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Abstract The eastern North American margin community seismic experiment (ENAM‐CSE) was conceived to target the ENAM Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) primary site with a suite of both active‐ and passive‐source seismic data that would shed light on the processes associated with rift initiation and evolution. To fully understand the ENAM, it was necessary to acquire a seismic dataset that was both amphibious, spanning the passive margin from the continental interior onto the oceanic portion of the North American plate, and multiresolution, enabling imaging of the sediments, crust, and mantle lithosphere. The ENAM‐CSE datasets were collected on‐ and offshore of North Carolina and Virginia over a series of cruises and land‐based deployments between April 2014 and June 2015. The passive‐source component of the ENAM‐CSE included 30 broadband ocean‐bottom seismometers (OBSs) and 3 onshore broadband instruments. The broadband stations were deployed contemporaneously with those of the easternmost EarthScope Transportable Array creating a trans‐margin amphibious seismic dataset. The active‐source portion of the ENAM‐CSE included several components: (1) two onshore wide‐angle seismic profiles where explosive shots were recorded on closely spaced geophones; (2) four major offshore wide‐angle seismic profiles acquired with an airgun source and short‐period OBSs (SPOBSs), two of which were extended onland by deployments of short‐period seismometers; (3) marine multichannel seismic (MCS) data acquired along the four lines of SPOBSs and a series of other profiles along and across the margin. During the cruises, magnetic, gravity, and bathymetric data were also collected along all MCS profiles. All of the ENAM‐CSE products were made publicly available shortly after acquisition, ensuring unfettered community access to this unique dataset.

The value of deep‐water sedimentary successions as reliable records of environmental change has been questioned due to their long response times and sediment pathways leading to complex responses to climatic change and tectonic signals over differing timescales. We studied the Gulf of Corinth, Greece, to test the value of deep‐water stratigraphic successions as records of external controls on sediment flux in a setting with short response times and transport distances. The confinement of the rift basin allows for a near‐complete accounting of clastic sediment volumes. The recent acquisition of high‐resolution seismic reflection data, utilisation of International Ocean Discovery Programme Expedition 381 cores and a robust chronological framework, enable evaluation of the stratigraphy at a high temporal resolution. Combining borehole and high‐resolution seismic reflection data, distinct seismic units can be correlated to multiple paleoenvironmental proxies, permitting quantification of sediment flux variation across successive glacial–interglacial cycles at ca. 10 kyr temporal resolution. Trends in average sediment flux since ca. 242 ka show ca. 2–9 times greater sediment flux in cooler glacials compared to warmer interglacial conditions. The Holocene is an exception to low sediment flux for the interglacials, with ca. 5 times higher rates than previous interglacials. The short and steep configuration of the Sythas canyon and its fan at the base of an active submarine normal fault results in deep‐sea deposition at all sea‐level stands. In contrast, adjacent canyon systems shut down during warm intervals. When combined with palynology, results show that periods of distinct vegetation re‐organisation correlate to sediment flux changes. The temporal correlation of sediment flux to palynology in the Gulf of Corinth over the last ca. 242 kyr is evidence that variability of sediment supply is largely governed by climate‐related changes in hinterland catchments, with sea‐level and tectonics being second‐order controls on sediment flux variability. [ABSTRACT FROM AUTHOR]

We develop a 3‐D isotropic shear velocity model for the Alaska subduction zone using data from seafloor and land‐based seismographs to investigate along‐strike variations in structure. By applying ambient noise and teleseismic Helmholtz tomography, we derive Rayleigh wave group and phase velocity dispersion maps, then invert them for shear velocity structure using a Bayesian Monte Carlo algorithm. For land‐based stations, we perform a joint inversion of receiver functions and dispersion curves. The forearc crust is relatively thick (35–42 km) and has reduced lower crustal velocities beneath the Kodiak and Semidi segments, which may promote higher seismic coupling. Bristol Bay Basin crust is relatively thin and has a high‐velocity lower layer, suggesting a dense mafic lower crust emplaced by the rifting processes. The incoming plate shows low uppermost mantle velocities, indicating serpentinization. This hydration is more pronounced in the Shumagin segment, with greater velocity reduction extending to 18 ± 3 km depth, compared to the Semidi segment, showing smaller reductions extending to 14 ± 3 km depth. Our estimates of percent serpentinization from VS reduction and VP/VS are larger than those determined using VP reduction in prior studies, likely due to water in cracks affecting VS more than VP. Revised estimates of serpentinization show that more water subducts than previous studies, and that twice as much mantle water is subducted in the Shumagin segment compared to the Semidi segment. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments. Plain Language Summary: This study uses seismic data from the 2018–2019 Alaska Amphibious Community Seismic Experiment and other land stations to image the 3‐D seismic velocity structure of the Alaska subduction zone. The analysis combines constraints from both Rayleigh waves and converted body waves. The results provide insight into the distinct lateral variations observed for many properties of the subduction zone. Thick, low‐velocity forearc crust is found beneath the Kodiak and Semidi segments, which may be related to the higher seismic coupling in these regions. The Bristol Bay Basin has a thin crust with a high velocity lower layer, suggesting a dense mafic lower crust emplaced by the extensional processes that formed the basin. Low velocities in the incoming plate near the trench in the Shumagin segment indicate pronounced mantle hydration, extending to about 18 km below the Moho. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments. Key Points: Crustal thickness of the inner forearc (35–42 km) generally exceeds that of the volcanic arc, but becomes variable in the Shumagin segmentThe Shumagin segment has more incoming plate mantle hydration than the Semidi segment, aligning with abundant plate bending normal faultsHydration extends to depths of 18 km below the Moho, indicating more water subducts than most previous estimates [ABSTRACT FROM AUTHOR]

Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the incoming lithosphere. Debate remains about the relative importance of pre‐existing faults, plate curvature and other factors controlling the extent and style of bending‐related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending faulting as the orientation of the abyssal‐hill fabric with respect to the trench and plate curvature vary along the margin. Here, we characterize faulting between longitudes 161°W and 155°W using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus should have limited impact on the expression of bend‐related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ∼156°W, bending faults parallel magnetic anomalies, implying that bending faults reactivate pre‐existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156°W, no bending faults are observed. Summed fault throws increase to the west, including where pre‐existing structure orientations are constant (between 157 and 161°W), suggesting that another factor such as the increase in slab curvature must influence bending faulting. However, the westward increase in summed fault throws is more abrupt than expected for gradual changes in slab bending alone, suggesting potential feedbacks between pre‐existing structures, slab dip, and faulting. Plain Language Summary: Subduction zones are plate boundaries where two tectonic plates converge, and the oceanic plate is bent and forced below the other plate. Oceanic plates are faulted as they bend, and these "bending faults" are thought to be important for controlling the deep water cycle on Earth and influencing the generation of large earthquakes in subduction zones. The amount and style of bending faulting varies between and within subduction zones around the world, and debate remains about what causes this variability. Possible controls include the overall curvature of the oceanic plate as it bends and subducts and pre‐existing weaknesses in the oceanic plate from when it formed. We use bathymetry data across the Alaska subduction zone to characterize bending faults here and understand the controls on their formation. This is an ideal study area because the curvature of the plate and the pre‐existing weaknesses vary in this region. The amount of bending faulting increases abruptly to the west and appears to result from a feedback between favorably oriented pre‐existing weaknesses and increased curvature of the oceanic plate. These results can be used to understand bending faulting in other subduction zones. Key Points: Bathymetry data reveal variations in the orientation and summed throws of bending faulting outboard of the Alaska subduction zoneThe westward increase in the number and summed throws of bending faults is due to favorably oriented pre‐existing structures and increased slab dipVariable orientations of bend faulting and volcanic constructs updip of 2020 M7.6 intraplate earthquake implies complex stresses in the slab [ABSTRACT FROM AUTHOR]

Half‐graben basins bounded by border faults typify early‐stage continental rifts. Deciphering the role that intra‐rift faults play in rift basin development is challenging as patterns of early‐stage faulting are commonly overprinted by subsequent deformation; yet the characterization of these faults is crucial to understand the fundamental controls on their evolution, their contribution to rift opening, and to assess their seismic hazard. By integrating multiple offshore seismic reflection data sets with age‐dated drill core, late‐Quaternary and cumulative faulting patterns are characterized in the Central and South Basins of the Malawi (Nyasa) Rift, an active, early‐stage rift system. Almost all intra‐rift faults offset a late‐Quaternary lake lowstand surface, suggesting they are active and should be considered in hazard assessments. Fault throw profiles reveal sawtooth patterns indicating segmented slip histories. Observed extension on intra‐rift faults is approximately twice that predicted from hanging wall flexure of the border fault, suggesting that intra‐rift faults accommodate a proportion of the regional extension. Cumulative and late‐Quaternary throws on intra‐rift faults are correlated with throw measured on the border fault in the Central Basin, whereas an anticorrelation is observed in the South Basin. Viewed in a regional context, these differences do not relate solely to the proposed southward younging of the rift. Instead, it is inferred that the distribution of extension is also influenced by variations in lithospheric structure and crustal heterogeneities that are documented along the rift axis. Key Points: We carried out a detailed fault analysis of the Malawi (Nyasa) Rift integrating drill core and seismic reflection data65% and 49% of the observed extension on intra‐rift faults in the Central and South Basins is explained by regional tectonic extensionSpatio‐temporal patterns of throw indicate the migration of extension onto intra‐rift faults may be ongoing in the Central Basin [ABSTRACT FROM AUTHOR]

Detailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and the distribution of igneous addition during continental rifting. The Eastern North American Margin (ENAM) is a volcanic passive margin that formed during the breakup of Pangea ∼200 Myr ago. The offshore, margin‐parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of syn‐rift magmatism. Previous widely spaced margin‐perpendicular studies seismically imaged igneous addition as seaward dipping reflectors (SDRs) and high velocity lower crust (HVLC; >7.2 km/s) beneath the ECMA. Along‐strike imaging is necessary to more accurately determine the distribution and volume of igneous addition during continental breakup. We use wide‐angle, marine active‐source seismic data from the 2014–2015 ENAM Community Seismic Experiment to determine crustal structure beneath a ∼370‐km‐long section of the ECMA. P‐wave velocity models based on data from short‐period ocean bottom seismometers reveal a ∼21‐km‐thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC (>7.2 km/s) alternate with two ∼30‐km‐wide areas where the average velocities are less than 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin, reflecting variable amounts of intrusion along‐strike. Our results suggest that magmatism during rifting was segmented. The HVLC discontinuities roughly align with locations of Mid‐Atlantic Ridge fracture zones, which may suggest that rift segmentation influenced later segmentation of the Mid‐Atlantic Ridge. Plain Language Summary: The East Coast of the United States is a passive margin that formed during continental breakup of Pangea, the most recent supercontinent. Although passive margins are generally not locales of active faulting and magmatism, by investigating their current structure, we gain insight into processes during past rifting. We know that extensive volcanism and magmatic addition to the Earth's crust occurred during the breakup of Pangea, but we do not fully understand how the distribution changes from north to south along the margin. To study these rocks, which are buried 10–20 km beneath the seafloor, we use ocean bottom seismometers to determine the sound speed of rocks beneath the surface, which can tell us about composition and rock type. We found that the thickness and extent of magma‐derived rocks are variable along the margin, with gaps up to 30 km wide. This variability is likely related to processes happening in the continental rift which may have influenced the structure of the Mid‐Atlantic Ridge that formed after rifting ended. Key Points: High velocity lower crust beneath the East Coast Magnetic Anomaly is variable along‐strike and discontinuous in some placesThe lower crust variability reflects variable intrusion into lower continental crust and represents magmatic segmentation during riftingThe high velocity lower crust discontinuities are coincident to Mid‐Atlantic Ridge fracture zones, suggesting a rift‐to‐ridge connection [ABSTRACT FROM AUTHOR]

Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Model (MSSM; 10.5281/zenodo.5599616), which describes the seismogenic properties of faults that formed during ongoing east African rifting in Malawi. We first use empirically derived constraints to geometrically classify active faults into section, fault, and multifault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that dated lake cores indicate is 75 ka. Elsewhere, slip rates are constrained from advancing a systems-based approach that partitions geodetically derived rift extension rates in Malawi between seismogenic sources using a priori constraints on a regional strain distribution and a hanging wall flexural extension in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability in logic tree outcomes used in these calculations. Sources in the MSSM are 5–269 km long, which implies that large-magnitude (Mw 7–8) earthquakes may occur in Malawi. However, low slip rates (0.05–2 mm yr -1) mean that the frequency of such events will be low (recurrence intervals of ∼103 –10 4 years). We also find that, for 9 out of 11 faults in Lake Malawi's North Basin, differences in the slip rates, when estimated independently from the geodetic data and the offset seismic reflector, are not statistically significant. The MSSM represents an important resource for investigating Malawi's increasing seismic risk and provides a framework for incorporating active fault data into seismic hazard assessment elsewhere in the East African Rift and other tectonically active regions. [ABSTRACT FROM AUTHOR]

The amount of magmatism that accompanies the extension and rupture of the continental lithosphere varies dramatically at rifts and margins around the world. Based on widely spaced geophysical transects, some margins are known to preserve a transition from magmatically robust to magmatically starved rifting along strike, but the nature of the transition is unknown. Wide-angle seismic data from the Black Sea provide the first direct observations of such a transition and show that it is abrupt, occurring over only ~20-30 km, and coincides with a transform fault. This abrupt transition cannot be explained solely by gradual along-margin variations in mantle properties, since these would be expected to result in a smooth transition from magma-poor to magma-rich rifting over hundreds of kilometers. We suggest that the abruptness of the transition results from the development of three-dimensional (3-D) melt migration due to along-strike variations in extension and thus the thickness of the lithosphere at the time of rifting. Localized magmatic addition attributed to melt focusing has been observed in modern mid-ocean ridges and active rift environments, but here we show that such processes can also produce abrupt along-strike changes from magma-poor to magma-rich rifting.

Syn‐rift deep‐water muds and mudstones preserve a relatively complete stratigraphic record of tectonic and climatic events. This paper investigates mud‐dominated deposits and stratigraphy using core from International Ocean Discovery Program (IODP) Expedition 381 sites M0078 and M0079 in the Gulf of Corinth, Greece. Millimetre‐scale logging defined several bed types: homogeneous and laminated mud beds, bioturbated beds, a variety of graded beds, and rare matrix‐supported conglomerates and slumps. Homogeneous muds and light grey to black laminated muds record deposition from distal, waning low density turbidity currents and terminal mud‐rich quasi‐laminar or laminar plug flows. Graded beds, interpreted as turbidites, range from beds several millimetre to a few centimetres of mud with silt to fine sand bases, to metre‐scale mud beds with coarser sand and pebble bases. Conglomerate and slumped beds record cohesive debris flows, transitional flows and slope failure. Three stratal package types are distinguished: bioturbated, bedded and laminated, recording distinct hydrological conditions. Bioturbated packages record interglacial marine conditions with well oxygenated waters. Bedded packages record hemipelagic processes and low energy density underflows in a mainly dysoxic, stratified, lacustrine setting (glacial phases). In laminated packages, white mm‐scale laminae of calcite or aragonite from varved, hemipelagic sediments demonstrating seasonal variability in a dysoxic non‐marine or transitional setting. Rift stratigraphy is linked to eustatically controlled connections to the global ocean across rift segment boundaries. The ca. 780 to 330 ka succession is dominated by laminated packages with thin bioturbated packages and distinct conglomerates and slumps, suggesting high sills, making ocean connections brief and transitional to lacustrine conditions prolonged. The ca. 330 ka to present succession shows well developed bioturbated and bedded packages, separated by thin laminated packages, suggesting brief transitions and well‐developed marine conditions due to lower sills. Results indicate that structurally controlled rift segment boundaries exert a first‐order control on syn‐rift stratigraphic evolution, with fault segment growth and linkage driving intra‐rift facies and sequence variability. [ABSTRACT FROM AUTHOR]

High score recovery at Ocean Drilling Program (ODP) Leg 210, Site 1276, provided a high-resolution porosity-depth relationship and an equally impressive age-depth model based on first and last occurrences of microfossils. Site 1276 was drilled over transitional crust in the Newfoundladn nonvolcanic margin, offshore Canada, between known continental crust on the west and apparent oceanic crust on the east as identified by seafloor-spreading magnetic anomalies M3 to M0 (Baremian--Aptian, 129.8--124.8 Ma). At Site 1276, two diabase sills were drilled at depths equivalent to the U reflection, a bright reflection that overlies transitional crust interpreted from seismic reflection profiles throughout the Newfoundland Basin. The sills were emplaced within uppermost Aptian fine- to coarse-grained sediments, 100-200 m above basement as estimated from seismic reflection data. Magma emplacement occured at shallow levels within the sediment column, as evidenced by: (1) the occurence of vesicles in the sill, and (2) compaction-induced folding of calcite veins that were emplaced near vertically in the sediments and are assumed to be coeval with the intrusion. By calculating the degree of shortening during vein emplacement, the age of magma emplacement can be deduced. From the porosity-age curve, the age of still emplacement is estimated to be 82.5--109.1 Ma, consistent with recent [sup.40Ar]/[sup.39Ar] radiometric dating of the upper sill that gave ages of 105.95 [+ or -] 1.78 Ma and 104.7 [+ or -] 1.7 Ma. The source of magmatism responsible for the diabase sills is necessarily postrift, and the sills are temporarily equivalent to alkali basalts dredged sills is necessarily postrift, and the sills are temporarily equivalent to alkali basalts dredged from the Newfoundland Seamonts. The simplest explanation for the Site 1276 diabases and the widespread distribution of the U reflection relates to the migration of the Azores, Madeira, and Canary plumes across the Newfoundland Basin between 80 and 120 Ma. Keywords: Newfoundland Basin extension, early postrift magmatism, Ocean Drilling Program.

We present the Malawi Active Fault Database (MAFD), an open‐access (https://doi.org/10.5281/zenodo.5507190) geospatial database of 113 fault traces in Malawi and neighboring Tanzania and Mozambique. Malawi is located within the East African Rift's (EAR) Western Branch where active fault identification is challenging because chronostratigraphic data are rare, and/or faults are buried and so do not have a surface expression. The MAFD therefore includes any fault that has evidence for displacement during Cenozoic East African rifting or is buried beneath the rift valley and is favorably oriented to the regional stresses. To identify such faults, we consider a multidisciplinary data set: high‐resolution digital elevation models, previous geological mapping, field observations, seismic reflection surveys from offshore Lake Malawi, and aeromagnetic and gravity data. The MAFD includes faults throughout Malawi, where seismic risk is increasing because of population growth and its seismically vulnerable building stock. We also investigate the database as a sample of the normal fault population in an incipient continental rift. We cannot reject the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law, which is consistent with Malawi's relatively thick seismogenic layer (30–40 km), low (<8%) regional extensional strain, and regional deformation localization (50%–75%) across relatively long hard‐linked border faults. Cumulatively, we highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the EAR and similar continental settings both to understand the regional seismic hazard and tectonic evolution. Plain Language Summary: Earthquakes represent the occurrence of slip along cracks in the Earth's crust. Therefore, mapping these cracks, or "faults," is important when assessing earthquake hazards. However, faults are challenging to identify as they may not be visible at the surface. Fault mapping also requires recognizing which faults have slipped in earthquakes in the recent geologic past, as these "active" faults are the most likely faults to have future earthquakes. Here, we describe how we identified active faults in Malawi, which is located along the tectonically active East African Rift. Faults under Lake Malawi were mapped using images of lake sediments that were generated from sound waves. Onshore, some faults were mapped from their expression in the landscape. Other faults, not visible at the surface, were identified from aeromagnetic data, which image the spatial distributions of magnetic minerals in the Earth's crust. Faults are considered active if that show evidence for slip during East African rifting in Malawi. We combined the active faults identified from these analyses into the Malawi Active Fault Database, a freely available geospatial database. We suggest that this database will be useful for seismic hazard planning in Malawi, where population growth and vulnerable buildings are increasing earthquake risk. Key Points: Digital elevation models, offshore seismic reflection surveys, and aeromagnetic data are combined to identify active faults in MalawiMapped faults are incorporated into the Malawi Active Fault Database, an open‐access geospatial databaseThe mapped faults follow a power law length distribution, which is consistent with strain localization onto a few long (>50 km) faults [ABSTRACT FROM AUTHOR]

Subduction zone architecture and properties are thought to control megathrust slip behavior, but few constraints on crustal structure and megathrust properties are available at sufficient resolution and depth, hindering understanding of linkages between structure and behavior. Here we present a P‐wave seismic velocity model based on wide‐angle seismic data integrated with collocated reflection imaging in the weakly coupled Shumagin Gap in the Alaska subduction zone, where a M7.8 occurred in July 2020. We show that this earthquake occurred near and below the Moho of the overriding plate where the megathrust is characterized by a 3‐ to 5‐km‐thick reflection band interpreted to represent tectonic mixing. The rheological heterogeneity of the plate boundary near and below the Moho could account for abundant interplate seismicity, repeated M7.x events, and patchiness of the 2020 rupture. Velocity variations in the overriding continental crust imply changes in rigidity that could further influence megathrust slip. Plain Language Summary: Slip on subduction zone plate boundary faults (megathrusts) produces large and destructive earthquakes, but many questions remain about what controls the size and character of these events. To address this question, we used seismic imaging data to determine subduction zone configuration and properties offshore Alaska in the area of a magnitude 7.8 earthquake that occurred in July 2020. We constrain the thickness of the crust overlying the megathrust, variations in strength within the crust, and changes in the total width and complexity of the fault zone. The M7.8 earthquake occurred on a part of the fault overlain by both continental crust and upper mantle, where imaging shows the fault zone may be particularly heterogeneous, which could explain the patchiness of slip during this event. Variations in strength of the overriding plate may also influence both shallow and deep fault slip in this subduction zone. Key Points: The megathrust intersects the continental Moho in the Shumagin Gap at ∼33 kmMegathrust heterogeneity near and below continental Moho could explain patchy slip in 2020 M7.8 event and abundant megathrust seismicityChanges in seismic velocity in the overriding crust imply changes in rigidity that could influence megathrust slip behavior [ABSTRACT FROM AUTHOR]

Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Database (MSSD), which describes the seismogenic properties of faults that have formed during East African rifting in Malawi. We first use empirical observations to geometrically classify active faults into section, fault, and multi-fault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that is estimated to be 75 ka based on dated core. Elsewhere, slip rates are constrained from advancing a 'systems-based' approach that partitions geodetically-derived rift extension rates in Malawi between seismogenic sources using a priori constraints on regional strain distribution in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability of outcomes from a logic tree used in these calculations. We find that for sources in the Lake Malawi's North Basin, where slip rates can be derived from both the geodetic data and the offset seismic reflector, the slip rate estimates are within error of each other, although those from the offset reflector are higher. Sources in the MSSD are 5-200 km long, which implies that large magnitude (MW 7-8) earthquakes may occur in Malawi. Low slip rates (0.05-2 mm/yr), however, mean that the frequency of such events will be low (recurrence intervals ~103-104 years). The MSSD represents an important resource for investigating Malawi's increasing seismic risks and provides a framework for incorporating active fault data into seismic hazard assessment in other tectonically active regions. [ABSTRACT FROM AUTHOR]

The in situ state and properties of sediments entrained along subduction megathrusts exert key controls on their mechanics and slip behavior. Low seismic velocity and high Vp/Vs are hypothesized to indicate highly elevated fluid pressure, and are invoked as conditions in the source areas of slow earthquakes and tremor. We report on Vp and Vs measurements for exhumed metasediments from Kodiak Island, AK, representative of materials along the modern megathrust. Our data reveal anisotropy of ∼8–28% in Vp and ∼6.5–8% in Vs at effective stresses ranging from ∼1 to 90 MPa, with lower wavespeeds perpendicular to the dominant fabric. The fabric‐normal velocities are sufficiently low to explain observations from regional geophysical surveys and are consistent with rock physics‐based models that incorporate small (<1%) crack porosity. We suggest that low velocity at ∼8–20 km depth along megathrusts may arise simply from the presence of foliated metasediments, without requiring near‐lithostatic fluid pressure. Plain Language Summary: The fluid pressure conditions along the plate interface in subduction zones are inferred from measurements of the speed of seismic waves through rocks (seismic velocity), which are often slower in narrow zones at the plate interface relative to the accretionary wedge and the subducting oceanic crust. These speeds are sensitive to variations in physical properties of the material they pass through, allowing us to infer information about the state and structure of the subduction zone. At the regional scale, zones of low seismic velocity are often interpreted to contain high fluid pressure and thus low effective stress. We conducted acoustic velocity measurements in the laboratory on exhumed rocks from Kodiak Island, AK, that are representative of rocks along the present‐day plate interface and find that our measurements of velocity matched those made along the modern Eastern Aleutian margin. In addition, we were able to reproduce the slower velocities by measuring perpendicular to the rock's dominant fabric. These velocities were also consistent with results from a physics‐based model for rocks with the same mineral composition. Based on our experimentation and modeling of velocities in these rocks, we conclude that elevated pore pressure is not required for velocities observed at depth. Key Points: Foliated and elastically anisotropic metamorphosed sediments are likely entrained along the plate interface at upper seismogenic depthsVelocities of metasediments measured normal to fabric in the laboratory are consistent with those reported from regional seismic surveysLaboratory measurements of velocity show that extremely elevated pore pressure is not required to generate low velocities in metasediments [ABSTRACT FROM AUTHOR]

The Lake Malawi (Nyasa) Rift, in the East African Rift System (EARS), is an ideal modern analogue for the study of extensional tectonic systems in low strain rate settings. The seismically active rift contains the 700-m-deep Lake Malawi, one of the world's oldest and largest freshwater lakes with one of the most diverse endemic faunal assemblages on Earth. Modern and reprocessed legacy multichannel seismic-reflection data are constrained by velocity information from a wide-angle seismic experiment to evaluate variability in extension, segmentation, and timing of fault development along the 550-km-long rift zone. Fault geometries and patterns of synrift sediment fills show that the Lake Malawi Rift is composed of three asymmetric rift segments, with intervening accommodation zone morphologies controlled by the degree of overlap between segment border faults. Most extension occurs on the basin border faults, and broadly distributed extension is only observed at one accommodation zone, where no border fault overlap is observed. Structural restorations indicate a weakly extended rift system (~7 km), with diminishing values of extension and thinner rift fill from north to south, suggesting a progressively younger rift to the south. There is no evidence of diking, sill injection, or extrusives within the synrift fill of the Lake Malawi Rift, although the volcanic load of the Rungwe magmatic system north of the lake and related subsidence may explain the presence of anomalously thick synrift fill in the northernmost part of the lake. The thickest synrift depocenters (~5.5 km) are confined to narrow 10- to 20-km-wide zones adjacent to each rift segment border fault, indicating concentration of strain on border faults rather than intrarift faults. Intrarift structures control axial sediment delivery in the North and Central rift segments, focusing sediment into confined areas resulting in localized overpressure and shale diapirs. The asymmetric, basement-controlled relief was established early in rift development. When overprinted with frequent high-amplitude hydroclimate fluctuations, which are well documented for this basin, the resulting highly variable landscape and lake morphometry through time likely impacted the diverse endemic faunas that evolved within the basin. New seismic-reflection data, augmented by wide-angle seismic data and age constraints from drill core, offer the most highly resolved 3D view to date of latest Cenozoic extensional deformation in East Africa and provide a foundation for hazards analysis, resource assessments, and constraining deformation in a low strain rate, magma-poor active rift. [ABSTRACT FROM AUTHOR]

The North Basin of the Malawi Rift is an active, early‐stage rift segment that provides the opportunity to quantify cumulative and recent faulting patterns in a young rift, assess contributions of intrarift faults to accommodating rift opening, and examine controls on spatial patterns of faulting. Multichannel seismic reflection data acquired in Lake Malawi (Nyasa) in 2015 together with legacy multichannel seismic data image a system of synthetic intrarift faults within this border‐fault‐bounded, half‐graben basin. A dense wide‐angle seismic reflection/refraction dip profile acquired with lake bottom seismometer data constrains sediment velocities that are used to convert fault throws from travel time to depth. Observed extension on intrarift faulting in the northern and central parts of the North Basin is approximately twice what would be predicted for hanging wall flexure, implying that the intrarift faults contribute to basin opening. The cumulative throw on intrarift faults is higher in the northern part of the rift segment than the south and is anticorrelated with throw on the border fault, which is largest in the southern part of the North Basin. This change in faulting coincides with a change in the orientation of the North Basin from a N‐S trend in the south to a NNW‐SSE trend in the north. We infer that the distribution of extension is influenced by rift orientation with respect to the regional extension direction. Almost all intrarift faults substantially offset late Quaternary synrift sediments, suggesting they are likely active and need to be considered in hazard assessments. Key Points: Intrarift faulting contributes to rift openingThe style and amount of rift faulting change along strike in North BasinNearly all intrarift faults have been active in the last ~75 kyr [ABSTRACT FROM AUTHOR]

Although the deep, wide basins of the Western rift, Africa, have served as analogues for the evolution of half‐graben basins, the geometry and kinematics of the border, intrabasinal, and transfer fault systems have been weakly constrained. Despite the >100‐km‐long fault systems bounding basins, little was known of seismicity patterns or the potential for M > 7.5 earthquakes. Using our new local earthquake database from the 2013‐2015 Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) seismic array (57 onshore, 32 lake‐bottom stations) and TANGA14 (13 stations), we examine the kinematics and extension direction of the Rungwe Volcanic Province and northern Malawi rift. We relocated earthquakes using a new 1‐D velocity model and both absolute and double‐difference relocation methods. Local magnitudes of 1,178 earthquakes within the array are 0.7 < ML < 5.2 with a b‐value 0.77 ± 0.03, and magnitude of completeness ML 1.9. Focal mechanism solutions for 63 earthquakes reveal predominantly normal and oblique‐slip motion, and full moment tensor solutions for ML 4.5, 5.2 earthquakes have centroid depths within 2 km of catalog depths. The preferred nodal planes dip more than 40° from surface to >25‐km depths. Extension direction from local earthquakes and source mechanisms of teleseismically detected earthquakes are approximately N58°E and N65°E, respectively, refuting earlier interpretations of a NW‐SE transform fault system. The low b‐value indicating strong coupling across crustal‐scale border faults, border fault lengths >100 km, and evidence for aseismic deformation together indicate that infrequent M > 7.5 earthquakes are possible within this cratonic rift system. Key Points: Steep nodal planes of earthquake focal mechanisms correspond to projections of border and intrabasinal faults to depths of 25 kmExtension direction across Rungwe volcanic province and Malawi rift is ENE, refuting interpretations of a NW‐SE transform fault systemLow b‐value, fault lengths >100 km, seismogenic layer 25‐30 km, and aseismic deformation suggest that infrequent M>7.5 earthquakes are possible [ABSTRACT FROM AUTHOR]

The Southeastern United States is an ideal location to understand the interactions between mountain building, rifting, and magmatism. Line 2 of the Suwannee suture and Georgia Rift basin refraction seismic experiment in eastern Georgia extends 420 km from the Inner Piedmont to the Georgia coast. We model crustal and upper mantle VP and upper crustal VS. The most dramatic model transition occurs at the Higgins‐Zietz magnetic boundary, north of which we observe higher upper crustal VP and VS and lower VP/VS. These observations support the interpretation of the Higgins‐Zietz boundary as the Alleghanian suture. North of this boundary, we observe a low‐velocity zone less than 2 km thick at ~5‐km depth, consistent with a layer of sheared metasedimentary rocks that forms the Appalachian detachment. To the southeast, we interpret synrift sediments and decreasing crustal thickness to represent crustal thinning associated with the South Georgia Rift Basin and subsequent continental breakup. The correspondence of the northern limit of thinning with the interpreted suture location suggests that the orogenic suture zone and/or the Gondwanan crust to the south of the suture helped localize subsequent extension. Lower crustal VP and VP/VS preclude volumetrically significant mafic magmatic addition during rifting or associated with the Central Atlantic Magmatic Province. Structures formed during orogenesis and/or extension appear to influence seismicity in Georgia today; earthquakes localize along a steeply dipping zone that coincides with the northern edge of the South Georgia Basin and the change in upper crustal velocities at the Higgins‐Zietz boundary. Key Points: Wide‐angle seismic data show upper crustal velocity change at Higgins‐Zietz boundaryRifting to form the South Georgia Basin localized south of the tectonic suture at the Higgins‐Zietz boundaryRecent earthquakes localize along the Higgins‐Zietz orogenic and rift‐related boundary [ABSTRACT FROM AUTHOR]

The crustal structure surrounding the northern Malawi rift and beneath the Rungwe Volcanic Province (RVP) has been investigated using teleseismic earthquakes recorded on SEGMeNT broad-band seismic stations to determine the extent to which the crust has been modified by Cenozoic rifting and magmatism. The SEGMeNT network included 57 broad-band seismic stations deployed in northern Malawi and southern Tanzania between August 2013 and October 2015. Estimates of crustal thickness, shear wave velocity and Poisson's ratio have been obtained by modelling P -wave receiver functions using the H – k stacking method and jointly inverting receiver functions with Rayleigh wave phase velocities. These estimates are used to investigate the extent of magmatic modification to the crust, indicated by changes in Poisson's ratio, and the geometry of crustal thinning along the northern margins of the Malawi rift and beneath the RVP. The average crustal thickness for the four stations in the RVP is 39 km, the average Poisson's ratio is 0.28 (Vp / Vs  = 1.83), and the average crustal shear wave velocity is 3.6 km s–1. Although the RVP has been a site of ongoing magmatism since at least 17 Ma and is associated with a pronounced low velocity zone in the mantle, our results show little evidence that the bulk composition or thickness of the crust beneath the RVP has been significantly modified by magmatism or extension. However, Poisson's ratios of 0.29–0.31 (Vp / Vs  = 1.85–1.91) at three of the stations in the RVP, where there is also no evidence for higher Vs, may indicate the presence of partial melt in the crust. The average crustal thickness of Proterozoic terranes surrounding the northern end of the Malawi rift ranges from 38 to 42 km. For most of the terranes, average Poisson's ratios are between 0.25 and 0.26 (Vp / Vs  = 1.73–1.76), with the exception of the Irumide Belt, which has an average Poisson's ratio of 0.23 (Vp / Vs  = 1.68). The average crustal shear wave velocities for all the terranes are either 3.6 or 3.7 km s–1. These results indicate a bulk felsic to intermediate crustal composition for all terranes, consistent with previous results, and reveal that there is little, if any, crustal thinning beneath the uplifted flanks of asymmetric basins within the northern Malawi rift or beneath the RVP. Consequently, crustal thinning in the northern Malawi rift must be highly focused beneath the centres of rift basin segments, consistent with models of rift flank topography and gravity observations. [ABSTRACT FROM AUTHOR]

P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low‐velocity zone (LVZ) with δVp = ~−1.5–2.0% and δVs = ~−2–3% centered beneath the RVP, a lower‐amplitude LVZ (δVp = ~−1.0–1.3% and δVs = ~−0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower‐amplitude anomaly at ~−10° to −11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting. Plain Language Summary: P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. A low‐velocity anomaly is imaged under the RVP and northern Malawi rift. We attribute the low‐velocity anomaly to flow of warm mantle from the African superplume to the southwest of the study area, which has migrated around the side of thick Bangweulu Craton lithosphere and upwelled beneath the thinner mobile belt lithosphere to the east of the Bangweulu Craton. The observation that volcanism began in the RVP prior to the onset of rifting suggests that the lithosphere beneath the Malawi rift may have been thermally weakened prior to rifting. Key Points: Low‐velocity anomaly is imaged under Rungwe Volcanic Province and northern Malawi riftLow‐velocity anomaly is attributed to upwelling of warm mantle around side of Bangweulu Craton lithosphereLithosphere beneath the Malawi rift may have been weakened prior to rifting [ABSTRACT FROM AUTHOR]

Variations in pore-fluid pressure along subduction megathrusts are often invoked to explain differences in fault slip behavior between margins. However, many other parameters vary between subduction zones, making it difficult to isolate the causes of elevated pore fluid pressures and their effects on megathrust behavior. Here we show evidence from pre-stack depth migrations of multichannel seismic reflection data along the subduction zone off the Alaska Peninsula for significant, systematic along-strike variations in the thickness, continuity, P-wave velocity, and estimated pore-fluid pressure of the subducting sediment layer that lines the shallow plate interface within 25 km of the trench. These variations appear to correlate with changes in seismicity, locking, and earthquake rupture history. The currently locked and seismically quiet Semidi segment has a continuous, thick (600-900 m) subducted sediment layer that is characterized by low seismic velocities and elevated pore pressure (λ* ≈ 0.4-0.8). The subducted sediment in the neighboring Shumagin Gap, a region with low geodetic coupling and abundant small earthquakes, is thinner, irregular and has lower pore pressure (λ* < 0.2-0.3). We suggest that the thicker and weakly faulted sediment layer entering the trench at Semidi is associated with a continuous and overpressured sediment layer lining the shallow plate interface, but forms a large coherent asperity as it dewaters and consolidates at greater depths, thus favoring large earthquakes. In contrast, thinner incoming sediment disrupted by outer rise faulting at Shumagin results in a plate interface without elevated pore-fluid pressures, but which is heterogeneous and complex at all depths, contributing to creep and frequent small earthquakes. [ABSTRACT FROM AUTHOR]

Drilling during ODP Leg 210 penetrated two post-rift sills (dated as ∼105.3 and ∼97.8 Ma) in the deep sediments overlying basement of the continent–ocean transition zone on the magma-poor Newfoundland margin. The sill emplacement post-dated the onset of seafloor spreading by at least 7–15 Myr. The shallower of the two sills coincides with the high-amplitude U reflection observed throughout the deep Newfoundland Basin, and strong reflectivity in the sub-U sequence suggests that a number of other sills are present there. In this paper, we use multichannel seismic reflection data and synthetic seismograms to investigate the nature, magnitude and extent of this post-rift magmatism in the deep basin. Features observed in seismic profiles that we attribute to sill injection include high-amplitude reflections with geometries characteristic of intrusions such as step-like aspect; abrupt endings, disruptions and junctions of reflections; finger-like forms; differential compaction around possible loci of magma injection and disruption of overlying sediments by apparent fluid venting. Interpreted sills occur only over transitional basement that probably consists of a mixture of serpentinized peridotite and highly thinned continental crust, and they cover an area of ∼80 000 km2. From analysis of synthetic seismograms, we estimate that sill intrusions may comprise ∼26 per cent of the sub-U high-reflectivity sequence, which yields a crude estimate of ∼5800 km3 for the total volume of sills emplaced by post-rift magmatism. This is significant for a margin usually described as ‘non-volcanic’. We discuss competing hypotheses about the source of the magmatism, which is still uncertain. [ABSTRACT FROM AUTHOR]

A geophysical survey was conducted over a hydrocarbon prospect in the North Celtic Sea Basin using a small array of ocean-bottom seismographs (OBSs). The purpose of this study was to determine the ratio of compressional (P)- to shear (S)-wave velocity of consolidated sedimentary rocks in order to constrain possible subsurface variations in pore-fluid content. The ratio of V P and V S is known to be particularly sensitive to lithology, porosity and pore-fluid content, making it a useful parameter for evaluating hydrocarbon prospects. OBSs offer a relatively cheap and time-effective means of acquiring multi-component data compared with ocean-bottom cables. In this contribution, we demonstrate the ability of an OBS survey comprising three pairs of two OBSs spaced at 1.6 km to recover lateral variations in the V P/ V S ratio. A key requirement of this type of study is that S waves will be generated by mode conversions in the subsurface, since they cannot be generated in nor travel through fluids. In this survey, the contrast in physical properties of the hard seabed of the North Celtic Sea Basin provided a means of generating converted S waves. Two-dimensional ray-tracing and forward modelling was used to create both V P and V S models along a profile crossing the Blackrock prospect in the North Celtic Sea Basin. These models comprise four layers and extend to a maximum depth of 1.1 km. The observed northward decrease in the V P/ V S ratio at depths of 500–1000 m below the seafloor in the study area is interpreted to represent lateral variation in the amount of gas present in the pore space of Upper Cretaceous chalks and shales overlying the prospective reservoir. [ABSTRACT FROM AUTHOR]