Outer Rise Research Articles

Explicit role of seamount subduction in upper plate deformation as exemplified in the Ryukyu subduction zone

The Ryukyu subduction zone is typical of the subduction of an oceanic ridge (seamount or plateau) and back-arc rifting. How the upper plate responds to the combined effect of subduction of a bathymetric high and back-arc spreading is not fully understood. We investigate the combined effect by calculating the T-axis from regional focal mechanisms. Three patterns are observed from the outer rise to the back arc, showing a coherent trench-normal T-axis along the strike. They can be readily explained by outer-rise plate bending, forearc upper-plate bending induced by seamount subduction, and back-arc spreading associated with slab rollback. A distinct pattern of trench-parallel T-axes along the strike is also observed in the arc. This can be attributed to the difference in the extension rate between forearc extension and back-arc rifting of the upper plate, where the rate of back-arc extension driven by slab rollback is faster than that of forearc trench-normal extension attributed to seamount subduction. We also examine the T-axis patterns in other systems with seamount subduction, including the southern Manila subduction zone and the southern Central to northern South American subduction zones, displaying similar trench-normal T-axes in the forearc. Therefore, our observations have a broader implication in upper-plate deformation in response to seamount subduction.

Structural analysis and evolution of large Venusian coronae: Insights from low-angle faults at coronae rims

We analyzed topography, fracture patterns, and faults of the asymmetric Atahensik Corona (700 × 900 km diameter), formerly known as Latona Corona, and their surrounding troughs using Magellan SAR imagery, and compare the results with the smaller, ovoid Didilia (400 × 450 km diameter) and Pavlova coronae (550 × 650 km diameter) to get insights on corona formation on Venus. Atahensik contains a high density of radial, oblique, and concentric fractures, the latter are inferred to be the youngest fractures. A high density of concentric fractures particularly occurs along the outer rise and indicates elastic downward bending of this part of the lithosphere in the later stage of corona formation. Along the steep inner slopes of Atahensik's arcuate troughs, large-scale faults are exposed that dip gently towards the corona center and crosscut all fractures. We propose that these low-angle faults were initially formed as thrust planes but subsequently became reactivated as low-angle normal faults, thereby exposing parts of their fault surfaces. Such faults have been identified not only along the arcuate troughs of Atahensik but also occur in Dali Chasma, northwest of Atahensik Corona.A phenomenological formation model of large coronae is presented: corona initiation starts with radial fracturing, which is caused by the dike emplacement and thermal uplift of the corona center due to the rise of a hot asthenospheric mantle plume. Uplift and lateral plume spreading steepen the outer rim of the uplift and cause intense radial fracturing of a central volcanic edifice and the corona's outer rim. This intermediate stage is preserved in several less-evolved coronae such as Didilia and Pavlova Coronae. The fractured ridge thrusts outward onto an intact and cooler lithosphere along strongly localized thrust planes. The overthrusted, cooler lithosphere is elastically bent downward and forms arcuate troughs and associated outer rises with numerous concentric fractures along their crest line. The fractured ridge annulus of the corona is supported by the intact and thickened lithosphere surrounding the corona. The present morphology of Atahensik Corona indicates subsequent subsidence in its central part due to declining plume activity and reduced thermal buoyancy. Reactivation of the thrusts as low-angle normal faults results from the subsidence of the corona interior, a gravitational instability of the elevated corona annulus, and a lack of shortening. The evolutionary sequence derived on the basis of structural data is in agreement with geodynamic models on corona formation involving a bending lithosphere at the plume margin.

Upper-mantle seismic anisotropy in the southwestern North Island, New Zealand: Implications for regional upper-mantle and slab deformation

We employed shear-wave splitting analysis on both teleseismic SKS and S waves, and S waves from deep (150–250 km) local earthquakes collected from a dense array with 43 temporary broadband seismic stations and nine long-term seismic stations centered at Mount Taranaki to characterize the upper-mantle dynamics in the southwestern North Island of New Zealand, in areas previously unexamined for shear-wave splitting. We observed predominantly trench-parallel fast polarizations and strikingly large delay times over 3 s from teleseismic analysis. In contrast, local S analysis yielded a sharp transition of fast-polarization from trench-parallel in the northeast to trench-normal in the southwest. Trench-parallel fast-polarization from teleseismic analysis may be attributed to sub-slab trench-parallel flow or to trench-parallel fractures in the subducting slab. More importantly, we attribute large delay times to deep upper-mantle (200–400 km depth) deformation, possibly associated with the dynamic interaction between the downgoing slab and the 410-km discontinuity or with the lithosphere delamination near the Taranaki-Ruapehu line. In contrast, the trench-parallel anisotropy from the local S waves in the northeast could be caused by fluid-bearing cracks in the crust of the Taupō Volcanic Zone and/or by trench-parallel fractures in the subducting slab resulting from outer rise bending. The abrupt change to trench-normal may be related to stress variations in the downgoing slab at different depths.

Vp/Vs structure and Pn anisotropy across the Louisville Ridge, seaward of the Tonga-Kermadec Trench

The Pacific Plate within the collision zone between the Louisville Ridge and the Tonga-Kermadec Trench was formed at the Osbourn Trough, a paleo spreading center that became inactive during the Cretaceous. In this region, the trench shallows from a depth of 8–11 km to ∼6 km below sea surface, while the outer rise topography is obscured by Louisville seamounts that rise 4–5 km above the adjacent seafloor. We derive 2-D P-wave (Vp) and S-wave (Vs) velocity-depth models along a wide-angle seismic profile oriented sub-parallel to the trench axis, intersecting the 27.6°S seamount. The seismic profile is located in the down-going Pacific Plate eastwards from the trench axis (∼100 km distant at the south end and ∼ 150 km at the north end), where bending-related faulting is limited or absent. Using the derived P- and S-wave velocity-depth models we calculate the corresponding Vp/Vs ratio model which shows values of 1.7–1.85 throughout the oceanic crust either side of the Louisville Ridge where it is unaffected by magmatism associated with its formation. This range of observations lies within those documented by laboratory measurements on basalt, diabase, and gabbro. Conversely, in the vicinity of the summit of 27.6°S seamount, the relatively elevated Vp/Vs (∼1.9) ratio observed can be attributed to water-saturated cracks within the shallow sub-seabed section of the intrusive core. Beneath the seamount the uppermost mantle has a Vp ranging from 8.0 to 8.9 km/s. Comparing our P-wave model with a pre-existing model running sub-perpendicularly along the Louisville Ridge axis, we observe an anisotropy of up to ∼6% at a depth of 3–4 km below the Moho. The predominant orientation of the faster axis follows the direction of paleo spreading flow when the plate was formed at the Osbourn Trough.

Patterns of Causative Faults of Normal Earthquakes in the Fluid‐Rich Outer Rise of Northeastern Japan, Constrained With 3D Teleseismic Waveform Modeling

AbstractAccurate earthquake source parameters are crucial for understanding plate tectonics, yet, it is difficult to determine these parameters precisely for offshore events, especially for outer‐rise earthquakes, as the limited availability of direct P or S wave data sets from land‐based seismic networks and the unsuitability of simplified 1D methods for the complex 3D structures of subducting systems. To overcome these challenges, we employ an efficient hybrid numerical simulation method to model these 3D structural effects on teleseismic P/SH and P‐coda waves and determine the reliable centroid locations and focal mechanisms of outer‐rise normal‐faulting earthquakes in northeastern Japan. Two M6+ events with reliable locations from ocean bottom seismic observations are utilized to calibrate the 3D velocity structure. Our findings indicate that 3D synthetic waveforms are sensitive to both event location, thanks to bathymetry and water reverberation effects, and the shallow portion of the lithospheric structure. With our preferred velocity model, which has Vs ∼16% lower than the global average, event locations are determined with uncertainties of <5 km for horizontal position and <1 km for depth. The refined event locations in a good match between one of the nodal strikes and the high‐resolution bathymetry, enabling the determination of the causative fault plane. Our results reveal that trench‐ward dipping normal faults are more active, with three parallel to the trench as expected, while five are associated with the abyssal hills. The significant velocity reduction in the uppermost lithosphere suggests abundant water migrating through active normal faults, enhancing both mineral alteration and pore density.

Brittle Deformation of Damaged Mafic and Ultramafic Rocks and Their Implications on Plate Bending

AbstractThe effect of damage on the brittle deformation of mafic and ultramafic rocks has been investigated by performing triaxial deformation experiments on thermally cracked and intact rock samples. The investigation was performed by recording the axial and lateral strains during deformation while simultaneously capturing the ultrasonic velocity, and electrical resistivity. While the peak strength is presumably controlled by the stiff intrinsic fractures, the crack opening mode also showed critical effects on the attained peak strength. The pore pressure distribution showed an apparent control over the dynamic Young's modulus as the ratio between the dynamic and static modulus of thermally cracked rocks is significantly higher than that of intact rocks. The compliant nature and the higher inelastic volumetric strain of the thermally cracked samples further indicated a possible explanation to the steep dipping plates and the taller topographic heights at the trench outer rise systems of old subduction zones.

Modeled Flooding by Tsunamis and a Storm Versus Observed Extent of Coral Erratics on Anegada, British Virgin Islands—Further Evidence for a Great Caribbean Earthquake Six Centuries Ago

AbstractModels of near‐field tsunamis and an extreme hurricane provide further evidence for a great precolonial earthquake along the Puerto Rico Trench. The models are benchmarked to brain‐coral boulders and cobbles on Anegada, 125 km south of the trench. The models are screened by their success in flooding the mapped sites of these erratics, which were emplaced some six centuries ago. Among 25 tsunami scenarios, 19 have megathrust sources and the rest posit normal faulting on the outer rise. The modeled storm, the most extreme of 15 hurricanes of category 5, produces tsunami‐like bores from surf beat. In the tsunami scenarios, simulated flow depth is 1 m or more at all the clast sites, and 2 m or more at nearly all, given either a megathrust rupture 255 km long with 7.5 m of dip slip and M8.45, or an outer‐rise rupture 130 km long with 11.4 m of dip slip and M8.17. By contrast, many coral clasts lie beyond the reach of simulated flooding from the extreme hurricane. The tsunami screening may underestimate earthquake size by neglecting trees and shrubs that likely impeded both the simulated flows and the observed clasts; and it may overestimate earthquake size by leaving coastal sand barriers intact. The screening results broadly agree with those from previously published tsunami simulations. In either successful scenario, the average recurrence interval spans thousands of years, and flooding on the nearest Caribbean shores begins within a half‐hour.

Stress State and Earthquake Triggering on the Outer Rise of the Southern Vanuatu Subduction Zone, Southern New Caledonia

ABSTRACT An analysis of earthquakes recorded in southern New Caledonia (SNC) over 14 months during 2018–2019 reveals focal mechanisms consistent with a normal-faulting stress state. The minimum principal stress is perpendicular to the Vanuatu subduction zone (VSZ), which is 200 km away, and is highly oblique to the local topographic ridge of New Caledonia, which may induce additional tension. An Mw 7.5 earthquake occurred in VSZ on 5 December 2018, and focal mechanisms appear to be different to those before the big earthquake. Significant increase in seismicity rates in both VSZ and SNC are observed following this large earthquake. A strong correlation between local and subduction zone seismicity rates is confirmed by analyses of seismic records before and after large subduction zone earthquakes 200–350 km away during the period of 2000–2018. The local seismicity rate and seismic hazard in SNC is about four times higher immediately after a large subduction earthquake, and Omori decay returns it to background levels after about 30 days. The triggering mechanisms remains unclear, but our study provides the first observations and a framework for future work.

Plate bending earthquakes and the strength distribution of the lithosphere

SUMMARY This study investigates the dynamics and constitutive behaviour of the oceanic lithosphere as it bends and yields during subduction. Two main observational constraints are considered: the maximum bending moment that can be supported by the lithosphere, and the inferred neutral plane depth in bending. We particularly focus on regions of old lithosphere where the ‘apparent’ neutral plane depth is about 30 km. We use subduction modelling approaches to investigate these flexural characteristics. We reassess bending moment estimates from a range of previous studies, and show a significant convergence towards what we call the ‘intermediate’ range of lithosphere strength: weaker than some classical models predict, but stronger than recent inferences at seamounts. We consider the non-uniqueness that arises due to the trade-offs in strength as well background (tectonic) stress state. We outline this problem with several end-member models, which differ in regard to relative strength in the brittle and ductile regimes. We evaluate the consistency of these models in terms of a range of constraints, primarily the seismic expression of the outer rise. We show that a 30 km neutral plane depth implies that net slab pull is not greater than about 2 TN m−1. In contrast, models with low brittle strength imply that regions with a 30 km neutral plane depth are under moderate net axial compression. Under these conditions, reverse faulting is predicted beneath the neutral plane at depths >30 km. We show that moderate variations in background stress have a large impact on the predicted anelastic dissipation. We suggest brittle reverse faulting is a marginal phenomenon which may be inhibited by moderate changes in background stress.

Fault geometry of M6-class normal-faulting earthquakes in the outer trench slope of Japan Trench from ocean bottom seismograph observations

Since the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the outer trench slope area from the trench to the outer rise along the Japan Trench. Concerns regarding large earthquakes and associated tsunamis have also arisen. Based on aftershock distributions, several outer trench slope normal-faulting earthquakes (hereinafter referred to as outer-rise earthquakes) are likely related to the rupture of multiple faults. However, few observations have clearly shown how multiple faults act during outer-rise earthquakes. During the ocean bottom seismograph (OBS) observations in the outer trench slope area of the central Japan Trench from September 2017 to July 2018, three M6-class normal-faulting earthquakes (Mw 6.2 on September 20, Mw 6.2 on October 06, and Mw 6.0 on November 12) occurred around the OBS network. The near-field OBS observations provided detailed information on hypocenter locations and focal mechanisms of the mainshocks and aftershocks, including immediately after the mainshocks. We investigated the fault configurations of normal-faulting earthquakes based on OBS observations. During the September 2017 earthquake, the mainshock ruptured high-angle normal faults with a dip angle of 65°. Off-fault aftershock activities that were not directly related to the mainshock rupture and could be explained by the stress changes caused by the mainshock were confirmed. However, hypocenter distributions and focal mechanisms of the main and aftershocks of the October and November 2017 earthquakes suggest that the mainshock ruptured multiple faults with various dipping directions, angles, and strike orientations. The complicated fault geometry should be considered a possible fault model for large outer-rise earthquakes and related tsunamis.

The effect of along-strike variable plate deflection on bending stress and seismicity at the southern Mariana Trench

Bending-induced faults are often recognized as sources of extensional or compressional earthquakes in the outer rise region, and have been extensively investigated in 2-D plate flexural models. However, such 2D models have difficulty in explaining observed earthquakes caused by reactivation of pre-exsiting fabrics inherited from mid-ridge, especially when the fabrics is oblique to the subduction. Here, we develop a 3-D flexural model to investigate the plate deflection at the southernmost Mariana subduction zone. The Particle Swarm Optimization (PSO) method was used to invert flexural parameters of the subducting plate. It shows that the boundary loading near the Challenger Deep is nearly twice that in the region further east. Under this deformation, the bending stresses within the outer rise vary significantly along the strike of the subduction zone. We find that the location of abrupt stress changes coincides with the outer rise earthquakes which were considered to be reactivation of remnant faults. We proposed a model to illustrate the influence of along-strike variation of plate deflection on bending stress and outer rise earthquakes: The variation of plate deflection along the trench may cause the change of the direction of the maximum extensional bending stress in the variable zone which provides greater possibilities for reactivation of preexisting weak faults, resulting in unevenly distributed outer rise earthquakes, reactivation of remnant fabric or earthquake.

Elastic Properties of Thermally Treated Diabase and Peridotite: Implications Toward the Elastic Properties of Oceanic Lithosphere

AbstractSupported by laboratory experimental results, the reduction in elastic thickness () and flexural rigidity () at trench outer rise, which enables bending, is attributed to damage. A series of laboratory experiments were performed on intact and thermally treated diabase and peridotite (400°C–800°C) under increasing confining pressures (3–200 MPa) at room temperature. Simultaneous porosity and elastic wave velocity (P and S) measurements were obtained under dry and fluid saturated conditions. The velocity and porosity inversions indicated a significant variation in crack properties (crack density, aspect ratio) and reduction in Young's modulus, of thermally treated samples in comparison to the intact samples. Hence the experimental results suggested weakening in elastic properties of a damaged oceanic lithosphere. Subsequently created lithospheric strength profiles showed a strength reduction in wet models in comparison to dry models. Under the assumption of different cooling models, the aging of the plate resulted in an increase in strength with deeper neutral zones. The outcome of the strength profiles was utilized to estimate and , which are also dependent on the elasticity of the lithosphere. The results showed a reduction in with increasing damage and suggest that the bending at the subduction zone is facilitated by weakened elastic properties of a damaged lithosphere. While most of the geophysical observations of were able to be constrained by the proposed damaged models, some shallow observations indicated enhanced damage triggered either by increased depth to brittle‐ductile transition or by large bending related faulting events.

Along-Strike Variation of Seismicity Near the Extinct Mid-Ocean Ridge Subducted Beneath the Manila Trench

AbstractThe change in seismic activity is explored using data recorded by ocean-bottom seismometers (OBSs) and permanent seismic stations near the extinct Mid-Ocean ridge of the South China Sea (SCS) and the Manila trench. We apply the machine learning–based algorithm EQTransformer to the OBS dataset for seismic event detection and phase picking and then evaluate the precision and compare the time residuals between automatic and manual picks. We derive a catalog of earthquakes in the region and find bending-fault earthquakes in the outer rise at the northern of the Huangyan (Scarborough) Seamount chain, where no historical seismicity was reported in the routine catalog. Abundant outer-rise earthquakes occurred on both sides of the Huangyan (Scarborough) Seamounts chain, but the focal depths vary along the trench. The Wadati–Benioff zone of the eastward subducted SCS oceanic lithosphere can be clearly identified. The focal depths are down to ∼100 km near Luzon island at ∼16° N but deepen southward to a depth of ∼180 km at ∼14° N. Dips of the slab also steepen from north to south, indicating along-strike changes in the geometry of the Manila megathrust.

Enhanced Regional Earthquake Catalog with Alaska Amphibious Community Seismic Experiment Data

Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) comprised 75 ocean-bottom seismometers and 30 land stations and covered about 650 km along the segment of the subduction zone that includes Kodiak Island, the Alaska Peninsula and the Shumagin Islands between May 2018 and September 2019. This unprecedented onshore-offshore dataset provided an opportunity to compile a greatly enhanced earthquake catalog for the region by both increasing the number of detected earthquakes and improving the accuracy of their source parameters. We use all available regional and AACSE campaign seismic data to compile an earthquake catalog for the region between Kodiak and the Shumagin Islands including the Alaska Peninsula (51° N–59° N, 148° W–163° W). We apply the same processing and reporting standards to additional picks and events as the Alaska Earthquake Center currently uses for compilation of the authoritative regional earthquake catalog. Over 7200 events (both newly detected and previously reported) have been processed with AACSE data. We added about 30% more events, 60% more phase picks, lowered the magnitude of completeness by about 0.2 on average across the region, and improved location errors. All data have been published in public data archives. In addition, we test the machine-learning earthquake detection and picking algorithm EarthquakeTransformer (EQT) on the AACSE seismic dataset, comparing EQT-determined P and S picks with the new catalog. EQT is entirely trained on land data, whereas AACSE is amphibious. Overall, EQT finds 59% of P and 63% of S arrivals in the catalog within 300 km epicentral distance. The percent of catalog picks detected by EQT varies inversely with earthquake epicentral distance, and EQT performs particularly poorly on data from earthquakes recorded by instruments in the outer rise.

Tectonic deformation at the outer rise of subduction zones

SUMMARY Fluids associated with subducting slabs play a crucial role in regulating the dynamics of water discharge, subsequent arc magmatism and intermediate-depth earthquakes in subduction zones. The incoming slab mantle hydration is primarily determined by deep normal faulting due to plate bending at the trench. However, the controlling factors on the outer rise faulting pattern, and the correlation between the inherited outer rise deformation and the intermediate-depth earthquakes, remain to be understood. Here we present high-resolution viscoelasto-plastic numerical models of free subduction for slab bending-related faulting prior to subduction. Our model results show that plastic weakening and friction coefficient of the slab mantle exhibit a significant impact on fault pattern, while plate age and elasticity have a minimal bearing for mature slabs. The brittle bending faults result in large positive pressure gradients in the vertical direction, facilitating seawater infiltrating into the subducting slabs, which corroborates previous numerical models. The faults reaching 15–30 km beneath the Moho coincide with the width of the double seismic zone in subduction zones. We anticipate that water pumped into the slab mantle along the faults, with decreasing water content along the depth, can explain the relatively sporadic lower plane earthquakes.

Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

AbstractThe hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H2O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H2O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths.

Fine Structure of the Subducting Slab and the 2022 M 7.4 Fukushima–Oki Intraslab Earthquake

Abstract The 16 March 2022 M 7.4 Fukushima–Oki earthquake is the largest one among forearc intraslab earthquakes in Japan since 2000. These subcoast events can cause severe damage to the local society because of their proximity to inhabited areas. However, their generating mechanism is still not clear. Here, we present 3D high-resolution seismic tomography of the source zone of four large intraslab events (M ≥7.0) during 2003–2022 in northeast Japan, which is obtained by inverting high-quality arrival-time data recorded at both onshore and offshore seismic stations. Aftershocks of the subcoast intraslab earthquakes are mainly distributed in gaps of high-velocity bodies with high Poisson’s ratio and at the upper ∼20 km depth of the subducting Pacific slab. Our results indicate that the four large intraslab events were caused by rupturing of buried hydrated faults that formed at the outer rise and dehydration embrittlement on the fault planes.

Correction: Seismic velocity structure and its implications for oceanic mantle hydration in the trench–outer rise of the Japan Trench

Correction: Seismic velocity structure and its implications for oceanic mantle hydration in the trench–outer rise of the Japan Trench

The Role of Crustal Accretion Variations in Determining Slab Hydration at an Atlantic Subduction Zone

AbstractWe present a 2D P‐wave velocity model from the outer rise region of the Lesser Antilles island arc, the first wide‐angle seismic study of outer rise processes at an Atlantic subduction zone. The survey consists of 46 OBS receivers over a 174 km profile with velocities resolved to 15 km below top basement. The final velocity model, produced through tomographic inversion, shows a clear decrease in the velocity of the lower crust and upper mantle of the incoming plate as it approaches the trench. We attribute this drop to outer rise bend‐related hydration, similar to Pacific cases, but superimposed on spatial variations in hydration generated at the slow‐spreading ridge axis. In thin, tectonically controlled crust formed under magma‐poor spreading conditions the superposition of these sources of hydration results in compressional velocities as low as 6.5 km s−1 beneath the PmP reflector. In contrast, segments of crust interpreted as having formed under magma‐rich conditions show velocity reductions and inferred hydrous alteration more like that observed in the Pacific. Hence, variations in the style of crustal accretion, which is observed on 50–100 km length scales both along and across isochrons, is a primary control over the distribution of water within the slab at Atlantic subduction systems. This heterogeneous pattern of water storage within the slab is likely further complicated by along strike variations in outer rise bending, subducting fracture zones and deformation at segment ends and may have important implications for our understanding of long‐term patterns of hazard at Atlantic subduction systems.

Imbalanced Moment Release Within Subducting Plates During Initial Bending and Unbending

AbstractInternal deformation within the downgoing plate in subduction zones to accommodate the bending of the plate as it starts to subduct is reflected in widespread intraplate seismicity. This seismicity, extending from the outer rise and outer trench slope down to intermediate depths within the slab, exhibits a wide variety of focal mechanism types, often indicative of the accumulation and recovery of down‐dip curvature through the combination of normal‐faulting and thrust‐faulting earthquakes. In the idealized case, where the bulk internal deformation is recovered and slabs descend as a straight plate into the deeper mantle, we might expect the seismic moment released in both extension and compression to balance. However, a number of factors may complicate this: the thermal, compositional, and rheological evolution of the slab as it subducts, changes in the proportion of deformation accommodated seismically, and whether the slab undergoes any permanent deformation. These factors may result in intraslab deformation and seismicity unrelated to the slab curvature. Here, we assess earthquake moment release in intraslab settings around the world, focusing on those subduction systems with relatively simple slab geometries. Consideration of several regions around the western Pacific and eastern Indian Ocean indicates that substantially more deformation is accommodated seismically during bending than during unbending, and that in both settings, significantly more moment release reflects down‐dip extension than down‐dip compression. This suggests that, although the location of seismicity is clearly related to changes in slab curvature, there is a component of permanent, unrecovered down‐dip extension in many subducting slabs.