust is thinner beneath the Neoproterozoic Scranton rift by ∼5–7 km, coincident with a ∼40 mGal Bouguer gravity high. Across the Neoproterozoic Rome Trough, the crust thins by ∼4–5 km, coincident with a ∼10 mGal Bouguer gravity high. Density models for these rifts show that the rift-related crustal thinning is sufficient to explain the gravity anomalies. The Vs models obtained for stations in the rifts indicate little, if any, mafic layering in the mid-crust and only a modest amount of mafic layering in the lower crust. In the northwestern portion of the Appalachian Basin in northeastern Ohio and northwestern Pennsylvania within the Elzevir block, another area of anomalously thick crust (50–52 km) is found. This region is not associated with any known tectonic structures or boundaries or a gravity anomaly. The lower ∼5–10 km of the crust in this region is characterized by high (>3.9 km s−1) shear wave velocities and thus appears to be mafic. The origin of anomalous crustal structure in all four areas is best attributed to Precambrian tectonic events that predate the formation of the Appalachian Basin, indicating that the crystalline crust beneath and surrounding the basin was not significantly affected by the Palaeozoic basin-forming orogenic events, a finding which supports the use of plate flexure models for understanding basin formation.

 

Ice shelves are assumed to flow steadily from their grounding lines to the ice front. We report the detection of ice‐propagating extensional Lamb (plate) waves accompanied by pulses of permanent ice shelf displacement observed by co‐located Global Navigation Satellite System receivers and seismographs on the Ross Ice Shelf. The extensional waves and associated ice shelf displacement are produced by tidally triggered basal slip events of the Whillans Ice Stream, which flows into the ice shelf. The propagation velocity of 2,800 m/s is intermediate between shear and compressional ice velocities, with velocity and particle motions consistent with predictions for extensional Lamb waves. During the passage of the Lamb waves the entire ice shelf is displaced about 60 mm with a velocity more than an order of magnitude above its long‐term flow rate. Observed displacements indicate a peak dynamic strain of 10⁻⁷, comparable to that of earthquake surface waves that trigger ice quakes.

 

The origin of the Cameroon Volcanic Line (CVL), which is difficult to explain with traditional plate tectonics and mantle convection models because the volcanism does not display clear age progression, remains widely debated. Existing seismic tomography models show anomalously slow structure beneath the CVL, which some have interpreted to reflect upper mantle convective processes, possibly associated with edge‐driven flow related to the neighboring Congo Craton. However, mid‐ and lower mantle depths are generally not well resolved in these models, making it difficult to determine the extent of the anomalous CVL structure. Here, we present a new P‐wave velocity model for the African mantle, developed with the largest collection of travel‐time residuals recorded across the continent to date and an adaptive model parameterization. Our extensive data set and inversion method yield high resolution images of the mantle structure beneath western Africa, particularly at the critical mid‐ and lower mantle depths needed to further evaluate processes associated with the formation of the CVL. Our new model provides strong evidence for a connection between the African Large Low Velocity Province, centered in the lower mantle beneath southern Africa, and the continental portion of the CVL. We suggest that seismically slow material generated near the core‐mantle boundary beneath southern Africa moves northwestward under the Congo Craton. At the northern edge of the craton, the hot, buoyant material rises through the upper mantle, causing the CVL volcanism. Consequently, CVL magmatism can be linked to large‐scale mantle processes rooted in the deep mantle.

 

We investigate the occurrence of repeating glacial seismicity near the grounding line of the Foundation Ice Stream and further upstream using continuous broadband seismic data collected by Polar Earth Observing Network (POLENET/A‐NET) stations from 2014 through 2019. Through manual identification and cross‐correlation analysis, 2,237 discrete icequakes (1.5  M_L  2.6) are detected in two spatial clusters, one located at the grounding line of the Foundation Ice Stream (2,219 event detections) and a second located further upstream proximal to a subglacial ridge (18 event detections). Seismicity is predominantly concentrated in the Schmidt Hills, located adjacent to the grounding line of the Foundation Ice Stream, and shows clear ocean tide modulation. Seismic events primarily occur during spring tides, and, on a shorter timescale, concurrent with the rising tide preceding daily maximum high tide. The seismicity can be attributed to stick‐slip motion and fracturing that preferentially occur during rising tides. Seismicity located further upstream in the southern portion of the Foundation Ice Stream most likely reflects basal stick‐slip processes associated with the subglacial topographic high.

 

The force balance that drives and maintains continental rifting to breakup is poorly understood. The East African Rift (EAR) provides an ideal natural laboratory to elucidate the relative role of plate driving forces as only lithospheric buoyancy forces and horizontal mantle tractions act on the system. Here, we employ high‐resolution 3D thermomechanical models to test whether: (a) the anomalous, rift‐parallel surface deformation observed by Global Navigation Satellite System (GNSS) data in the EAR are driven by viscous coupling to northward mantle flow associated with the African Superplume, and (b) the African Superplume is the dominant source mechanism of anomalous rift‐parallel seismic anisotropy beneath the EAR. We calculate Lattice Preferred Orientations (LPO) and surface deformation from two types of mantle flow: (a) a scenario with multiple plumes constrained by shear wave tomography and (b) a single superplume model with northward boundary condition to simulate large‐scale flow. Comparison of calculated LPO with observed seismic anisotropy, and surface velocities with GNSS and plate kinematics reveal that there is a better fit with the superplume mantle flow model, rather than the tomography‐based (multiple plumes) model. We also find a relatively better fit spatially between observed seismic anisotropy and calculated LPO with the superplume model beneath northern and central EAR, where the superplume is proposed to be shallowest. Our results suggest that the viscous coupling of the lithosphere to northward mantle flow associated with the African Superplume drives most of the rift‐parallel deformation and is the dominant source of the first‐order pattern of the observed seismic anisotropy in the EAR.

 

Abstract Ocean swell interacting with Antarctic ice shelves produces sustained (approximately, 2×106 cycles per year) gravity-elastic perturbations with deformation amplitudes near the ice front as large as tens to hundreds of nanostrain. This process is the most energetically excited during the austral summer, when sea ice-induced swell attenuation is at a minimum. A 2014–2017 deployment of broadband seismographs on the Ross Ice shelf, which included three stations sited, approximately, 2 km from the ice front, reveals prolific swell-associated triggering of discrete near-ice-front (magnitude≲0) seismic subevents, for which we identify three generic types. During some strong swell episodes, subevent timing becomes sufficiently phase-locked with swell excitation, to create prominent harmonic features in spectra calculated across sufficiently lengthy time windows via a Dirac comb effect, for which we articulate a theoretical development for randomized interevent times. These events are observable at near-front stations, have dominant frequency content between 0.5 and 20 Hz, and, in many cases, show highly repetitive waveforms. Matched filtering detection and analysis shows that events occur at a low-background rate during all swell states, but become particularly strongly excited during large amplitude swell at rates of up to many thousands per day. The superimposed elastic energy from swell-triggered sources illuminates the shelf interior as extensional (elastic plate) Lamb waves that are observable more than 100 km from the ice edge. Seismic swarms show threshold excitation and hysteresis with respect to rising and falling swell excitation. This behavior is consistent with repeated seismogenic fracture excitation and growth within a near-ice-front damage zone, encompassing fracture features seen in satellite imagery. A much smaller population of distinctly larger near-front seismic events, previously noted to be weakly associated with extended periods of swell perturbation, likely indicate calving or other larger-scale ice failures near the shelf front. 

Abstract Observations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data. 

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO₂produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.