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The Lithosphere-Asthenosphere Boundary (LAB) beneath oceanic plates is generally imaged as a sharp seismic velocity reduction, suggesting the presence of partial melts. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. Recent geophysical studies suggest its persistence with down-going old and cold slabs, but whether or not it is commonly present remains unclear, especially for young and warm slabs such as in the Cascadia subduction zone. Here we provide evidence for its presence at Cascadia in the form of a large (9.8 ± 1.5 % ) decrease in shear-wave velocity over a very small (<3 km) depth interval. Similarly large and sharp seismic velocity reduction at the bottom of both old and young slabs, as well as along the base of oceanic plates before subduction, possibly represents widespread presence of melts. The melt-rich sub-slab LAB may strongly influence subduction dynamics and viscoelastic earthquake cycles. This study provides seismic evidence for the presence of partial melts along the base of Cascadia's subducting slab, with implications to lithosphere-asthenosphere decoupling that potentially influences subduction dynamics and earthquake cycles. [ABSTRACT FROM AUTHOR]

At the northern Cascadia subduction zone, the subducting Explorer and Juan de Fuca plates interact across a transform deformation zone, known as the Nootka fault zone (NFZ). This study continues the Seafloor Earthquake Array Japan Canada Cascadia Experiment to a second phase (SeaJade II) consisting of nine months of recording of earthquakes using ocean-bottom and land-based seismometers. In addition to mapping the distribution of seismicity, including an MW 6.4 earthquake and aftershocks along the previously unknown Nootka Sequence Fault, we also conducted seismic tomography, which delineates the geometry of the shallow subducting Explorer plate (ExP). We derived hundreds of high-quality focal mechanism solutions from the SeaJade II data. The mechanisms manifest a complex regional tectonic state, with normal faulting of the ExP west of the NFZ, left-lateral strike-slip behaviour of the NFZ, and reverse faulting within the overriding plate above the subducting Juan de Fuca plate. Using data from the combined SeaJade I and II catalogs, we have performed double-difference hypocentre relocations and found seismicity lineations to the southeast of, and oriented 18° clockwise from, the subducted NFZ, which we interpret to represent less active small faults off the primary faults of the NFZ. These lineations are not optimally oriented for shear failure in the regional stress field, which we inferred from averaged focal mechanism solutions, and may represent paleo-configurations of the NFZ. Further, active faults interpreted from seismicity lineations within the subducted plate, including the Nootka Sequence Fault, may have originated as conjugate faults within the paleo-NFZ. [ABSTRACT FROM AUTHOR]

Viscoelastic dislocation theory is important to understanding fundamental geodynamics and validating numerical models in the study of earthquake deformation. Available mathematical solutions differ in assumed Earth geometry and formulation of gravity terms, but the main challenge they commonly face is Inverse Laplace Transform (ILT). Limitations in previously used ILT methods tend to degrade the performance and/or accuracy of the solutions. To overcome these limitations, we have derived new Green's functions for a layered spherical Earth based on the Fixed‐Talbot ILT method. The new solution uses a precise density‐gravity relation, includes pre‐stress advection, buoyancy, and self‐gravitation, invokes the bi‐viscous Burgers rheology, and yields deformational and gravitational changes both at the surface and in the interior of the Earth. The Fixed‐Talbot ILT method, which involves a left‐open parabolic integration path in the complex plane, greatly improves computing efficiency and allows convenient use of a large number of Earth layers. Comparison with other existing solutions demonstrates the excellent performance of the new solution. Taking the postseismic deformation of the 2004 Sumatra Mw 9.2 earthquake as an example, we illustrate how to integrate numerically our Green's functions to represent realistic slip distribution over a three‐dimensionally curved fault. The combination of computational efficiency, mathematical completeness in modeling gravitational effects, adaptability to any radial structure of the Earth, and ability to provide results at any depth including the surface enables a powerful tool for forward and inverse modeling of viscoelastic earthquake cycles in a self‐gravitating spherical Earth. Plain Language Summary: Because of Earth's viscoelastic behavior, earthquake‐induced deformational and gravitational changes evolve with time, providing important geodetic signals for the study of earthquake processes. Precise mathematical solutions to describe these changes can be obtained for laterally uniform Earth models, but the efficiency, adaptability, and/or accuracy of the solutions are often hindered by limitations in a key step of deriving these solutions: the Inverse Laplace Transform (ILT). In this work, we derive a new solution by using an advanced method, termed the Fixed‐Talbot method, for the ILT. Our solutions differ from most other available solutions also in the precise formulation of the density‐gravity relation and full inclusion of all the gravity effects. By numerically comparing with other existing solutions, we demonstrate the excellent performance and advantages of the new solution. Using the postseismic deformation of the 2004 Sumatra Mw 9.2 earthquake as an example, we illustrate how to handle arbitrary slip distributions on a three‐dimensionally curved subduction megathrust by numerically integrating the point‐source solution. The new solution provides a powerful and convenient tool for modeling earthquake cycles. Key Points: New solution for deformation and gravitation response of self‐gravitating, viscoelastic, and layered spherical Earth to shear dislocationFixed‐Talbot method for inverse Laplace transform improves efficiency, adaptability, and accuracy of earthquake deformation modelingExamples provided to demonstrate solution performance for complex Earth stratification and distributed slip on 3D‐curved megathrust fault [ABSTRACT FROM AUTHOR]

Following great subduction earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Understanding both the horizontal and vertical components in the same geodynamic framework presents challenges. Here, by modeling short‐term (a few years) postseismic viscoelastic relaxation (VER) and afterslip following synthetic and real subduction earthquakes, we demonstrate that the complexity of the vertical deformation can be explained in simple terms. Along a margin‐normal profile, VER results in an up‐down‐up trisegment, long‐wavelength pattern common to most megathrust earthquakes, including near‐trench uplift, midway subsidence, and near‐arc uplift, with locations controlled by coseismic fault slip. The magnitude of the first two segments is controlled mainly by oceanic mantle viscosity, and the third by mantle wedge viscosity. In contrast with VER, afterslip results in an up‐down bimodal pattern of variable wavelengths specific to individual earthquakes. Its site‐specific and heterogeneous nature is primarily responsible for the complexity in vertical deformation, but its effect can be adequately modeled using a simple elastic model. If the coast is near the megathrust rupture zone, variable combinations of the VER and afterslip effects lead to either uplift or subsidence. If the coast is in the near‐arc segment of VER deformation, uplift usually occurs. Modeling the common VER process enables the identification of site‐specific afterslip, which helps to understand the mechanism of afterslip in the context of the broad spectrum of fault slip behavior. Our results also have important implications to deciphering coastal paleoseismic records to constrain coseismic versus postseismic deformation of ancient earthquakes. Plain Language Summary: How the Earth surface deforms after great subduction earthquakes yields important information on the physical properties of the mantle and the megathrust fault. A few years after these earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Our work is focused on understanding this complexity. The postseismic deformation consists of two components. One is due to the gradual response of the mantle, called viscoelastic relaxation, and the other is due to the continuing slip of the megathrust around the rupture zone, called afterslip. We show that the two components have different spatiotemporal characteristics governed by different parameters of the subduction zone system. The combined effects of the two components at different distances from the earthquake rupture zone explain the observed complexity. Understanding the mechanisms of postseismic deformation helps us to understand mantle and fault properties and to properly interpret geological data that constrain deformation due to ancient earthquakes. Key Points: Viscoelastic relaxation (VER) after great subduction earthquakes generates a common up‐down‐up postseismic deformation pattern from trench to arcSite‐specific, heterogenous afterslip causes complexity of coastal vertical postseismic deformation, depending on coast‐rupture distanceSeparately modeling VER and afterslip helps to understand rheology, fault slip behavior, and paleoseismic records [ABSTRACT FROM AUTHOR]

A smooth subducting seafloor usually leads to a smooth megathrust, which promotes great earthquakes. We present a case study at the northern Manila Trench to emphasize that the process depends also on sediment accretion and deformation. Here, a very rugged igneous basement is hidden beneath a smooth sediment cover on the incoming seafloor. Seismic imaging indicates that most of the sediment is scraped off at the trench, resulting in an uneven décollement interrupted by partially exposed subducted seamounts. New thermal models confirm the presence of large frictional heating, also consistent with a rough and strong megathrust. Our findings in conjunction with the low degree of megathrust locking reported in the literature are compatible with the notion of strong creep. Consequent implications to regional seismic and tsunami hazards call for further investigations, especially with new seafloor geodetic monitoring and improved paleoseismic studies. Plain Language Summary: Great earthquakes, usually accompanied with large tsunamis, tend to occur in subduction zones where the megathrust fault is smooth. In appraising the potential of such great earthquakes, it is customary to use the ruggedness of the incoming seafloor before subduction to represent the roughness of the megathrust. In this work, we conduct a case study at the northern Manila Trench subduction zone to demonstrate that a smooth incoming seafloor does not always lead to a smooth megathrust. If the seafloor smoothness owes to a thick sediment cover, a rugged basement hidden beneath the sediment can still lead to a rough megathrust if most of the sediment is scraped off when the seafloor is subducted. We show seismic imaging results that indicate sediment scraping‐off at the trench to expose basement relief such as seamounts. The lack of a very smooth megathrust suggests lower potential for great earthquakes, but an accurate assessment of regional seismic and tsunami hazards requires other independent observations that help to constrain strain accumulation across the trench and earthquake history. Key Points: Seismic imaging shows that rugged igneous basement hidden beneath smooth sediment on subducting seafloor can lead to rough megathrustThermal modeling constrained by heat flow observations indicates large frictional heating, consistent with a very rough megathrustPossibility of a strong‐creep megathrust raises scientific questions in assessing seismic and tsunami hazards at northern Manila Trench [ABSTRACT FROM AUTHOR]

Subduction megathrust ruptures that breach the trench, such as in the 2011 M = 9 Tohoku‐oki earthquake, can be very tsunamigenic. However, whether buried ruptures are intrinsically less tsunamigenic has not been fully investigated. Here, we conduct this investigation by studying the mechanics of seafloor deformation and the resultant tsunami runup. With a trench‐breaching rupture, deformation is dominated by the rigid‐body translation of the frontal upper plate, and seafloor uplift is enhanced by the horizontal motion of the sloping seafloor. With a buried rupture, the rigid‐body motion is reduced, but the shortening of the upper plate due to seaward slip termination causes elastic thickening to enhance tsunamigenic seafloor uplift. By combining a finite‐element deformation model and a shallow‐water equation tsunami model, we systematically test various subduction zone geometrical and slip parameters to study the trade‐off between rigid‐body translation and elastic thickening in causing seafloor uplift and tsunami runup. To isolate the effect of slip depth, we compare scenarios with the same peak slip and rupture width. We find that very shallow ruptures, including those breaching the trench, are generally less tsunamigenic than deeper ruptures. Given peak slip, tsunamigenic potential is maximized if the rupture is not too shallow or too deep but is buried to moderate depths. Our model tests using variable hypothetical rupture depths suggest that, with a slip magnitude as large as in the 2011 Tohoku‐oki earthquake, tsunami runups as high as the observed would occur even if the rupture were fully buried. Plain Language Summary: During subduction earthquakes, sudden seafloor deformation due to megathrust slip raises seawater to generate tsunamis. If the slip breaches the trench, seawater is raised by the seaward motion of the sloping seafloor. If the near‐trench part of fault refuses to slip, in a scenario called buried rupture, the seafloor will bulge to raise seawater. Here we combine computer models of seafloor deformation and tsunami propagation to study how the two types of rupture control tsunami runup. We find that, given slip size and location, their tsunamigenic potential is similar owing to a trade‐off mechanism. With buried rupture, tsunami generation is weakened by lessened motion of the sloping seafloor but strengthened by increased seafloor bulging. The effect is the opposite for a trench‐breaching rupture. We also find that if a rupture is buried to an optimal moderate depth, the bulging effect and tsunami size are maximized. These findings clarify that the key reason for a large tsunami is how large, not just how shallow, the megathrust slip is. This knowledge helps to understand why some recent megathrust earthquakes, including the 2011 M = 9 Tohoku‐oki earthquake, were so tsunamigenic. For tsunami hazard assessment, the tsunamigenic potential of buried ruptures should not be underrated. Key Points: Tsunamigenic seafloor uplift due to megathrust slip reflects a trade‐off between rigid‐body translation and elastic thickeningModerately buried slip is usually more tsunamigenic than trench‐breaching slip of the same size, because of enhanced elastic thickeningWhat determines tsunami size, such as in the 2011 Tohoku‐oki tsunami, is mainly how large, not just how shallow, the offshore slip is [ABSTRACT FROM AUTHOR]

Geophysical observations including surface heat flow data indicate the subducting slab becomes fully coupled to the overlying mantle wedge at ∼70–80 km depth. This maximum depth of decoupling (MDD) separates cool, stagnant forearc mantle from warmer, convecting mantle capable of generating arc magmas. Thermodynamic calculations demonstrate that talc is stable in H2O‐undersaturated parts of the mantle wedge where its stability is controlled by the pressure‐dependent, fluid‐absent reaction: talc + forsterite = antigorite + enstatite, which occurs at pressures ∼2–2.5 GPa (∼70–80 km depth) and temperatures <650°C. At shallower depths, H2O‐undersaturated portions of the basal mantle wedge contain talc, which experimental studies show dramatically weakens rocks. At greater depths, talc is restricted to silica‐rich portions of the mantle wedge. The common MDD in subduction zones may reflect the downdip transition from a talc‐present decoupled shear zone to a talc‐absent fully coupled interface along the base of the mantle wedge. Plain Language Summary: In many subduction zones, the relative strength of the plate interface compared to the overlying mantle changes dramatically at 70–80 km depth. At shallower levels, the interface acts as a weak shear zone decoupling the subducting plate from the overlying cool, stagnant mantle. At deeper levels, the strength of the shear zone increases such that subducting plate drives convection in the overlying mantle creating the high temperatures needed for arc magmatism. We propose the change in interface strength may be related to the stability of talc, a very weak mineral, in the forearc mantle. At shallower levels, talc is stable in a wide range of H2O‐undersaturated mantle compositions. At deeper levels, the stability field of talc is dramatically reduced and restricted to silica‐rich regions of the mantle. Key Points: Talc, a very weak mineral, is stable in H2O‐undersaturated ultramafic (mantle) rocks at P < ∼2–2.5 GPa (70–80 km depth) and T < ∼650°CAt pressures greater than ∼2–2.5 GPa, talc breaks down via the P‐dependent reaction: talc + forsterite = antigorite + enstatiteThe breakdown of talc may explain the change in subduction‐interface rheology at 70–80 km depth where full slab‐mantle coupling begins [ABSTRACT FROM AUTHOR]

The Cascadia subduction zone (CSZ) is an exceptional geologic environment for recording evidence of land-level changes, tsunamis, and ground motion that reveals at least 19 great megathrust earthquakes over the past 10 kyr. Such earthquakes are among the most impactful natural hazards on Earth, transcend national boundaries, and can have global impact.Reducing the societal impacts of future events in the US Pacific Northwest and coastal British Columbia, Canada, requires improved scientific understanding of megathrust earthquake rupture, recurrence, and corresponding hazards. Despite substantial knowledge gained from decades of research, large uncertainties remain about the characteristics and frequencies of past CSZ earthquakes. In this review, we summarize geological, geophysical, and instrumental evidence relevant to understanding megathrust earthquakes along the CSZ and associated uncertainties. We discuss how the evidence constrains various models of great megathrust earthquake recurrence in Cascadia and identify potential paths forward for the earthquake science community. Despite outstanding geologic records of past megathrust events, large uncertainty of the magnitude and frequency of CSZ earthquakes remains. This review outlines current knowledge and promising future directions to address outstanding questions on CSZ rupture characteristics and recurrence. Integration of diverse data sets with attention to the geologic processes that create different records has potential to lead to major progress. [ABSTRACT FROM AUTHOR]

Dense Global Navigation Satellite System (GNSS) observations enable the development of megathrust interseismic locking models for the southern Kurile subduction zone where many great earthquakes have occurred. Inversion of these data assuming uniform elastic Earth has yielded slip deficit rates that are unreasonably high and/or full locking depth that is unreasonably large. Using the finite element method, here we construct a new Kurile locking model that includes interseismic viscoelastic stress relaxation and non‐uniform compliance of the elastic upper plate. Inverting the same geodetic data using the new subduction zone model alleviates the previously seen unreasonable features in inferred megathrust locking state. In the new model, full locking extends to shallower depths than the downdip limit of some large megathrust earthquakes including the 2003 Mw 8.0 Tokachi‐oki earthquake, supporting the notion of the shrinking of the locked area before the earthquakes and/or propagation of seismic rupture into creeping areas as previously predicted by friction or dynamic rupture models. By modeling the effects of a few recent M 8 earthquakes, we show that postseismic transients of recent earthquakes, although second‐order, should be addressed in deriving megathrust locking models. The locking state near the trench cannot be resolved by the land‐based GNSS data regardless of the improved model rheology and structure, although independent observations, such as slow earthquakes, may be used to speculate on the near‐trench locking state in various part of the margin in the absence of seafloor geodetic observations. Key Points: Viscoelastic mantle relaxation and non‐uniform compliance of upper plate are of first order importance to interseismic deformationInvoking relaxation and non‐uniform compliance in geodetic inversion leads to shallower megathrust locking than in previous modelsImproved model structure and rheology do not improve resolution of near‐trench locking state, demonstrating need for seafloor geodesy [ABSTRACT FROM AUTHOR]

• A weak LAB beneath the slab affects deformation following megathrust earthquakes. • Stress relaxation in the low-viscosity LAB enhances landward motion beyond rupture. • Observed deformation can be explained by a thin LAB of low viscosity (<1 × 1017 Pa s). • The preferred LAB thickness of 10 km or less agrees with seismological inferences. • Mechanical effect of LAB with heterogeneous melt distribution deserves further study. The rheology of the seismically imaged Lithosphere-Asthenosphere Boundary (LAB) is an important new question in plate tectonics. If the LAB is associated with ponding of partial melts as inferred from many seismic studies, it should have a low viscosity that minimizes resistance to plate motion and subduction. Here, we propose that recently observed postseismic enhanced landward motion (ELM) in coastal areas outside the rupture zone of large subduction earthquakes provides the first geodetic evidence for a low-viscosity LAB beneath subducting oceanic lithosphere. Using three-dimensional viscoelastic finite element models, we demonstrate that a low-viscosity LAB readily leads to postseismic ELM over broad coastal areas far beyond the rupture zone. If the LAB thickness is assumed to be 10 km, key spatiotemporal characteristics of the observed ELM can be well explained by an LAB viscosity of 5 × 1016 Pa s, orders of magnitude lower than typical asthenospheric viscosities. A thicker LAB such as 20 km with a higher viscosity may also explain the postseismic ELM but is incompatible with existing, albeit limited, near-field vertical deformation observations. The small thickness and the low viscosity of the model LAB are consistent with inferences from seismological observations and support the notion of ponding melts. Our preliminary weak-LAB models also predict diagnostic deformation features that can be tested by future onshore and offshore geodetic observations. Whether a weak LAB is ubiquitously present before and after plate subduction and how its presence and spatial heterogeneity impact postseismic and plate-scale deformation are important research targets for future investigations. [ABSTRACT FROM AUTHOR]

What controls subduction megathrust seismogenesis downdip of the mantle wedge corner (MWC)? We propose that, in the region of the 2010 Mw = 8.8 Maule, Chile, earthquake, serpentine minerals derived from the base of the hydrated mantle wedge exert a dominant control. Based on modeling, we predict that the megathrust fault zone near the MWC contains abundant lizardite/chrysotile‐rich serpentinite that transforms to antigorite‐rich serpentinite at greater depths. From the MWC at 32–40 km depth to at least 55 km, the predominantly velocity‐strengthening megathrust accommodated dynamic propagation of the 2010 rupture but with small slip and negative stress drop. The downdip distribution of interplate aftershocks exhibits a gap around the MWC that can be explained by the velocity‐strengthening behavior of lizardite/chrysotile. Interspersed velocity‐weakening and dynamic weakening antigorite‐rich patches farther downdip may be responsible for increased abundance of aftershocks and possibly for some of the high‐frequency energy radiation during the 2010 rupture. Plain Language Summary: A subduction megathrust rupture may extend from the trench to beneath populated coastal area. To understand what controls the megathrust seismogenic behavior of its deeper part, we conduct a case study of the 2010 magnitude 8.8 Maule, Chile, earthquake and its aftershocks. With numerical thermal modeling, we find that the deep seisomogenic behavior may be controlled by serpentine materials derived from the base of the overlying hydrated mantle wedge. The deep part of the megathrust ruptured "passively" against increasing resistance (negative stress drop). Lower‐temperature serpentines lizardite and chrysotile that are known to facilitate aseismic slip may be responsible for an observed gap in aftershock distribution in the downdip direction. Higher‐temperature serpentine antigorite that is known to facilitate seismic slip may be responsible for increased aftershocks farther downdip and possibly radiation of high‐frequency energy during the 2010 Maule earthquake. Key Points: Seismogenic behavior of subduction megathrust may be affected by materials derived from the base of the serpentinized forearc mantle wedgeThermopetrologic modeling suggests abundance of lizardite and chrysotile around the mantle wedge corner but antigorite farther downdipSerpentine frictional behavior may explain distributions of stress drop and seismic energy radiation in 2010 rupture and aftershocks [ABSTRACT FROM AUTHOR]

In southern Kurile, interseismic contraction observed with Global Navigation Satellite Systems is faster in the volcanic arc than in the forearc. Here, we use a viscoelastic finite element model to investigate the physical process responsible for the localized deformation. On the basis of thermal and seismic observations that suggest the volcanic arc and backarc to be more compliant than a typical continental lithosphere, we assume a lower rigidity and/or smaller thickness for the arc and backarc. We find that the interseismic observations require the volcanic arc to be very compliant and the backarc to be moderately compliant. The trade‐off between lower rigidity and smaller thickness cannot be resolved by interseismic observations alone, but coseismic observations can help to set a lower limit for the rigidity. Our results indicate that properly accounting for the long‐wavelength lithospheric heterogeneity is important to accurately inferring megathrust locking state from geodetic observations. Plain Language Summary: When the megathrust of a subduction zone is locked, the neighboring crust is shortened like a spring to build up energy for future earthquakes. GPS monitoring of the rate of shortening helps us identify the locked zone. In Hokkaido, however, the rate of shortening is larger in the area hosting a chain of volcanoes, called the volcanic arc. To explain what amplifies the shortening rate, we use computer models to investigate the effect of the lateral variations in the properties of the crust. We find that the faster shortening occurs because the arc area, and to a lesser degree the area farther away from the trench, is more compliant, that is, it offers less resistance to the lateral compression due to megathrust locking. The greater compliance is consistent with what can be inferred from heat flow and seismic imaging in this area. Our results indicate that, in order to accurately infer megathrust locking from GPS observations, we must include the greater compliance in our model. Key Points: Interseismic GNSS velocities in southern Kurile suggest low rigidity and/or thin lithosphere in the arc and, to a lesser degree, backarcInterseismic data cannot resolve trade‐off between rigidity and thickness, but coseismic data provide additional constraints on rigidityAddressing the effect of compliant arc‐back area will greatly improve the estimate of megathrust locking state [ABSTRACT FROM AUTHOR]

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through. • Exhumed metamorphic oceanic rocks are derived from very young to very old slabs. • Exhumed slab rocks likely represent conditions not typical of normal subduction. • The rocks may not be directly comparable with models designed for normal subduction. • Shallow mantle wedge flow can make slab rocks hot at the beginning of subduction. • Warming of stalled slab can make slab rocks hot at the ending of subduction. [ABSTRACT FROM AUTHOR]

In a viscoelastic Earth, stresses slowly built up due to fault locking are relaxed concurrently during the entire interseismic period. This interseismic stress relaxation causes crustal deformation much farther away from the locked fault than can be explained using elastic models that neglect the relaxation. Here we develop a viscoelastic geodetic inversion model to address this problem at Cascadia. We invert ~500 horizontal velocity vectors based on continuous and campaign geodetic measurements over the past two decades. Ambiguities arising from long‐term rotation of upper‐plate crustal blocks are addressed by test‐correcting the geodetic velocities with two different block‐motion models. Fault back slip (i.e., slip deficit) Green's functions are derived using a Maxwell viscoelastic finite element model with realistic subduction zone structure and megathrust geometry. The preferred model features a narrow and shallow megathrust locked zone, consistent with earlier thermorheological reasoning. For an elastic model to fit the data to the same fidelity, megathrust locking has to extend to much greater depths. However, even with the viscoelastic model, the land‐based geodetic data still cannot resolve whether there is some creep (incomplete locking) in the shallowest part of the megathrust far offshore. Neither can the land data fully resolve along‐strike variations of the locking state. These ambiguities can be resolved only when adequate seafloor geodetic data are obtained. Key Points: Widespread interseismic deformation reflects viscoelastic relaxation but may be interpreted as deep locking by elastic modelsViscoelastic geodetic inversion models indicate megathrust locking at shallow depths at CascadiaRegardless of Earth rheology, near‐trench ambiguity of locking state can be resolved only by future seafloor observations [ABSTRACT FROM AUTHOR]

The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure‐temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction. Key Points: Exhumed rocks from subduction zones are on average warmer than models predictions for global subduction zonesArbitrary warming below the forearc makes the models conform with the rock record but conflicts with independent observationsThe discrepancy between the rock record and models is explained by preferential exhumation of oceanic crust under warm conditions [ABSTRACT FROM AUTHOR]

Because of a combination of new observational tools and a flurry of large megathrust earthquakes, tremendous progress has been made in recent years towards understanding the process of great subduction earthquakes at Cascadia and other subduction zones around the world. This review article attempts to clarify some of widely used geodynamic concepts and identify the most important scientific questions for future research related to megathrust behaviour. It is important to specify how the megathrust seismogenic zone has been defined when comparing data and models. Observations and concepts currently used to define the seismogenic zone include: (A) the stability transition in rate-and-state dependent friction; (B) the slip zone of large interplate earthquakes; (C) the distribution of small-medium earthquakes; and (D) the geodetically-determined zone of fault locking. Land-based geodetic measurements indicate that the Cascadia megathrust is locked to some extent, but the degree of locking is not well constrained. The near absence of detectable interplate seismicity, with the exception of a segment near 44.5°N and near the Mendocino Triple Junction, is presently interpreted to indicate full locking along most of Cascadia. Resolving the locking state requires seafloor geodetic measurements. The slip behaviour of the shallowest segment of the megathrust and its tsunamigenic potential are complex and variable. Structural studies combined with modeling have the potential to improve our understanding of the signature left in the structure by the slip history. For several reasons, but mostly because of interseismic viscoelastic stress relaxation, the downdip limit of megathrust locking cannot be reliably constrained by geodetic data. Independent information is needed on the composition and thermal state of fault zone materials. The spatial relationship between the seismogenic zone and the zone of Episodic Tremor and Slip (ETS) remains controversial. Observations from the Nankai subduction zone and the San Andreas Fault suggest that ETS does not mark a simple spatial transition from seismic to aseismic behaviour and that multiple transitions may be present because of petrological and rheological changes with depth. Coseismic rupture in the AD 1700 Cascadia earthquake has been shown to vary along strike, and it is important to investigate whether the position of boundaries between high slip and low slip are stationary with time (and therefore probably geologically controlled) and are reflected in current interseismic locking of the megathrust. [ABSTRACT FROM AUTHOR]

Abstract: Among the wide range of thermal, petrologic, hydrological, and structural factors that potentially affect subduction earthquakes, the roughness of the subducting seafloor is among the most important. By reviewing seismic and geodetic studies of megathrust locking/creeping state, we find that creeping is the predominant mode of subduction in areas of extremely rugged subducting seafloor such as the Kyushu margin, Manila Trench, northern Hikurangi, and southeastern Costa Rica. In Java and Mariana, megathrust creeping state is not yet constrained by geodetic observations, but the very rugged subducting seafloor and lack of large earthquakes also suggest aseismic creep. Large topographic features on otherwise relatively smooth subducting seafloor such as the Nazca Ridge off Peru, the Investigator Fracture Zone off Sumatra, and the Joban seamount chain in southern Japan Trench also cause creep and often stop the propagation of large ruptures. Similar to all other known giant earthquakes, the Tohoku earthquake of March 2011 occurred in an area of relatively smooth subducting seafloor. The Tohoku event also offers an example of subducting seamounts stopping rupture propagation. Very rugged subducting seafloor not only retards the process of shear localization, but also gives rise to heterogeneous stresses. In this situation, the fault zone creeps because of distributed deformation of fractured rocks, and the creep may take place as transient events of various spatial and temporal scales accompanied with small and medium-size earthquakes. This process cannot be described as stable or unstable friction along a single contact surface. The association of large earthquakes with relatively smooth subducting seafloor and creep with very rugged subducting seafloor calls for further investigation. Seafloor near-trench geodetic monitoring, high-resolution imaging of subduction fault structure, studies of exhumed ancient subduction zones, and laboratory studies of low-temperature creep will greatly improve our understanding of the seismogenic and creep processes and their hazard implications. [Copyright &y& Elsevier]

Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised. [ABSTRACT FROM AUTHOR]

Seamount subduction is a common process in subduction zone tectonics. Contradicting a widely held expectation that subducting seamounts generate large earthquakes, seamounts subduct largely aseismically, producing numerous small earthquakes. On rare occasions when they do produce relatively large events, the ruptures tend to be complex, suggesting multiple rupture patches or faults. We explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault. The complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures. [ABSTRACT FROM AUTHOR]

Vigorous fluid circulation maintained in newly subducted ocean crust significantly affects subduction zone temperatures on the Nankai margin, Japan. The shallow part of the igneous ocean crust is pervasively fractured and thus highly permeable, allowing vigorous hydro thermal circulation. This circulation has been recognized as an important control on the thermal budget and evolution of ocean crust worldwide. However, existing subduction zone thermal models either do not include hydrothermal circulation in ocean crust or assume that it abruptly stops upon subduction. Here we use a conductive proxy to incorporate the thermal effects of high Nusselt number fluid circulation in subducting crust into a subduction zone thermal model. Hydrothermal circulation reduces temperatures in the seismogenic zone of the Nankai margin plate boundary fault by ~20 °C at the updip limit of seismicity and ~100 °C at the downdip limit. With improved thermal models for subduction zones that include the effects of hydrothermal circulation in subducting crust, estimates of metamorphic reaction progress and interpretations of fault zone processes on various margins may need to be revisited. [ABSTRACT FROM AUTHOR]

Abstract: We inverted GPS velocities from 20 continuous and 53 campaign sites in the northern Cascadia subduction zone using a Bayesian inverse method to estimate the locking state of the plate interface. The results are consistent with previous estimates based on thermal arguments and forward modeling. They suggest that the completely locked segment of the plate interface is offshore and that the degree of locking gradually decreases landward. The very gradual transition from full locking to full slip approximates the effect of stress relaxation that is not included by the elastic model assumed for the inversion. [Copyright &y& Elsevier]

Shear-wave anisotropy in Earth's mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones. Knowledge of shear-wave anisotropy is important to understanding the structure and dynamics of the subduction zone mantle wedge. Here, the authors find unambiguous evidence that forearc anisotropy resides in the upper-plate crust, while weak anisotropy in the most seaward part of the mantle wedge indicates decoupling from the slab [ABSTRACT FROM AUTHOR]

The rupture-zone averaged static stress drop in the 2011 M=9 Tohoku-oki earthquake wasless than 5 MPa according to all published slip models, but it caused a reversal of the state ofstress in most of the offshore forearc, from predominantly compressive to predominantlyextensional. In this work, we demonstrate that this stress reversal has the followingimportant geodynamic implications. (1) The reversal unequivocally indicates avery weak subduction megathrust. In order to reproduce this reversal in a finiteelement model of force balance that quantifies the effects of gravity and megathrustfriction, the effective coefficient of friction of the megathrust has to be about 0.032. Alower value would make the forearc too extensional before the earthquake, and ahigher value would make it too compressive after the earthquake. (2) The very weakmegathrust results in very low differential stresses in the upper plate, consistent withpreviously reported sensitivity of the state of stress to small perturbations. Applying thedynamic Coulomb wedge model, we demonstrate that the inner wedge, and byinference the nearshore-onshore forearc, is in a stable state throughout subductionearthquake cycles, far from failure. (3) The outer wedge is normally in a stable state butmay reach an extensionally or compressively critical state during an earthquake,depending on the behavior of the shallow megathrust. Gravitational collapse of theouter wedge is prevented by a finite strength of the underlying shallow megathrust.Therefore, complete stress drop (i.e., effective coefficient of friction decreases tozero) is unlikely to have happened over the main part of the shallow megathrustexcept for limited local areas. (4) The presence of permanent deformation such asearthquakes and active faulting in the overall stable forearc can be explained bystructural and stress heterogeneities. We propose that for the most part, the upper platenear a subduction zone is much below its yield stress and is an elastic body, butpermanent deformation can locally occur in areas of low strength, high stress, orhigh fluid pressure. (5) The concept of dynamic Coulomb wedge (Wang and Hu,2006, JGR) needs to be expanded to address timescales much beyond subductionearthquake cycles. The strength of the megathrust varies over geological timescales,affecting the stability of the outer wedge and its response to earthquake rupture. [ABSTRACT FROM AUTHOR]

The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within ~30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest. [ABSTRACT FROM AUTHOR]

Large rupture of the shallowest portion of subduction thrust faults (megathrusts), such as during the 2011 moment magnitude (M w ) 9.0 Tohoku-oki earthquake, can generate the most devastating tsunamis. However, it remains unclear whether such trench-breaching rupture is typical of other subduction earthquakes. The main difficulty in answering this question is the common lack of near-trench geodetic monitoring in subduction zones worldwide. Seafloor and sub-seafloor fluid pressure measurements at two closely located borehole observatories in the Middle America trench have provided clear evidence for the absence of trench-breaching rupture during the 2012 M w 7.6 Costa Rica earthquake, and for the presence of substantial trench-breaching afterslip at slow rates after the rupture ( Davis et al., 2015 ). In this study, we compare postseismic seafloor pressure change at the trench with coastal Global Navigation Satellite System (GNSS) displacements. The same temporal characteristics of the deformation at the trench and coastal sites indicate that both offshore and onshore deformation were the consequence of afterslip that occurred over a wide spatial range updip of the rupture. By determining the co- and post-seismic slip distributions and inferring the associated shear stress changes on the megathrust, we show that the mechanical behaviour varies in the dip direction. The slip behaviour of the shallow megathrust at Costa Rica is consistent with conventional conceptual models, and contrasts with the behaviour of the shallowest megathrust during the Tohoku-oki event. [ABSTRACT FROM AUTHOR]

In order to investigate upper mantle and crustal anisotropy along the transpressive Queen Charlotte margin between the Pacific (PA) and North America (NA) plates, we conducted shear wave splitting analyses using 17 seismic stations in and around the island of Haida Gwaii, Canada. Despite the limited station coverage at present, our reconnaissance study does reveal a systematic pattern of mantle anisotropy in this region. Fast directions derived from teleseismic SKS -phase splitting are mostly margin-parallel (NNW–SSE) near the plate boundary but transition to predominantly E–W-trending farther away. We propose that the former is associated with the absolute motion of PA, and the latter reflects a transition from this direction to that of the absolute motion of NA. The broad width of the zone of transition from the PA to NA direction is probably caused by the very obliquely subducting PA slab that travels primarily in the margin-parallel direction. Anisotropy of Haida Gwaii based on local earthquakes features a fast direction that cannot be explained with regional stresses and is probably associated with local structural fabric within the overriding crust. Our preliminary shear wave splitting measurements and working hypotheses based on them will serve to guide more refined future studies to unravel details of the geometry and kinematics of the subducted PA slab, as well as the viscous coupling between the slab and upper mantle in other transpressive margins. [ABSTRACT FROM AUTHOR]

Abstract: Slow slip events, or “silent” earthquakes, may relieve a significant amount of stress at many subduction plate boundaries, both downdip of the limit of seismogenesis, and within the seismogenic zone itself in cases where seismic energy release accounts for only a fraction of the plate tectonic displacement rate (Schwartz and Rokosky, 2007). Slow slip has been identified in several instances downdip of the landward limit of the seismogenic zone and is often accompanied by seismic tremor or low-frequency earthquake activity along and above the plate interface (referred to as “episodic tremor and slip”, or ETS). Little is known, however, about the spatial distribution and history of slip between great earthquakes along the seismogenic thrust interface itself which lies mostly offshore. In this article we present formation pressure transients observed in two deep-sea boreholes near the toe of the subduction prism off Nicoya Peninsula, Costa Rica, which followed ETS events observed on shore by 1–2weeks. The signatures of the transients are consistent with local slip on the shallow part of the thrust interface, with the underthrusting plate experiencing relaxation and the outer prism experiencing contraction. The delay between the tremor activity and the pressure transients observed c. 100km seaward at the prism toe suggests either slow propagation across the seismogenic zone or delayed deformation at the outer part of the prism triggered by the slip beneath Nicoya. Such slip may serve generally to relieve stress at subduction zones, but also to increase stress in parts of the plate boundary where interseismic slip does not occur. [Copyright &y& Elsevier]

New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10−6 yr−1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr−1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes. [Copyright &y& Elsevier]

Abstract: With new data recovered in December 2011 and November 2012 from two Ocean Drilling Program (ODP) CORK (circulation obviation retrofit kit) borehole observatories in the toe of the Nankai subduction zone accretionary prism (Hole 808I) and in the subducting Philippine Sea plate (Hole 1173B) off Southwestern Japan, records of formation fluid pressure now span over 10yr. Over nearly this same period of time the Japanese terrestrial HiNet array has enabled detection of small earthquakes across the breadth of the accretionary prism. The records include several formation fluid pressure anomalies and concurrent local very-low-frequency earthquake (VLFE) swarms. In the subducting plate, pressure anomalies are most commonly slow negative steps that are inferred to reflect dilatational strain associated with slow slip on the subduction thrust fault in the areas of VLFE activity. In three instances, concurrent positive impulsive anomalies are observed in the prism; these are inferred to reflect contraction when slip reaches the location of Hole 808I. The spatial distribution of VLFEs suggests that slow slip may occur in patches that cumulatively span the seaward half of the subduction prism. Two anomalies occurred at times of the largest earthquakes in the region, the September 2004 Mw 7.7 off-Kii earthquake 220km to the northeast, and the March 2011 Mw 9.0 Tohoku earthquake roughly 900km to the northeast. In the subducting plate, the observed changes in pressure are abrupt and consistent in sign with the expected coseismic strain (contractional and dilatational, respectively). In the case of the Tohoku earthquake, the co-seismic pressure decrease was followed 12 days later by an additional but slow decrease, and by an impulsive but complex and long-lived increase in the accretionary prism Hole 808I. In this instance, co-seismic cross-strike stress may have triggered post-seismic slip and the deformation seen in the plate and prism. Given the great distance from the Tohoku epicenter and small change in stress estimated at the Nankai observatory sites (ca. 7kPa shear stress), triggering of local slip would require the outer subduction thrust and overlying prism in this region to be very fragile. To what degree this state might vary with time is not known, but tracking episodic slip in the seaward part of this and other subduction zones using formation-fluid-pressure and VLFE monitoring may provide valuable clues about the evolution of subduction faults through their thrust earthquake cycles. [Copyright &y& Elsevier]

Abstract: We have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones. The benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically. We propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure. A comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers. A minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model. [Copyright &y& Elsevier]

• Hypocenters within the subducted Explorer plate indicate slab deformation. • The oceanic slab is bending downward toward the northwest. • A complex sequence of focal mechanisms also indicates plate deformation. • Decreased seismic activity in the overriding plate indicates decoupling to the NW. • Deformation and decoupling could limit megathrust rupture propagation. At the northernmost extent of the Cascadia subduction zone, the Explorer plate subducts at approximately 2 cm/yr, less than half the rate of the Juan de Fuca plate to the south. The boundary between these two plates is known as the Nootka fault zone, which is one of the focuses of the Seafloor Earthquake Array Japan-Canada Cascadia Experiment (SeaJade). During this survey, an M W 6.4 earthquake occurred on 24 April 2014. This event and the subsequent aftershocks (referred to as the Nootka Sequence) reveal an approximately 40-km-long subducted fault within the Explorer Plate to the north of the Nootka fault zone. We infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the Nootka fault zone. The depth distribution and focal mechanisms of the aftershocks indicate significant margin-parallel deformation of the subducting plate. The subduction interface at the Nootka Sequence fault has been deflected downward to the northwest from a depth of approximately 15 – 25 km over a distance of 25 km. We propose two possible scenarios that are modified from previously suggested slab-tear model with induced margin-parallel mantle flow to explain the significant deformation of the young, warm subducting Explorer plate. To the northwest of this change in slab geometry, a lack of seismic activity above the plate interface indicates that the Explorer plate has partially decoupled from the overriding North America plate. We conclude that the geometric variation separating the southern Explorer plate from the north, along with decoupling and a possible intraslab tear, may be a significant combination to resist the propagation of a megathrust rupture across this boundary. [ABSTRACT FROM AUTHOR]

• Landward GNSS velocity in 1960 Mw 9 Chile earthquake area recently increased. • The only long-lived continuous GNSS site COYQ suggests that this change was rapid. • The rapid change cannot be explained by postseismic viscoelastic deformation alone. • The rapid change can be explained by a downdip widening of megathrust locked zone. After a great subduction earthquake, viscoelastic stress relaxation causes prolonged seaward motion of inland areas of the upper plate, as was observed around the turn of the century in the area of the 1960 Mw 9.5 Chile earthquake with Global Navigation Satellite System (GNSS) measurements. However, recent GNSS observations during 2010–2019 indicate a systematic decrease in the velocity of the seaward motion over a region covering the latitudinal range of the southern half of the 1960 rupture. Data from the only long-lived continuous site in this region (COYQ since 1997), situated over 200 km away from the trench, suggest that the decrease in the seaward velocity (or increase in the landward velocity) occurred within a few years prior to 2010. This rapid and regional change cannot be explained by viscoelastic relaxation. We thus propose that the change was caused by a relatively sudden downdip widening of the zone of locking along the megathrust. Using three-dimensional finite element modelling, we find that the observed velocity change cannot be otherwise explained, although the amount of the increase in locking cannot be uniquely determined because of trade-offs between, and uncertainties in, the various parameters involved. For example, the degree of the increase in locking is affected by the value of coseismic slip in 1960 in the southernmost part of the rupture zone. A postseismic deformation model with greater coseismic slip in accordance with the most recent coseismic slip model in the literature better fits COYQ data prior to 2005 and requires greater locking increase afterwards. A model with less coseismic slip requires less locking increase but an additional long-term slow slip event prior to 2005. The rapid surface velocity change and the inferred increase in megathrust locking several decades after a great earthquake present new challenges to the understanding of fault mechanics and subduction zone dynamics. [ABSTRACT FROM AUTHOR]

To accurately assess earthquake and tsunami hazards posed by the Cascadia Subduction Zone, it is critically important to know which area of the plate interface is locked and whether or not part of the energy is being released aseismically by slow creep on the fault. Deeper locking that extends further to the coast produces stronger shaking in population centers. Shallow locking, on the other hand, leads to bigger tsunamis. We will report on and discuss plans for a new amphibious Northern Cascadia Subduction Zone Observatory (NCSZO) that will leverage the existing NEPTUNE cabled seafloor observatory, which is operated by Ocean Networks Canada (ONC), and the onshore network of geodetic stations, which is operated by Natural Resources Canada (NRCan).To create a NCSZO we plan to (1) add a network of seven GPS-Acoustic (GPS-A) sites offshore Vancouver Island, (2) establish a Deformation Front Observatory, and (3) improve the existing onshore geodetic network. The GPS-A stations will provide the undisturbed motion of the Juan de Fuca (JdF) Plate (1), deformation of the JdF plate (2), deformation of the overriding plate (3-7) and a cabled laboratory to study the potential for continuous GPS-A measurements (6). The Deformation Front Observatory will be used to study transient slip events using seafloor pressure and tilt instruments and fluid flux meters.The majority of the offshore instruments will be installed in summer 2019. The station layout shown on the map (see poster) is a first draft. Finial locations that maximize scientific return and optimize logistics are currently under discussion. [ABSTRACT FROM AUTHOR]