Plate Interface In Subduction Zones Research Articles

Intraslab seismicity migration simultaneously with an interface slow slip event along the Ecuadorian subduction zone

Slow slip events (SSEs) are transient slip episodes taking place along the plate interface of subduction zones. They usually occur synchronously to tectonic tremors and/or seismic swarms, but the relationship between slip and seismicity remains unclear. Here, we study a well-instrumented seismic swarm-SSE sequence near a highly coupled segment along the Ecuadorian subduction zone. GPS data reveal that a one-week long SSE developed along the subduction interface at shallow depth (5–15 km) with an equivalent moment magnitude (Mw) of 6.3. During that period, a local temporary seismic network recorded >700 earthquakes with magnitudes ranging between 1.0 and 4.6. The distribution of seismicity illuminates a steep intraslab fault downdip of an oceanic relief, striking almost parallel to the trench. This seismicity migrates along strike of this active structure at a velocity of 10–15 km/day. This velocity is commonly observed in the case of slow slip propagation. Our observation suggests that the interplate SSE might have triggered a slow slip along the the intraslab active structure, which in turn drove the intraslab seismicity. In addition, at the northern part of the intraslab faults, a cluster of seismicity shows a radial expansion compatible with a fluid diffusion process. Overall, our study highlights the dynamical interaction between an interplate slow slip and an intraslab seismic swarm with the interplay of an intraslab aseismic slip propagation and fluid diffusion, both controlling the migration of seismicity.

Seismic Anisotropy of Mafic Blueschists: EBSD‐Based Constraints From the Exhumed Rock Record

AbstractSeismic anisotropy constitutes a useful tool for imaging the structure along the plate interface in subduction zones, but the seismic properties of mafic blueschists, a common rock type in subduction zones, remain poorly constrained. We applied the technique of electron backscatter diffraction (EBSD) based petrofabric analysis to calculate the seismic anisotropies of 14 naturally deformed mafic blueschists at dry, ambient conditions. The ductilely deformed blueschists were collected from terranes with inferred peak P‐T conditions applicable to subducting slabs at or near the plate interface in active subduction zones. Epidote blueschists display the greatest P wave anisotropy range (AVp ∼7%–20%), while lawsonite blueschist AVp ranges from ∼2% to 10%. S wave anisotropies generate shear wave splitting delay times up to ∼0.1 s over a thickness of 5 km. AVp magnitude increases with glaucophane abundance (from areal EBSD measurements), decreases with increasing epidote or lawsonite abundance, and is enhanced by glaucophane crystallographic preferred orientation (CPO) strength. Two‐phase rock recipe models provide further evidence of the primary role of glaucophane, epidote, and lawsonite in generating blueschist seismic anisotropy. The symmetry of P wave velocity patterns reflects the deformation‐induced CPO type in glaucophane—an effect previously observed for hornblende on amphibolite P wave anisotropy. The distinctive seismic properties that distinguish blueschist from other subduction zone rock types and the strong correlation between anisotropy magnitude/symmetry and glaucophane CPO suggest that seismic anisotropy may be a useful tool in mapping the extent and deformation of blueschists along the interface, and the blueschist‐eclogite transition in active subduction zones.

Pre‐Subduction Architecture Controls Coherent Underplating During Subduction and Exhumation (Nevado‐Filábride Complex, Southern Spain)

AbstractThe interplay between structural and metamorphic processes operating along the deep plate interface in subduction zones remains elusive as much of the geologic record is recycled into the mantle. In some cases, metamorphosed subducted rocks are underplated and exhumed to the surface, providing critical constraints on structural processes and the rheological evolution of subduction interfaces at convergent margins. One such exhumed high‐pressure/low‐temperature subduction complex is the Cenozoic Nevado‐Filábride Complex (NFC) in Southern Spain. This study presents new data from the NFC that elucidate the syn‐metamorphic deformation, stacking, and underplating of continental slivers along the subduction interface. The structurally lowest NFC dominantly comprises lithologically monotonous Paleozoic metamorphic basement rocks recorded by apatite U‐Pb ages and shows no evidence for large‐scale internal duplications suggesting it behaved as a coherent basement succession during subduction. In contrast, structurally higher levels of the NFC are characterized by the stacking of older‐on younger coherent slices and distinctly different metamorphic ages. These relationships document syn‐subduction structural repetitions and tectonic stacking of imbricate thin slivers (∼100s m) during subduction underplating. Structurally higher levels of the NFC exhibit both Eocene and Miocene metamorphic zircon rims and apatite ages, along with microstructures indicative of relatively higher temperature metamorphism. Large‐scale underplating and antiformal stacking of slivers in the subduction channel can provide buoyancy forces to underplate and assist exhumation. We demonstrate that the presubduction stratigraphic architecture is a key control on the style and timing of deformation and metamorphism, facilitating coherent subduction underplating.

B-value variations in the Central Chile seismic gap assessed by a Bayesian transdimensional approach

The b-value can be used to characterize the seismic activity for a given earthquake catalog and provide information on the stress level accumulated at active faults. Here we develop an algorithm to objectively estimate variations of b-value along one arbitrary dimension. To this end, we employ a Bayesian transdimensional approach where the seismic domains will be self-defined according to information in the seismic catalog. This makes it unnecessary to prescribe the location and extent of domains, as it is commonly done. We first show the algorithm’s robustness by performing regressions from synthetic catalogs, recovering the target models with great accuracy. We also apply the algorithm to a microseismicity catalog for the Central Chile region. This segment is considered a seismic gap where the last major earthquake with shallow slip was in 1730. Our results illuminate the downdip limit of the seismogenic zone and the transition to intraslab seismicity. In the along-strike direction, low b-value coincides with the extent of locked asperities, suggesting a high-stress loading at the Central Chile seismic gap. Our results indicate the reliability of the Bayesian transdimensional method for capturing robust b-value variations, allowing us to characterize the mechanical behavior on the plate interface of subduction zones.

Self-similar stochastic slip distributions on a non-planar fault for tsunami scenarios for megathrust earthquakes

Megathrust earthquakes that occur repeatedly along the plate interface of subduction zones can cause severe damage due to strong ground motion and the destructive tsunamis they can generate. We developed a set of scenario earthquakes to evaluate tsunami hazards and tsunami early warning systems for such devastating earthquakes. Although it is known that the slip distribution on a fault strongly affects the tsunami height distribution in near-field coastal areas, the slip distribution of future earthquakes cannot be exactly predicted. One way to resolve this difficulty is to create a set of scenario earthquakes in which a set of heterogeneous slip distributions on the source fault is stochastically generated based on a given slip probability density function (SPDF). The slip distributions generated in this manner differ from event to event, but their average over a large ensemble of models converges to a predefined SPDF resembling the long-term average of ruptures on the target fault zone. We created a set of SPDF-based scenario earthquakes for an expected future Mw 8.2 Tonankai earthquake in the eastern half of the Nankai trough, off southwest Japan, and computed the ensuing tsunamis. We found that the estimated peak coastal amplitudes among the ensemble of tsunamis along the near-field coast differed by factors of 3 to 9, and the earliest and latest arrivals at each observation site differed by 400 to 700 s. The variations in both peak tsunami amplitude and arrival time at each site were well approximated by a Gaussian distribution. For cases in which the slip distribution is unknown, the average and standard deviation of these scenario datasets can provide first approximations of forecast tsunami height and arrival time and their uncertainties, respectively. At most coastal observation sites, tsunamis modeled similarly but using a uniform slip distribution underpredicted tsunami amplitudes but gave earlier arrival times than those modeled with a heterogeneous slip distribution. Use of these earlier arrival times may be useful for providing conservative early warnings of tsunami arrivals. Therefore, tsunami computations for both heterogeneous and uniform slip distributions are important for tsunami disaster mitigation.Graphical abstract

Volatile (Li, B, F and Cl) mobility during amphibole breakdown in subduction zones

Amphiboles are ubiquitous minerals in the altered oceanic crust. During subduction, their breakdown is governed by continuous reactions up to eclogitic facies conditions. Amphiboles thus contribute to slab-derived fluid throughout prograde metamorphism and continuously record information about volatile exchanges occurring between the slab and the mantle wedge. However, the fate of volatile elements and especially halogens, such as F and Cl, in amphibole during subduction is poorly constrained. We studied metagabbros from three different localities in the Western Alps: the Chenaillet ophiolite, the Queyras Schistes Lustrés and the Monviso meta-ophiolitic complexes. These samples record different metamorphic conditions, from greenschist to eclogite facies, and have interacted with different lithologies (e.g. sedimentary rocks, serpentinites) from their formation at mid-oceanic ridge, up to their devolatilization during subduction. In the oceanic crust, the initial halogen budget is mostly stored in magmatic amphibole (F=300–7000ppm; Cl=20–1200ppm) or in amphibole corona (F=100–7000ppm; Cl=80–2000ppm) and titanite (F=200–1500ppm; Cl<200ppm) formed during hydrothermal seafloor alteration. It is thus the fate of these phases that govern the halogen fluxes between the crust and the overlying mantle and/or the plate interface in subduction zones. Li and B are poorly stored in the oceanic crust (<5ppm). In subduction zones, prograde metamorphism of metagabbros is first marked by the crystallization of glaucophane at the expense of magmatic and amphibole coronas. This episode is accompanied with a decrease of halogen concentrations in amphiboles (<200ppm of F and Cl) suggesting that these elements can be transferred to the mantle wedge by fluids. In the Queyras Schistes Lustrés complex, the intense deformation and the abundant devolatilization of metasedimentary rocks produce large fluid flows that promote rock chemical hybridization (metasomatic mixing with hybrid composition between metasedimentary rock and metagabbro) at the metasedimentary rock/metagabbro contacts. Such fluid/rock interactions result in a strong addition of Li in glaucophane (up to 600ppm) whereas halogen concentrations are unaffected. At eclogite facies conditions, metagabbros display low halogens concentrations (<20ppm of F and<100ppm of Cl) relative to altered oceanic crust (F=40–650ppm; Cl=40–1400ppm) suggesting that these elements are continuously released by fluids during the first 30–80km of subduction whatever the tectonic environment (e.g. slab, plate interface) and the considered fluid/rock interactions.

Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake

Research Article| May 01, 2013 Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake Jeremy E. Kozdon; Jeremy E. Kozdon Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305jekozdon@nps.edu *Now at Department of Applied Mathematics, Naval Postgraduate School, 833 Dyer Road, Monterey, California 93943. Search for other works by this author on: GSW Google Scholar Eric M. Dunham Eric M. Dunham Department of Geophysics and Institute for Computational and Mathematical Engineering, Stanford University, 397 Panama Mall, Stanford, California 94305edunham@stanford.edu Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2013) 103 (2B): 1275–1289. https://doi.org/10.1785/0120120136 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeremy E. Kozdon, Eric M. Dunham; Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1275–1289. doi: https://doi.org/10.1785/0120120136 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract There is strong evidence that the 11 March 2011 Tohoku earthquake rupture reached the seafloor. This is surprising because the shallow portion of the plate interface in subduction zones is thought to be frictionally stable, leading to the widely held view that coseismic rupture would stop several tens of kilometers downdip of the seafloor. Various explanations have been proposed to reconcile this seeming inconsistency, including dynamic weakening (e.g., thermal pressurization) and extreme stress release around shallow subducted seamounts. We offer a simpler explanation supported by 2D dynamic rupture simulations of the Tohoku earthquake. Our models account for depth‐dependent material properties and the complex geometry of the fault, seafloor, and material interfaces, based on seismic surveys of the Japan Trench. The fault obeys rate‐and‐state friction with standard logarithmic dependence of shear strength on slip velocity in steady state. In our preferred model, the uppermost section of the fault is velocity strengthening. Rupture nucleates on a deeper, velocity‐weakening section. Waves released by deep slip reflect off the seafloor, transmitting large stress changes to the upper section of the fault driving the rupture through the velocity‐strengthening region to the trench. We validate the model against seafloor deformation and 1‐Hz Global Positioning System (GPS) data. The seafloor displacements constrain the seismogenic depth and overall amount of slip, particularly near the trench. Our simulations reproduce many features in the GPS data, thereby providing insight into the rupture process and seismic wave field. Sensitivity to parameters is explored through an extensive suite of simulations. Neither static seafloor deformation nor onshore 1‐HzGPS data can uniquely determine near‐trench frictional properties due to trade‐offs with average stress drop. While conducted specifically for the Japan Trench region, our simulations suggest that rupture to the trench in megathrust events is quite possible, even if velocity‐strengthening properties extend tens of kilometers landward from the trench.Online Material: Mp4 movies of particle velocities. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effect of glacial-interglacial sea-level changes on the displacement and stress field in the forearc and along the plate interface of subduction zones

Abstract. Combined seismological, space-geodetic and numerical studies have shown that the seismicity at subduction zones may be modulated by tides and glacier fluctuations on timescales of 1–100 a, because these changes in loads on Earth's surface are able to alter the stress field in the upper plate and along the plate interface. Here we use a two-dimensional finite-element model of a subduction zone to investigate how glacial-interglacial sea-level changes affect the forearc region and the plate interface. The model results show that a sea-level fall by 125 m over 100 ka causes up to 0.7 m of vertical displacement, with the maximum uplift occurring between the trench and the coast. The uplift signal induced by the sea-level fall decreases to zero ~20 km landward of the coastline. A subsequent sea-level rise by 125 m over 20 ka causes subsidence, which is again most pronounced in the submarine part of the forearc. The sea-level changes cause horizontal displacements of up to 0.12 m, which are directed seaward during sea-level fall and landward during sea-level rise. With respect to the stress field, the sea-level changes lead to variations in the vertical stress and the shear stress of up to 1.23 MPa and 0.4 MPa, respectively. The shear stress variations are highest beneath the coast, i.e. in the area where the sea-level changes cause the strongest flexure. The resulting Coulomb stress changes on the plate interface are of the order of 0.2–0.5 MPa and indicate that earthquakes are promoted during sea-level fall and delayed during sea-level rise. Our findings imply that eustatic sea-level changes during glacial-interglacial periods may have induced displacements and stress changes that were large enough to affect the seismic cycle of subduction thrusts.

Inhibition of subduction thrust earthquakes by low-temperature plastic flow in serpentine

Serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults; however, recent frictional experiments have shown a marked increase in its shear strength with increasing depth, implying its deformation via unstable sliding at a plate interface in subduction zones. Here we report the results of a series of simple-shear experiments designed to determine the dominant deformation style of low-temperature serpentine species under P– T conditions that correspond to the mantle wedge corner in cool subduction zones (ca. P = 1 GPa and T = 200 °C). We found that with increasing shear strain, the dominant flow mechanism of the serpentinite evolves from (1) inhomogeneous semi-brittle flow by strain localization into sample-scale shear bands to (2) homogeneous ductile flow by intracrystalline deformation within individual serpentine grains, with the development of a planar shape fabric and a strong crystallographic-preferred orientation. TEM observations revealed that grain size reduction in lizardite crystals occurs due to (001) interlayer glide, represented by the occurrence of tiny sheet-like sectors oriented parallel to (001) liz lattice fringes, which evolved from large, elongated sectors with long axes oriented normal to (001) liz. Given that a low-viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains, the above results indicate that the presence of low-temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes, as stress is preferentially accommodated by plastic flow. This hypothesis explains the occurrence of an anomalous non-seismic region (devoid of large thrust earthquakes) in locally hydrated forearc mantle within northeast Japan.

Stress and deformation in subduction zones: insight from the record of exhumed metamorphic rocks

Abstract High pressure (HP) and ultrahigh (UHP) metamorphic rocks are exhumed from subduction zones at high rates on the order of plate velocity (cm/year). Their structural and microstructural record provides insight into conditions and physical state along the plate interface in subduction zones to depths of &gt;100 km. Amazingly, many identified (U)HP metamorphic rocks appear not to be significantly deformed at (U)HP conditions, despite their history within a high strain rate mega-shearzone. Other (U)HP metamorphic rocks seem to be deformed exclusively by dissolution-precipitation creep. Indications of deformation by dislocation creep are lacking, apart from omphacite in some eclogites. Available flow laws for dislocation creep (extrapolated to low natural strain rates, which is equivalent to no deformation on the time scales of subduction and exhumation, i.e., 1 to 10 Ma) pose an upper bound to the magnitude of stress as a function of temperature along the trajectory followed by the rock. Although the record of exhumed (U)HP metamorphic rocks may only be representative of specific types or evolutionary stages of subduction zones, for such cases it implies: (1) strongly localized deformation; (2) predominance of dissolution-precipitation creep and fluid-assisted granular flow in the shear zones, suggesting Newtonian behaviour; (3) low magnitude of differential stress; which (4) is on the order of the stress drop inferred for earthquakes; and (5) negligible shear heating. These findings are easily reconciled with exhumation by forced flow in a low viscosity subduction channel prior to collision, implying effective decoupling between the plates.