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A decade-long research collaboration has revealed that the split between Africa and North America roughly 200 million years ago was more drawn out than previously thought.

Intraplate earthquakes induced by anthropogenic fluid injection present unexpected seismic risk to previously quiescent or low seismicity-rate regions. Despite many studies of induced seismicity, there are relatively few with detailed openly accessible constraints on the interaction between seismic sources and subsurface structures. In this study of the Raton Basin, we refine source observations from a dense nodal array and constrain basin structure using teleseismic receiver functions. The cross-correlation-based relocated hypocentres and a new set of focal mechanisms light up active fault segments and show clear spatiotemporal patterns. The geometric complexity of reactivated fault clusters appears greatest near higher rate injection wells. Simpler normal fault structure is found farther from injection wells and near abrupt structural transitions suggested by receiver functions. While less induced seismicity in the crystalline basement is expected when injection is >1 km from the top of the basement (like Raton), our receiver function analysis identified a basin thickness ~3 km beneath the nodal array and lateral variations in sedimentary structures. Our results explain potential fluid connectivity between the injection depths focused at ~1–1.5 km below the surface and basement fault activity that begins at ~3 km and reaches peak activity at ~4–8 km depths. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

We conduct a high‐resolution teleseismic receiver function investigation of the subducting plate interface within the Alaskan forearc beneath Kodiak Island using data collected as part of the Alaska Amphibious Community Seismic Experiment in 2019. The Kodiak node array consisted of 398 nodal geophones deployed at ∼200–300 m spacing on northeastern Kodiak Island within the southern asperity of the 1964 Mw9.2 Great Alaska earthquake. Receiver function images at frequencies of 1.2 and 2.4 Hz show a coherent, slightly dipping velocity increase at ∼30–40 km depth consistent with the expected slab Moho. In contrast to studies within the northern asperity of the 1964 rupture, we find no evidence for a prominent low‐velocity layer above the slab Moho thick enough to be resolved by upgoing P‐to‐S conversions. These results support evidence from seismicity and geodetic strain suggesting that the 1964 rupture connected northern (Kenai) and southern (Kodiak) asperities with different plate interface properties. Plain Language Summary: We use 398 portable seismometers that were deployed as part of the Alaska Amphibious Community Seismic Experiment to image the boundary between the subducting Pacific plate and the base of the North American plate. The seismometers, spaced ∼200–300 m apart, were stationed on Kodiak Island in 2019 within the southern rupture area of the 1964 Mw9.2 Great Alaska earthquake. We analyze conversions from compressional to shear waves from distant earthquakes to understand the conditions of the plate interface. Our results show a dipping velocity increase at ∼30–40 km depth at the expected location of the Pacific slab crust‐mantle boundary. In contrast to prior results from the northern 1964 rupture zone, we do not find a low‐velocity layer on the subducting plate. Our results indicate that the 1964 rupture connected segments of the Alaskan subduction zone with different plate interface properties. Key Points: We present receiver function imaging from a dense three‐component nodal array deployment on Kodiak Island above the subducting Pacific PlateA clear slab Moho conversion is found but, in contrast to the Kenai Peninsula, there is no coherent low‐velocity layer atop the slabThe 1964 Great Alaska Earthquake ruptured across structural segments with different plate interface properties [ABSTRACT FROM AUTHOR]

[1] The St. Elias orogen is the result of~10 Myr of oblique convergence and flat-slab subduction in the Gulf of Alaska between North America and the Yakutat microplate. Extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge. Reflection seismic profiles across the offshore Pamplona zone fold-thrust belt, the frontal St. Elias orogenic wedge, provide constraints for quantifying Pleistocene deformation recorded in the glaciomarine Yakataga formation. The total amount of Pleistocene shortening observed varies from ~3 to 5 mm/yr, compared to the current GPS-derived Yakutat-North America convergence rate across the St. Elias orogen of ~45 mm/yr. Growth strata and kinematic fold analysis allow comparison of relative timing of fault activity, which reveals temporal and spatial shifting of active deformation during the glacial period: faulting localized adjacent to the coastline and at the current submarine deformation front. The abandoned, currently inactive region is colocated with the major glacial depocenter in the region, the Bering Trough. These observations imply that glacial processes such as sediment loading and focused erosion during advance-retreat cycles has a direct effect on the evolution of individual faults within the Pamplona zone and the overall deformation pattern in the offshore St. Elias margin. This information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during Pleistocene glacial-interglacial periods. Citation: Worthington, L. L., S. P. S. Gulick, and T. L. Pavlis (2010), Coupled stratigraphic and structural evolution of a glaciated orogenic wedge, offshore St. Elias orogen, Alaska, Tectonics, 29, TC6013, doi: 10.1029/2010TC002723.

We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24-27 km thick overlain by sedimentary units that are 4.5-7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ~1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0-5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault. doi: 10.1130/G31170.1

The complex postdetonation geologic structures that form after an underground nuclear explosion are hard to constrain because increased heterogeneity around the damage zone affects seismic waves that propagate through the explosion site. Generally, a vertical rubble‐filled structure known as a chimney is formed after an underground nuclear explosion that is composed of debris that falls into the subsurface cavity generated by the explosion. Compared with chimneys that collapse fully, leaving a surface crater, partially collapsed chimneys can have remnant subsurface cavities left in place above collapsed rubble. The 1964 nuclear test HADDOCK, conducted at the Nevada test site (now the Nevada National Security Site), formed a partially collapsed chimney with no surface crater. Understanding the subsurface structure of these features has significant national security applications, such as aiding the study of suspected underground nuclear explosions under a treaty verification. In this study, we investigated the subsurface architecture of the HADDOCK legacy nuclear test using hybrid 2D-3D active source seismic reflection and refraction data. The seismic data were acquired using 275 survey shots from the Seismic Hammer (a 13,000 kg weight drop) and 65 survey shots from a smaller accelerated weight drop, both recorded by ∼1000 three‐component 5 Hz geophones. First‐arrival, P‐wave tomographic modeling shows a low‐velocity anomaly at ∼200  m depth, likely an air‐filled cavity caused by partial collapse of the rock column into the temporary postdetonation cavity. A high‐velocity anomaly between 20 and 60 m depth represents spall‐related compaction of the shallow alluvium. Hints of low velocities are also present near the burial depth (⁠∼364  m⁠). The reflection seismic data show a prominent subhorizontal reflector at ∼300  m depth, a short‐curved reflector at ∼200  m⁠, and a high‐amplitude reflector at ∼50  m depth. Comparisons of the reflection sections to synthetic data and borehole stratigraphy suggest that these features correspond to the alluvium-tuff contact, the partial collapse cavity, and the spalled layer, respectively. [ABSTRACT FROM AUTHOR]

We demonstrate successful crustal imaging via teleseismic P-wave coda autocorrelation, using data recorded on a 261 station array of vertical-component high-frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P-wave coda of 30 teleseismic events and use phase-weighted stacking to yield seismic profiles comparable to low-passed versions of those produced via controlled-source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid-crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P-wave coda energy using modern large-array seismic data, and they corroborate previous interpretations of crustal structures in the study area. [ABSTRACT FROM AUTHOR]

Convergence and shallow subduction of the Yakutat microplate beneath North America has shaped the Pamplona zone fold and thrust belt in the northeastern Alaska subduction zone. Here, convergent tectonics and glaciomarine sedimentary processes have created patterns of deformation and deposition recorded by a shallow sedimentary sequence with varying fluid pressure, compaction, and fault activity. If the record of ice sheet advance is preserved on the seafloor and in the underlying sediments then bathymetric evidence of glaciomarine processes should overlap with evidence of overpressure, such as low velocity zones in the shallow sediments. In this study, we present a velocity model through the Bering trough as well as regional drainage shape characteristics. We use streamer tomography on a seismic survey from the St. Elias Erosion and Tectonics Project (STEEP) to determine a velocity model. We further inform our interpretation using physical properties relationships developed with data from core samples (IODP Expedition 341) taken near the seismic line. To document drainage morphology, we use smooth sheet bathymetry to identify a drainage network and query profile shapes at well distributed locations; where profile shape is determined by fit with a power function. Initial results suggest overlap between u-shaped drainage profiles towards the SE end of the Bering trough and low-velocity shallow sediments NE of the shelf break. In part, because this overlap occurs within the mapped extent of the last glacial maximum, we find our tomography results consistent with buildup of overpressure due to glacially driven loading of highly saturated sediments. [ABSTRACT FROM AUTHOR]

The purpose of this presentation will be to present further research of the region and to answer the following questions using seismic and gravitational modeling of White Sands National Monument (WHSA). WHSA is the largest gypsum dune field in the world and was formed ~7000 years ago with the retreat of Pleistocene Lake Otero: 1) What controls the location of the western dune front? The western dune front of WHSA has remained relatively fixed as shown by measurements from 1944 to 2010. This behavior is quite different from other dune fields in the region that show migration up to 35 m per year. The hypothesis is that the dune front is pinned to a now buried paleo-shoreline of Lake Otero; 2)What is the relationship between subsurface structure of the Tularosa Basin and the dune field? Fault and block geometries in our study area are currently unknown. Previous gravitational modeling has shown that the highest measurements stand in the deepest part of the basin. From this we can hypothesize that Lake Otero regressed towards the deepest part of the basin and the dune field migrates over the highest part of the basin along with the Jarilla Fault that flanks the eastern edge of the basin. Our methods include seismic and gravitational imaging and modeling that will provide a detailed image of the upper 50-100 meters and 101 km respectively. Expected outcomes, results, and conclusions of this project will provide further knowledge of the subsurface structure and the underlying basin for our study area of WHSA. [ABSTRACT FROM AUTHOR]

Southeastern Alaska is home to one of the most actively deforming continental margins in the world, and temperate glaciers covering the area contribute to high rates of erosion and offshore sedimentation, making the Neogene sediment covering the offshore continental shelf an ideal location to study interactions between climate and tectonics. We present a high-resolution two-dimensional P-wave velocity model of the glaciomarine sediment and underlying basement in the area and compare to coincident seismic reflection profiles in order to determine lithologic boundaries and correlate with stratigraphic units observed onshore and in offshore boreholes. Data used in this study includes ~115 km of multichannel seismic reflection and coincident ocean-bottom seismometer wide-angle reflection and refraction data. In Yakutat Bay, sediment velocities are high (~ 4 km/s) compared to velocities farther offshore (< 3 km/s), possibly indicating greater compaction of the sediments in the bay. In the central portion of the shelf, three distinct refractions from sedimentary layers are observed and are interpreted as the Yakataga, Poul Creek, and Kulthieth Formations. Northern profiles show only two refractions from sedimentary layers. Basement refractions are observed in the north at close offsets, evidence of shallow basement that could be indicative of uplift in the bay. Our velocity profile used in conjunction with seismic reflection data shows transitions in dominant depositional processes over time in the central portion of the block and helps to provide a regional framework of the geometries of the sedimentary packages and their spatial relationship to major tectonic features. [ABSTRACT FROM AUTHOR]

The use of ground-penetrating radar (GPR) can provide detailed, centimeter-scale resolution images of the subsurface to a depth of tens of meters. Alaska is one of the most seismically active areas in the United States, but few faults have undergone detailed studies of Quaternary stratigraphy and near-surface structure. In the summer of 2015, a pilot study collected and analyzed 2-D GPR data across the Denali Fault in central Alaska to help constrain near-surface deformation. A basic data processing flow and a 1-D, single-layer velocity model was applied in order to conduct preliminary interpretations. We observed clear reflections to a depth of ~7 m below the ground surface. Data collected at co-located paleoseismic trenching mimic the near-surface reflections and help ground-truth the GPR data. The combined data sets provide insight into the north-south limits of the deformation zone of the fault at this site, along with fault zone morphology at a shallow (<7 m) depth. However, farther from the trenching exposures, the GPR data were less clear as to whether it displayed any reflection patterns indicative of fault zones, which may be due to adverse environmental conditions for GPR implementation. The poor imaging leads to more speculative interpretations, making correlation between the trenching exposures and the GPR data difficult. This pilot study demonstrates the potential use of GPR in the characterization of faults and the advantages and disadvantages of using GPR in conjunction with trenching and outcrop interpretation. [ABSTRACT FROM AUTHOR]

In the last decades research has shown that climate activity can be an important component of large scale tectonics. Glacial erosion and deposition, forced by climate patterns, affect mass balance in large-scale tectonically active regions as well as offshore depositional basins. One ideal natural laboratory to study these climate-tectonic interactions is offshore glacial-marine deposits in SE Alaska where an offshore accretionary wedge records glacial and marine sedimentation as well as faulting. This location is ideal because the offshore depositional basin is near the onshore sediment source which facilitates a short time between erosion and deposition as well as a relatively closed system from source to basin. Thanks to previous research efforts there is extensive data including sediment core, well log measurements, bathymetry, and offshore seismic reflection surveys available to aide in the interpretation of climate- linked glacial activity and fault activity. Detailed stratigraphic and structural interpretation of seismic reflection lines collected off the coast of SE Alaska allow for sequence interpretation of fault activity, glacial deposition, and climate change. Interpretation of sedimentation patterns with special focus on extent and style of glacial sedimentation and fault activity link these processes in space and time. Applied spatial-statistics provides a quantitative link between glacial activity and faulting by testing for shape similarity and proximity of related geologic features. [ABSTRACT FROM AUTHOR]