North American Plate Research Articles

Mantle helium in Southern Quebec groundwater: A possible fossil record of the New England hotspot

The Monteregian Hills are an alignment of magmatic intrusions of Cretaceous age located in the St. Lawrence Lowlands, Quebec, Canada. Their origin is controversial and numerous studies have failed to decipher between a hotspot trail or sub-continental magmatism related to the opening of the North Atlantic Ocean. Here, we show that 17.7±9.6% of the helium of the modern to Holocene-aged groundwater from the regional aquifer is of mantle origin, with a 3He/4He (R) of up to 1.42 times the atmospheric ratio (Ra). It suggests that a fossil Monteregian Hills magmatic signal, diluted by local radiogenic helium and preserved in the Monteregian Hills intrusions, is leached locally by flowing modern or sub-modern groundwater. Helium isotopic measurements by pyrolysis in Monteregian Hills bulk rocks and clinopyroxene separates show R/Ra values of up to 4.96, suggesting that fossil mantle helium has been partially preserved in these rocks and their mineral phases. Monte Carlo simulations of a magma aging model shows that the initial 3He/4He ratio in these Cretaceous intrusions could have been between 21±10Ra and 33±28Ra (2σ), favoring the hypothesis that the Monteregian Hills are the product of the passage of the North American plate over the New England hotspot. This study raises the prospect of using modern groundwater as an archive of mantle He over a hundreds of millions of years timescale.

Investigating the formation of the Cretaceous Western Interior Seaway using landscape evolution simulations

Abstract Transient intraplate sedimentation like the widespread Late Cretaceous Western Interior Seaway, traditionally considered a flexural foreland basin of the Sevier orogeny, is now generally accepted to be a result of dynamic topography due to the viscous force from mantle downwelling. However, the relative contributions of flexural versus dynamic subsidence are poorly understood. Furthermore, both the detailed subsidence history and the underlying physical mechanisms remain largely unconstrained. Here, we considered both Sevier orogenic loading and three different dynamic topography models that correspond to different geodynamic configurations. We used forward landscape evolution simulations to investigate the surface manifestations of these tectonic scenarios on the regional sedimentation history. We found that surface processes alone are unable to explain Western Interior Seaway sedimentation in a purely orogenic loading system, and that sedimentation increases readily inland with the additional presence of dynamic subsidence. The findings suggest that dynamic subsidence was crucial to Western Interior Seaway formation and that the dominant control on sediment distribution in the Western Interior Seaway transitioned from flexural to dynamic subsidence during 90–84 Ma, coinciding with the proposed emplacement of the conjugate Shatsky oceanic plateau. Importantly, the sedimentation records require the underlying dynamic subsidence to have been landward migratory, which implies that the underlying mechanism was the regional-scale mantle downwelling induced by the sinking Farallon flat slab underneath the westward-moving North American plate. The simulated landscape evolution also implies that prominent regional-scale Laramide uplift in the western United States should have occurred no earlier than the latest Cretaceous.

Quaternary slip rates on the White Mountains fault zone, eastern California: Implications for comparing geologic to geodetic slip rates across the Walker Lane

AbstractThe White Mountains fault zone in eastern California is a major fault system that accommodates right-lateral shear across the southern Walker Lane. We combined field geomorphic mapping and interpretation of high-resolution airborne light detection and ranging (LiDAR) digital elevation models with 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene right-lateral slip rates on the White Mountains fault zone. Alluvial fans were found to have ages of 46.6 + 11.0/–10.0 ka and 7.3 + 4.2/–4.5 ka, with right-lateral displacements of 65 ± 13 m and 14 ± 5 m, respectively, yielding a minimum average slip rate of 1.4 ± 0.3 mm/yr. These new slip rates help to resolve the kinematics of fault slip across this part of the complex Pacific–North American plate boundary. Our results suggest that late Pleistocene slip rates on the White Mountains fault zone were significantly faster than previously reported. These results also help to reconcile a portion of the observed discrepancy between modern geodetic strain rates and known late Pleistocene slip rates in the southern Walker Lane. The total middle to late Pleistocene slip rate from the southern Walker Lane near 37.5°N was 7.9 + 1.3/–0.6 mm/yr, ∼75% of the observed modern geodetic rate.

The Northern Terminus of Cascadia Subduction

AbstractThe Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean‐bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40°). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low VP/VS (<1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates.

Spatial Variations in Crustal and Mantle Anisotropy Across the North American‐Caribbean Boundary on Haiti

AbstractHaiti, on the island of Hispaniola, is situated across the North American‐Caribbean plate boundary at the transition point between oblique subduction in the east and a transform plate boundary in the west. Here we use shear wave splitting measurements from S waves of local (0–50 km) and intermediate depth (50–150 km) earthquakes as well as SK(K)S phases from teleseismic earthquakes to ascertain good spatial and vertical resolution of the azimuthal anisotropic structure. This allows us to place new constraints on the pattern of deformation in the crust and mantle beneath this transitional region. SK(K)S results are dominated by plate boundary parallel (E‐W) fast directions with ~1.9 s delay times, indicating subslab trench parallel mantle flow is continuing westward along the plate boundary. Intermediate depth earthquakes originating within the subducting North American plate show a mean fast polarization direction of 065° and delay time of 0.46 s, subparallel to the relative plate motion between the Caribbean and North American plates (070°). We suggest a basal shear zone within the lower ductile crust and upper lithospheric mantle as being a potential major source of anisotropy above the subducting slab. Upper crustal anisotropy is isolated using shear wave splitting measurements on local seismicity, which show consistent delay times on the order of 0.2 s. The fast polarization directions indicate that the crustal anisotropy is controlled by the fault networks in close proximity to the major strike‐slip faults, which bisect the north and south of Haiti, and by the regional stress field where faulting is less pervasive.

Uplift and exhumation in Haida Gwaii driven by terrane translation and transpression along the southern Queen Charlotte fault, Canada

AbstractThe mountainous archipelago of Haida Gwaii abuts the transpressive Pacific–North American plate margin north of the Cascadia subduction zone (northwestern North America). Topography on Haida Gwaii has been attributed to either dynamic uplift supported by subduction initiation or crustal shortening driven by shear adjacent the plate-bounding Queen Charlotte fault. In order to resolve how intraplate strain is accommodated, we obtained thermochronometry data from 20 bedrock samples on Haida Gwaii, including zircon (U-Th)/He, apatite (U-Th-Sm)/He, and apatite fission-track dates. With dates ranging from 5 to 60 Ma, we interpret exhumation rates increasing in proximity to the Queen Charlotte fault and leading to a maximum of 6 km of exhumation since 20 Ma. The onset of exhumation significantly predates the purported initiation of subduction, precluding a direct relationship between subduction initiation and the development of topography in the archipelago. Instead, exhumation onset correlates with passage of the Yakutat terrane, suggesting that North America was deformed and Haida Gwaii uplifted during terrane translation. Steady or slightly decreasing exhumation rate since the Miocene is at odds with estimated increases to intraplate convergence over this time, ruling out crustal shortening in Haida Gwaii as the only response to transpression between North America and the Pacific. From this, we conclude that plate convergence is accommodated through basin inversion and internal shortening in the North American and Pacific plates as well as potential underthrusting of the Pacific plate beneath North America.

Active Faults and Seismicity of the Kulu–Tenka District, Magadan Oblast Based on Remote Sensing Results

Satellite imagery was used to identify active neotectonic faults. The areas of greater fault density coincide with concentration zones of earthquake epicenters, including the largest of them (K ≥ 10). The earthquakes are confined to intersections of deep NE and NW striking faults. The Kulu–Tenka region can be considered as the area of interaction between the zones of dynamic influence of faults that separate the Eurasian, North American, and Pacific plates.

The January 23, 2018 M7.9 Kodiak earthquake, Alaska: A consequence of slip partitioning in the outer rise region

The January 23, 2018 M7.9 earthquake ruptured conjugate set of strike-slip faults in the outer rise of the Alaska subduction zone, southeast of Kodiak Island. The complex nature of this earthquake is evident from the earthquake focal mechanism and aftershock relocation patterns which revealed rupture of multiple north-south and east-west trending conjugate fault planes. Here, we suggest that this earthquake occurred as a result of two different modes of interaction process. The stress triggering process reveals (i) the co-seismic and post-seismic deformation of the M9.2 1964 Alaska megathrust earthquake increased Coulomb failure stress of ∼10 kPa and ∼0.05 kPa respectively in the 2018 Kodiak earthquake source region, and (ii) the overall deformation is being accommodated in the outer rise region due to slip partitioning resulting from the oblique convergence between the Pacific and North American plates along the Alaska Aleutian megathrust.

The Santa Fe Intrusion and Other Magmatic Bodies Under the Chichón Volcano Area (Mexico): Inferences from Aeromagnetic and New Petrologic-Geochronologic Data

We review the current knowledge of the Pleistocene Modern Chiapanecan Volcanic Arc (MCVA). This arc is related to the subduction of the Cocos plate beneath the North American plate in the State of Chiapas, southeastern Mexico. The MCVA consists of large intrusive bodies, domes, eroded volcanic landforms, and the active El Chichon, which produced the disastrous 1982 eruption, the deadliest in Mexico’s recorded history. The available geological knowledge, and new geological and aeromagnetic data on the arc, reveals a system composed of a sizeable intrusive body called the Santa Fe diorite, and small-size volcanoes such as El Chichon and Catedral, and extinct volcanoes associated with volcaniclastic deposits. A 3D-inversion of the aeromagnetic anomalies indicates that the Santa Fe diorite is a large intrusive body (27 km long, 4 km wide with a minimum volume of 1662 km3) while small volcanoes such as El Chichon have small-size magma chambers (~ 7 km3). Interestingly, our models of the causative bodies for the aeromagnetic anomalies suggest that the El Chichon volcano, as well as of other volcanic areas in the region, are not linked directly to the Santa Fe intrusive. However, new 40Ar/39Ar dates for samples from the Santa Fe intrusive (2.2 Ma), the Catedral volcano (1.6 Ma), and a mafic enclave (1.09 Ma) hosted in 1982 Chichon deposits, along with the aeromagnetic anomalies and geochemical data confirm that these extrusive and intrusive structures belong to the MCVA. The chemistry of these structures suggests that magmas generated in the upper mantle by the subduction system evolved through different processes, such as crustal contamination for the Santa Fe diorite and Catedral volcano, and crystal fractionation for El Chichon volcano.

Characterization of the tectonic origins of historical and modern seismic events and their societal impact on the Azores Archipelago, Portugal

Abstract The Azores archipelago is located in the North Atlantic Ocean and consists of nine volcanic islands distributed along a general WNW–ESE trend, crossing the Mid-Atlantic Ridge (MAR) in the zone where three lithospheric plates (Eurasian, Nubian and North American) come across. To the west of the MAR, at the North American plate, the Corvo and Flores islands emerge from a present-day, relatively stable geological setting, while the other islands are located in an important seismically and volcanically active zone, corresponding to the boundary between the Eurasian and Nubian tectonic plates. Since the settlement of the Azores archipelago, in the mid-fifteenth century, several destructive earthquakes, such as the 1522 earthquake that severely affected São Miguel Island, and seismic crises with a significant destructive impact, have affected the region with intensities up to X (European Macroseismic Scale of 98 – EMS-98). The location of such events, either inland or offshore, is mainly related to the WNW–ESE structural system, which extends from the MAR to the Gloria Fault (GF). The instrumental seismic activity registered since 1997 has allowed improvements to the characterization of the present-day boundary between the Eurasian and Nubian plates.

AS ROCHAS VULCÂNICAS DA ILHA SÃO JORGE, AÇORES (PORTUGAL): PETROGRAFIA E GEOQUÍMICA

A Ilha São Jorge faz parte do Grupo Central do Arquipélago dos Açores (Portugal), situado próximo à junção tríplice, entre as placas litosféricas Norte-Americana, Euro-Asiática e Africana, na região do Atlântico Norte. A vulcanoestratigrafia da ilha é composta por três complexos vulcânicos denominados Topo, Rosais e Manadas. Segundo a bibliografia, a idade da Ilha São Jorge é 1,3 ± 0,0035 Ma (40Ar/39Ar). Com base na petrografia, as rochas apresentam granulometria média a fina, texturas porfirítica, microporfirítica, glomeroporfirítica, intergranular, pilotaxítica, traquítica e intersertal. De acordo com dados geoquímicos, as rochas forma classificadas como basanitos, basaltos alcalinos e hawaítos, compostos por olivina, augita, plagioclásio e minerais opacos, que ocorrem como fenocristais ou inseridos na matriz. Algumas amostras de basalto alcalino e hawaíto apresentam kaersutita, interpretada como xenocristal. Texturas de reabsorção, embainhamento, zonação e bordas de reação são feições comuns em fenocristais e indicam que o magma sofreu descompressão rápida, com variação brusca de temperatura e pressão. A interpretação geoquímica permitiu concluir que as lavas apresentam afinidade alcalina sódica e foram geradas por baixas taxas de fusão parcial, a partir de um magma mantélico enriquecido. Os dados geoquímicos são compatíveis com os de magmas gerados em ambiente geotectônico do tipo intraplaca oceânico (OIB).

Cenozoic tectonostratigraphic evolution of the strike-slip Cauto-Guacanayabo Basin, Eastern Cuba

An integrated study of Cauto-Guacanayabo strike-slip basin has allowed establishing its tectonostratigraphic evolution. This work uses subsurface data (wells, seismic and gravimetric) and surface data (digital terrain model and geological mapping) to investigate the relations between sedimentary infill and tectonic activity. In the sedimentary infill, four tectonosequences have been recognized, which are constituted by 19 lithostratigraphic units which range in age from middle Eocene to Holocene, and are delimited by regional unconformities at the basin scale. In seismic reflection profiles, the thickness of the sedimentary record can exceed 2600 ms of double time (TWT), while the thickness cut by the boreholes reaches 2500 m in the northeast part of the basin. Each tectonosequence shows different geometries, thickness and number of depocenters. The predominant geometries in a mapping view are rectangular and elongated in the northeast-southwest direction. This evidences the complexity and compartmentalization of the sedimentary infill in the Cauto-Guacanayabo Basin. The coexistence of extensive and compressive structures, close to each other in the Cauto-Guacanayabo Basin, is associated with local transtensive and transpressive regimes. These regimes have been controlled by the left lateral activity of the Cauto-Nipe strike-slip fault and the Guacanayabo-Nipe tectonic corridor. The compressive structures are mainly located in the southern part to the northeast – southwest and east-northeast – west-southwest directions. In the northeast part of the basin, extensive structures with north – south, northeast – southwest and possibly northwest – southeast directions are common, while in the Gulf of Guacanayabo, there is evidence of lesser deformation. The structural patterns indicate that the most intense activity of the Guacanayabo-Nipe tectonic corridor took place during the middle Eocene. The tectonic activity of the corridor begins to wane in the upper Oligocene, which coincides with the transfer of slip to the Orient Fault, the current boundary between the Caribbean and North American plates.

Geophysical analysis of the 30 July 1972 Sitka, Alaska, earthquake sequence

AbstractThe 1972 Mw 7.6 Sitka earthquake is the largest historical event along the southeastern Alaska portion of the strike-slip Queen Charlotte fault, the transform boundary between the Pacific and North American plates. The fault is one of the fastest moving transform boundaries in the world, having accumulated enough slip since 1972 to produce an event of comparable size in the near future. Thus, understanding the controls on the rupture process of the 1972 mainshock is important for seismic hazard assessment in Alaska. Following the mainshock, the U.S. Geological Survey installed a network of portable seismographs that recorded over 200 aftershocks. These locations were never published, and the original seismograms and digital phase data were misplaced. However, we were able to scan paper copies of the phase data, convert the data to digital form, and successfully relocate 87 aftershocks. The relocations show two clusters of aftershocks along the Queen Charlotte fault, one ∼40 km north of the mainshock epicenter and the other just south of the mainshock, both regions adjacent to portions of the fault that experienced maximum moment release in 1972. Many of the northern aftershocks locate east of the Queen Charlotte fault. This pattern is similar to aftershocks observed in the 2013 Mw = 7.5 Craig, Alaska earthquake. Recent and pre-1971 (1925–1970) seismicity indicates that the regions where aftershocks clustered remained active through time. Gravity, magnetic, and bathymetric anomalies suggest that the structural variations in both the Pacific and North American plates (e.g., age, density, rock type, and thickness) play roles in rupture nucleation and termination along the northern Queen Charlotte fault.

The Contemporary Seismic Deficit in the Kuril–Kamchatka Subduction Zone

Based on an analysis of seismicity for the period of 1900–2018, the seismic potential of the Kuril–Kamchatka subduction zone is estimated, and the geometry of interplate coupling of the North American and Pacific lithospheric plates and the rate of seismic underthrusting of the latter beneath the former are specified. In order to identify areas of modern seismic deficits in the subduction zone, an approach based on the cumulative displacement of interplate earthquakes with Mw ≥ 7.0 is proposed. Together with other methods, such an approach contributes to a more accurate seismic hazard assessment of the region.

Shear Wave Splitting and Mantle Flow Beneath Alaska

AbstractShear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion.

Hydrochronology of a proposed deep geological repository for low- and intermediate-level nuclear waste in southern Ontario from U–Pb dating of secondary minerals: response to Silurian and Cretaceous events

A record of fluid flow has been documented within a Paleozoic carbonate platform sequence by U–Pb dating of calcite in veins and vugs from rock core sampled through a shallowly dipping sequence of sedimentary rocks beneath the Bruce nuclear site, Ontario, Canada. Secondary calcite from >650 m deep Ordovician carbonate rocks yields a Silurian age of 434 ± 5 Ma possibly related to infiltration of seawater from overlying evaporitic basins as well as hydrothermal solutions that infiltrated from below. In contrast, near-surface Devonian rocks mostly give vein infill ages over the range of 80–100 Ma with evidence for younger infill down to 50 Ma. Vein calcite samples previously dated from surface outcrops of Ordovician carbonate exposed up to 500 km to the east yielded similar U–Pb ages. Coincidence of near-surface vein calcite ages indicates widespread vein emplacement synchronous with a change in direction of motion of the North American plate as well as possible erosional unroofing following passage of the region over the Great Meteor hotspot approximately 125 Myr ago. Deeper carbonate formations have remained apparently impermeable to post-Paleozoic disturbance despite these perturbations.

Strong seismic anisotropy in the deep upper mantle beneath the Cascadia backarc: Constraints from probabilistic finite-frequency SKS splitting intensity tomography

The volcanically active High Lava Plains (HLP) region is a striking tectonomagmatic feature of eastern Oregon, in the backarc of the Cascadia subduction zone. It features young (<12 Ma), bimodal volcanic activity; the rhyolitic volcanism has a spatiotemporal trend that is oblique to that expected from the absolute motion of the North American plate. Several models have been proposed to explain the tectonic evolution of Cascadia backarc and the relationships between upper mantle processes and volcanic activity; however, consensus remains elusive. Because seismic anisotropy in the upper mantle reflects processes such as mantle flow and partial melting, constraints on anisotropic structure can shed light on the connections between mantle dynamics and tectonomagmatic activity in the Cascadia backarc. Anisotropy is often constrained via SKS splitting measurements; however, their interpretation is typically ambiguous because they lack depth resolution. Here we present new constraints on upper mantle anisotropy beneath the HLP region from probabilistic finite-frequency SKS splitting intensity tomography, which provides both lateral and depth constraints on anisotropic structure. Our technique is based on a Markov chain Monte Carlo approach to searching parameter space, and we use finite-frequency sensitivity kernels to relate model perturbations to splitting intensity observations. We use data from broadband stations of the dense High Lava Plains experiment, which provide good resolution of upper mantle anisotropic structure, as demonstrated via resolution tests. We find evidence for particularly strong seismic anisotropy in the deep upper mantle (200-400 km depth) beneath the Cascadia backarc, suggesting that flow in the deep upper mantle, rather than alignment of partial melt in the shallow mantle, provides the first-order control on shear wave splitting delay times. Our model provides additional support for the idea that mantle flow beneath the Cascadia backarc is controlled by rollback subduction. Our results suggest that anisotropy in the deep upper mantle may be more important to the interpretation of SKS splitting measurements in some settings than commonly appreciated, and provides an avenue for reconciling apparently contradictory constraints on anisotropic structure from surface waves and SKS splitting.

CCAF-DB: the Caribbean and Central American active fault database

Abstract. A database of ∼250 active fault traces in the Caribbean and Central American regions has been assembled to characterize the seismic hazard and tectonics of the area, as part of the Global Earthquake Model (GEM) Foundation's Caribbean and Central American Risk Assessment (CCARA) project. The dataset is available in many vector GIS formats and contains fault trace locations as well as attributes describing fault geometry and kinematics, slip rates, data quality and uncertainty, and other metadata as available. The database is public and open source (available at: https://github.com/GEMScienceTools/central_am_carib_faults, last access: 23 March 2020), will be updated progressively as new data become available, and is open to community contribution. The active fault data show deformation in the region to be centered around the margins of the Caribbean plate. Northern Central America has sinistral and reverse faults north of the sinistral Motagua–Polochic fault zone, which accommodates sinistral Caribbean–North American relative motion. The Central Highlands in Central America extend east–west along a broad array of normal faults, bound by the Motagua–Polochic fault zone in the north and trench-parallel dextral faulting in the southwest between the Caribbean plate and the Central American forearc. Faulting in southern Central America is complicated, with trench-parallel reverse and sinistral faults. The northern Caribbean–North American plate boundary is sinistral off the shore of Central America, with transpressive stepovers through Jamaica, southern Cuba and Hispaniola. Farther east, deformation becomes more contractional closer to the Lesser Antilles subduction zone, with minor extension and sinistral shear throughout the upper plate, accommodating oblique convergence of the Caribbean and North American plates.

Илин-Тасское (Абыйское) землетрясение 14 февраля 2013 г. с Mw=6.7 (северо-восток Якутии)

Information on the strong Ilin-Tass (Abyi) earthquake recorded on February 14, 2013, in Northeastern Yakutia with Мw.=6.7 is provided. It arose to the Chersky seismotectonic zone (SZCH), which is part of the Arctic-Asian seismic belt that separates the Eurasian and North American lithospheric plates in Northeast Asia. The intensity of the shock at the epicenter corresponded to 9. The instrumental and macroseismic data, the focal mechanism, and the seismotectonic situation in the epicenter region are analyzed. The interconnections of the earthquake with the large regional Ilin-Tass fault are established. It is concluded that the occurrence of the Ilin-Tass (Abyi) earthquake occurred as a result of thrust displacements along the mentioned fault during the collision of the Eurasian and North American plates under compression conditions.

Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

AbstractStructural heterogeneities in subduction zones can affect slip behaviors of the megathrust faults and the generation of intraslab earthquakes. In this work we study the 3‐D seismic structures (Vp, Vs, and Poisson's ratio) in and around the source zones of the 2018 Anchorage intraslab earthquake (Mw 7.1) and the 1964 Alaska megathrust earthquake (Mw 9.2). The Anchorage earthquake occurred in an anomalous zone within the subducting Yakutat/Pacific plate with a higher Poisson's ratio than the normal slab. Above the source zone, the overriding North American plate shows a low Vs and a high Poisson's ratio. These features indicate that strong dehydration occurs in the source zone and released fluids ascend into the overlying crust. Two areas with long‐term slow slip events in the Upper and Lower Cook Inlet predominantly exhibit a high Poisson's ratio in the lowermost portion of the crust and the cold nose of the mantle wedge, whereas a low Poisson's ratio zone is revealed between them, suggesting that their segmentation is possibly related to localized slab‐releasing fluids. In the Prince William Sound, the rupture of the 1964 Great Alaska earthquake initiated beneath a high‐V and high Poisson's ratio zone of the overlying crust and the large slips occurred beneath a low‐Vs and high Poisson's ratio zone, suggesting that lateral heterogeneities of the overriding plate may have played an important role in the nucleation and rupture processes of the Great Alaska earthquake.