Subducting Plate Interface Research Articles

Subduction plate interface shear stress associated with rapid subduction at deep slow earthquake depths: example from the Sanbagawa belt, southwestern Japan

Abstract. Maximum shear stress along an active deformation zone marking the subduction plate interface is important for understanding earthquake phenomena and is an important input parameter in subduction zone thermomechanical modeling. However, such maximum shear stress is difficult to measure directly at depths more than a few kilometers and is generally estimated by simulation using a range of input parameters with large associated uncertainties. In addition, estimated values generally represent maximum shear stress conditions over short observation timescales, which may not be directly applicable to long-timescale subduction zone modeling. Rocks originally located deep in subduction zones can record information about deformation processes, including maximum shear stress conditions, occurring in regions that cannot be directly accessed. The estimated maximum shear stress is likely to be representative of maximum shear stress experienced over geological timescales and be suitable to use in subduction zone modeling over timescales of millions to tens of millions of years. In this study, we estimated maximum shear stress along a subduction plate interface by using samples from the Sanbagawa metamorphic belt of southwestern (SW) Japan, in which slivers of mantle-wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the zone of active deformation along the subduction plate interface. To obtain estimates of maximum shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks – quartz is the main component of the rocks we collected and its deformation stress is assumed to be roughly representative of the stress experienced by the surrounding rock and plate interface deformation zone. Maximum shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer) and the apparent grain size of dynamically recrystallized quartz in a thin section to an appropriate piezometer. Combined with information on sample deformation depth, estimated from the P–T (pressure–temperature) path and deformation temperatures, it is suggested that there was nearly constant maximum shear stress of 15–41 MPa in the depth range of about 15–30 km, assuming plane stress conditions even when uncertainties related to the measurement direction of thin section and piezometer differences are included. The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquake domain. Estimated maximum shear stress may not only be useful for long-timescale subduction zone modeling but also represent the initial conditions from which slow earthquakes in the same domain nucleated.

Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

AbstractThe architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation‐based kinematics, to delineate the wedge growth history with four décollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded décollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high‐friction patch in the basal décollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low‐angle, long‐lived forethrust and multiple cycles of frontal accretion on short‐lived forethrusts at the deformation front. Our findings suggest that décollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of ∼105 yr in natural accretionary systems. This result should be considered when investigating modern subduction zones.

Formation of the eclogites of the Atbashi complex, Kyrgyzstan, in a subduction zone mélange diapir

Much debate exists concerning mechanisms of crustal material transfer from subducting slab to overlying mantle. Formation of mélange rocks by physical mixing of slab components within subduction plate interface is predicted to transfer their compositional signal to source of arc magmas by ascending as diapirs from slab-top. Despite being supported conceptually and through modeling, existence of these diapirs in global subduction architecture remains inconclusive. Here we use petrological observations, thermometry and thermodynamic modeling, combined with geochemical constraints and compilation of massive existing data, to investigate eclogites from a deeply buried mélange “package” in Kyrgyz Tianshan, southern Altaids. We find that various slab components physically mixed to form eclogitic mélange rocks at threshold depth of the subarc (i.e., ≥85 km). Index mineralogical and Pressure-Temperature records indicate a thermal history with substantial heating after peak burial to condition crossing wet solidus. Such translation, toward hot corner of mantle wedge, is short-lived around several hundred thousand to few million years, serving as first tangible evidence substantiating mélange diapirs propagate and dynamically mix with overlying mantle. Contemporaneous Late Carboniferous flare-up of regional arc magmatism with mélange diapir melting signal also advocates that non-negligible process of mantle wedge hybridization by buoyant mélange materials, to transfer volatile, generate arc lavas and regulate terrestrial geochemical cycles, stands.

Substantial Upper Plate Faulting Above a Shallow Subduction Megathrust Earthquake: Mechanics and Implications of the Surface Faulting During the 2016 Kaikoura, New Zealand, Earthquake

AbstractThe 2016 moment magnitude 7.8 Kaikoura, New Zealand, earthquake occurred at the southern end of the Hikurangi subduction zone where the upper plate above the shallow megathrust is exposed sub‐aerially. As a result, the substantial co‐seismic deformation in the upper plate above the megathrust rupture was observed geologically and geodetically. We explore the relationship between this surface faulting and the subduction megathrust rupture and find that the greatest upper plate fault slip occurred coincident (in time and location) with the megathrust rupture. Models of Coulomb stress change demonstrate that these surface faults become positively loaded as the upper plate rebounds during the megathrust event, favoring fault slip. In addition, during the megathrust rupture these faults terminate against an uncoupled subduction plate interface. We simulate the effects of decoupling at the base of these faults and find that very large fault slip is an expected consequence of this decoupling, allowing near‐complete strain release. In contrast, typical strike‐slip faults, pinned at their base, would have lower amounts of fault slip. These two conditions—increased Coulomb stress and basal decoupling—combine to produce the extreme co‐seismic upper plate faulting observed above the shallow Kaikoura megathrust earthquake. Similar conditions occur in other global subduction zones, but in most subduction zones the region above the coupled megathrust is underwater and poorly observed. Our analysis of the Kaikoura earthquake indicates a need to reevaluate patterns of strain accumulation and release in these regions, rather than assuming simple models of elastic rebound.

Detection of Deep Low‐Frequency Tremors From Continuous Paper Records at a Station in Southwest Japan About 50 Years Ago Based on Convolutional Neural Network

AbstractSince deep low‐frequency tremors are considered to be associated with large earthquakes that occur adjacently on the same subducting plate interface, it is important to investigate tremors that occurred before the establishment of modern seismograph networks such as the High Sensitivity Seismograph Network (Hi‐net). We propose a deep‐learning solution to detect evidence of tremors in scanned images of paper seismogram records from over 50 years ago. In this study, we fine‐tuned a convolutional neural network (CNN) based on the Residual Network, which was pre‐trained using images of synthetic waveforms from our previous study, using a data set comprised of images generated from real seismic data recorded digitally by Hi‐net to facilitate a supervised analysis. The fine‐tuned CNN was able to predict the presence or absence of tremors in the Hi‐net images with an accuracy of 98.64%. Gradient‐weighted Class Activation Mapping heatmaps created to visualize model predictions indicated that the CNN's ability to detect tremors is not degraded by the presence of teleseisms. Once validated using the Hi‐net images, the CNN was applied to paper seismograms recorded from 1966 to 1977 at the Kumano observatory in southwest Japan, operated by Earthquake Research Institute, The University of Tokyo. The CNN showed potential for detecting tremors in scanned images of paper seismogram records from the past, facilitating downstream tasks such as the creation of new tremor catalogs. However, further training using an augmented data set to control for variables such as inconsistent plotting pen thickness is required to develop a universally applicable model.

A combination of tides and nontidal variations in ocean bottom pressure may generate interannual slip fluctuations in the transition zone along a subduction plate interface

The tidal triggering of earthquakes has been studied for many years. The discovery of slow earthquakes in the early 2000s, including slow slip, has urged scientists to investigate the tidal responses of these earthquakes due to their sensitivity to weak stress perturbations. Previous studies have shown that slow earthquakes correlate with diurnal and semidiurnal tides and seasonal variations in surface loads more clearly than ordinary earthquakes. However, little is known about long-term responses to external stresses. In this paper, based on a widely accepted frictional law for faults, a mechanism is proposed by which nontidal variations in ocean bottom pressure, when combined with tides, promote the occurrence of slow earthquakes. Because slow earthquakes accompany a slip on the plate interface, this mechanism allows one to estimate slip modulations. A one-degree-of-freedom slip model is constructed and applied to Ise Bay in the Tonankai region of southwestern Japan, where large-scale ocean mass redistributions have occurred. The model calculated with parameters determined from the observation of tectonic tremors is quantitatively consistent with the slip during 1997–2013 inferred from GNSS data, suggesting that the decrease in the sea-level change in approximately 2006 could cause the acceleration of a slip observed after that. This result implies that the decreases in sea level in approximately 1996 and 2014 could also cause subsequent slip accelerations. These three slip acceleration periods temporally coincide with the increases in background seismicity in a shallower portion of the plate interface. These changes in seismicity are common to shallow earthquakes in the Tokai area, and a similar model can reproduce them. Further studies are expected to reveal causality between shallow earthquakes and long-term slip fluctuations based on modeling that considers changes in the frictional property along the plate interface.

Subduction Zone Interface Structure Within the Southern MW9.2 1964 Great Alaska Earthquake Asperity: Constraints From Receiver Functions Across a Spatially Dense Node Array

AbstractWe conduct a high‐resolution teleseismic receiver function investigation of the subducting plate interface within the Alaskan forearc beneath Kodiak Island using data collected as part of the Alaska Amphibious Community Seismic Experiment in 2019. The Kodiak node array consisted of 398 nodal geophones deployed at ∼200–300 m spacing on northeastern Kodiak Island within the southern asperity of the 1964 Mw9.2 Great Alaska earthquake. Receiver function images at frequencies of 1.2 and 2.4 Hz show a coherent, slightly dipping velocity increase at ∼30–40 km depth consistent with the expected slab Moho. In contrast to studies within the northern asperity of the 1964 rupture, we find no evidence for a prominent low‐velocity layer above the slab Moho thick enough to be resolved by upgoing P‐to‐S conversions. These results support evidence from seismicity and geodetic strain suggesting that the 1964 rupture connected northern (Kenai) and southern (Kodiak) asperities with different plate interface properties.

An Experimental Study of Chlorite Stability in Varied Subduction Zone Lithologies with Implications for Fluid Production, Melting, and Diapirism in Chlorite-Rich Mélange Rocks

AbstractFour ultramafic bulk compositions comprising only natural minerals were used to constrain the stability field of chlorite in a variety of subducted, chlorite-rich rocks through an examination of key chlorite dehydration reactions relevant to the sub-arc. Seventy-four piston cylinder experiments were conducted at a range of pressures (1.0–5.0 GPa) and temperatures (500°C–1150°C).Bulk 1 represents a chlorite mélange (Mg# = 0.94) typically formed in the subduction channel. This composition was used to examine the terminal chlorite reactions to olivine, orthopyroxene, and spinel at low pressure and to olivine, garnet, and spinel at high pressure. Chlorite achieves a thermal maximum stability at 2.0 GPa, 850°C; at 3.0 GPa, 850°C; and at 5.0 GPa, 760°C. The terminal chlorite breakdown reaction rises at a much steeper Clapeyron slope than shown in previous studies. Bulk 2 contains additionally antigorite and tremolite, to constrain phase relations in more fertile compositions. Chlorite reacts with clinopyroxene at ~100°C lower temperatures and with orthopyroxene at ~20°C–60°C lower temperatures than the terminal chlorite breakdown. The reactions have a subparallel Clapeyron slope and none of the three chlorite dehydration reactions crosses the antigorite breakdown reaction up to 5 GPa. This demonstrates that chlorite is the most stable carrier of H2O to high temperatures in subducted ultramafic rocks. Chlorite mélanges that form at the subduction plate interface will dehydrate at 850°C–800°C, 80–120 km depth for intermediate to hot subduction geotherms and liberate 10–12 wt.% of H2O, triggering wet melting in associated sediments. For cold subduction geotherms, chlorite dehydration occurs at 780°C–740°C, 120–170 km depth. Interaction of such fluids with sediments will likely produce a supercritical fluid phase. No melting in the ultramafic rocks has been observed at the chlorite breakdown reactions. Wet melting of the chlorite mélange at 3 GPa occurred between 1100°C and 1150°C.The stability of chlorite in more Fe-rich mélanges (bulk Mg# = 0.50 and 0.68, respectively) were conducted at 3.0 GPa and revealed thermal maxima at 650°C and 765°C, respectively. Collectively, the thermal stability of chlorite is dependent upon the Mg# of the bulk composition and spans over 200°C at sub-arc depths. The density of run products was calculated to test the validity of the chlorite mélange diapir model. With the progressive breakdown of chlorite, ultramafic chlorite mélanges transform into garnet peridotite, thereby losing any buoyancy they initially possessed. This makes the likelihood of mélange diapirs as a major transport mechanism through the sub-arc unfeasible.

Along-arc heterogeneous rheology inferred from post-seismic deformation of the 2011 Tohoku-oki earthquake

SUMMARYThe Japan forearc plays a crucial role in modulating the post-seismic deformation in response to the enormous stress perturbation induced by the 2011 Tohoku-oki earthquake. Dense geodetic observations across Japan have revealed coupled interactions between afterslip on the subducting plate interface and viscous deformation within the mantle wedge, and detailed numerical models can provide further profound insights into the forearc rheology. Recent studies have revealed the presence of a stagnant section in the forearc mantle of the Tohoku subduction zone, and here we investigate the associated along-arc variation of the stagnant part of the mantle wedge (cold nose) across Japan. We utilize a newly deployed geodetic network along a corridor in the Fukushima–Niigata region and compare the surface deformation pattern to that of the Miyagi–Yamagata corridor close to the main rupture area. We present a 3-D rheological model using laboratory-derived constitutive laws to simulate the geodetic observations including displacement fields and their time-series. Our results suggest along-arc heterogeneity in the forearc mantle rheology; specifically, we find a narrower cold nose in the Miyagi region and a wider one in the Fukushima forearc. The geodetic inferences on the forearc variation are consistent with along-arc spatial heterogeneity in the cut-off depth for shallow earthquakes as well as comparative measurements of the respective geothermal gradients between the Miyagi and Fukushima regions.

Development of a detection method for short-term slow slip events using GNSS data and its application to the Nankai subduction zone

Using global navigation satellite system (GNSS) data to detect millimeter-order signals of short-term slow slip events (S-SSEs) and to estimate their source parameters, especially duration, is challenging because of low signal-to-noise ratio. Although the duration of S-SSEs in the Nankai subduction zone has been estimated using tiltmeters, its regional variation has never been quantitatively studied. We developed an S-SSE detection method to estimate both the fault model and duration with their errors based on the detection methods developed by previous studies and applied it to a 23-year period of GNSS data in the Nankai subduction zone. We extracted S-SSE signals by calculating correlation coefficients between the GNSS time series and a synthetic template representing the time evolution of an S-SSE and by computing the average of correlation coefficients weighted by the predicted S-SSE signals. We enhanced the signals for duration estimation by stacking GNSS time series weighted by displacements calculated from the estimated fault model. By applying the developed method, we detected 284 S-SSEs from 1997 to 2020 in the Nankai subduction zone from Tokai to Kyushu and discussed their regional characteristics. The results include some newly detected S-SSEs, including events accompanying very low-frequency earthquakes and repeating earthquakes in offshore Kyushu. Our study provides the first geodetic evidence for synchronization of S-SSEs and other seismic phenomena in offshore Kyushu. We estimated the cumulative slip and duration, and their error carefully. We also estimated the average slip rate by dividing the cumulative slip by the cumulative duration. This study clarified that the average slip rate in western Shikoku was approximately twice as that in eastern Shikoku and Kyushu. These regional differences were statistically significant at the 95% confidence interval. Multiple factors can influence the regional characteristics of S-SSEs, and we speculate that the subducting plate interface geometry is one of the dominant factors.Graphical

Toward Waveform‐Based Characterization of Slab & Mantle Wedge (SAM) Earthquakes

AbstractEarthquakes in subduction zones occur in the slab mantle, in the subducting crust, on the subduction plate interface, and, in some cases, in the mantle wedge–regions that are separated by strong seismic discontinuities. These discontinuities are typically imaged with techniques using teleseismic waves, while local earthquakes are located based on arrival times. While this combination of imaging and earthquake location provides a good initial overview of where the earthquakes are located, the uncertainties associated with the two approaches are too large (i.e., few kilometers) to robustly identify on which side of a discontinuity (with thickness 100 m) the earthquakes occurred. Here we investigate how the waveforms of local earthquakes, which contain secondary phases arising from wave scattering at discontinuities, can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three‐step approach and includes an application to data from western Greece. First, to identify characteristic secondary phases, we analyzed synthetic seismograms from a generic 2‐D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three‐component seismograms. Third, to identify individual secondary phases in the data, we matched their timing to arrivals computed in a 3‐D velocity model. We identified on average two to three secondary arrivals per station. These include P‐ and S‐reflections from the plate interface which indicate hypocenters in the mantle wedge, and P‐reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust.

Constraints on Element Mobility During Deformation Within the Seismogenic Zone, Shimanto Belt, Japan

AbstractSubduction interfaces exhibit a variety of slip behaviors, including megathrust and slow earthquakes. Field observations are consistent with crack‐seal deformation, in which tensile cracks are sealed by fluid‐transported solute. However, there are few constraints on the mass fluxes and length and time scales of such deformation and attendant increases in cohesion within the seismogenic zone. Here, we present a systematic geochemical investigation of mass transport associated with development of crack‐seal veins in the Shimanto Belt—an accretionary complex that preserves a record of plate boundary slip behavior at temperatures relevant to the seismogenic zone: 150–350°C. These mélanges show evidence for shear across decameter‐scale zones of deformation dominated by anastomosing scaly fabrics and pervasive veins. We use meso‐ and microstructural observations with geochemical observations of scaly fabrics and crack‐seal veins to evaluate the role of silica redistribution in healing fracture porosity along the plate interface and modulating slip behavior. Crack‐seal veins contain primarily quartz with albite and calcite, and vein textures provide evidence for partial sealing. Bulk rock analyses determined that the amount of phyllosilicates, specifically illite and chlorite, increases with temperature. A simple mass‐balance model based on the immobile chemical component TiO2 shows a systematic trend in mobility for all three mélanges and increased element mobility as a function of temperature. Scaly fabrics and veins show compositional evidence for locally sourced mineral redistribution. This study supports a model where development of scaly slip surfaces and fracture healing through temperature‐dependent mineral redistribution can impact slip behavior and fluid flow along the subduction plate interface.

Subslab heterogeneity and giant megathrust earthquakes

The nucleation and rupture processes of giant megathrust earthquakes (M ≥ 9.0) in subduction zones are still controversial. Most previous studies have focused on the subducting plate interface, and the structure beneath the subducting slab and its influence on earthquake generation remain unclear. Here, we present high-resolution seismic velocity tomography beneath six regions where giant earthquakes have occurred. Subslab low-velocity (slow) anomalies are revealed, which may reflect hot mantle upwelling. The giant earthquake hypocentres are generally located above the edges of the slow anomalies or above the gaps between them. Large coseismic slips of the giant earthquakes mainly occurred above gaps between the slow anomalies. We suggest that differential buoyancy force between the slow anomalies and their gaps may be an important factor for earthquake nucleation, and the rupture extent of a giant earthquake may be constrained by the slow anomalies. Hence, it is necessary to conduct seismic tomography to investigate the detailed subslab structure, which may help to pinpoint the potential location and damage zone of a future giant earthquake. Mantle heterogeneity beneath subducting plates may influence giant megathrust earthquakes, according to seismic tomography of the subslab structure beneath six megathrusts that have ruptured in M ≥ 9.0 earthquakes.

Short-term interaction between silent and devastating earthquakes in Mexico

Either the triggering of large earthquakes on a fault hosting aseismic slip or the triggering of slow slip events (SSE) by passing seismic waves involve seismological questions with important hazard implications. Just a few observations plausibly suggest that such interactions actually happen in nature. In this study we show that three recent devastating earthquakes in Mexico are likely related to SSEs, describing a cascade of events interacting with each other on a regional scale via quasi-static and/or dynamic perturbations across the states of Guerrero and Oaxaca. Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stress produced by seismic waves of the great 2017 (Mw8.2) Tehuantepec earthquake, which strongly disturbed the SSE cycles over a 650 km long segment of the subduction plate interface. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable.

Metasomatism and deformation of block-in-matrix structures in Syros: The role of inheritance and fluid-rock interactions along the subduction interface

The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present study documents an example of a block-in-matrix ‘mélange’ metasomatized along the subduction interface (Kampos-Lia mélange zone, Cycladic Blueschists, Syros island). We show that this particular mélange zone is a preserved fragment of discontinuous oceanic domain associated with the Pindos basin, which underwent dominant exhumation-related deformation with top to the east shearing. A large part of the ‘mélange’ structure is inherited from the initial intraoceanic setting and should not be considered as a tectonic ‘mélange’. Through exhaustive, meter-scale mapping of the nature of blocks and matrix we show that metasomatism dominantly occurs at the contact between metavolcanic layers and serpentinite, with diffusion of Ca from the metavolcanics to the matrix and diffusion of Mg in the opposite direction. Most of the metavolcanic layers and blocks (mafic and carbonate) are only partly digested contrary to the ultramafic matrix which has been largely metasomatized and forms a tremolite-chlorite-talc schist, i.e. a ‘hybrid’ rock with intermediate chemical composition. Geochemical data demonstrate that metasomatic element mobilization (e.g. Li, B, U, LILE and REE) varies at the scale of the unit, suggesting a complex evolution of fluid composition at the kilometer scale. We suggest that the fluid which intruded the Kampos subunit was derived from the subducting slab and caused an enrichment in Li, B, U, LILE (Rb, K, Na, Ba, Eu), MREE and HREE. Mineralogical and P-T constraints indicate that the dominant part of metasomatism and rock hybridization occurred during exhumation, at ~ 1 GPa, well after peak burial and detachment from the slab (~ 2 GPa). Due to the absence of major tectonic mixing and the scarce preservation of prograde and peak metasomatic reactions, this metasomatism may be representative of fore-arc hybridization processes at depths of ~ 35 km. Deeper hybridization processes as hypothesized to produce a potential source for arc magmas are not well preserved in the studied examples. However, by changing the mineralogy of the matrix, metasomatism changes the rheological properties of the mélange and could thus manipulate the rheology of the subduction interface and influence exhumation processes.

Effect of normal stress on the frictional behavior of brucite: application to slow earthquakes at the subduction plate interface in the mantle wedge

Abstract. We report the results of friction experiments on brucite under both dry and wet conditions under various normal stresses (10–60 MPa). The final friction coefficients of brucite were determined to be 0.40 and 0.26 for the dry and wet cases, respectively, independent of the normal stress. Under dry conditions, velocity-weakening behavior was observed in all experiments at various normal stresses. Under wet conditions, velocity weakening was observed at low normal stress (10 and 20 MPa), whereas velocity strengthening was determined at a higher applied normal stress. Microstructural observations of recovered experimental samples indicate localized deformation within a narrow shear band, implying that a small volume of brucite can control the bulk frictional strength in an ultramafic setting. Among serpentinite-related minerals, weak and unstable frictional behavior of brucite under hydrated mantle wedge conditions may play a role in slow earthquakes at the subduction plate interface in the mantle wedge.

Spatio-temporal characteristics of frictional properties on the subducting Pacific Plate off the east of Tohoku district, Japan estimated from stress drops of small earthquakes

The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the 2011 M9 Tohoku earthquake. We analyzed 1142 earthquakes with 4.4 le M_{W} le 5.0 that occurred in 2003 through 2018 and obtained spatio-temporal pattern of stress drop on the Pacific Plate that subducts beneath the Okhotsk Plate. Here we show that small earthquakes at edges of a region with a large slip during the 2011 Tohoku earthquake had high values of stress drop, indicating that the areas had a high frictional strength and suppressed the coseismic slip of the 2011 Tohoku earthquake. In addition, stress drops of small earthquakes in some of the areas likely decreased after the 2011 Tohoku earthquake. This indicates that the frictional strength decreased at the areas due to the following aftershocks of the 2011 Tohoku earthquake, consistent with a high aftershock activity. This also supports that the frictional properties on a subducting plate interface can be monitored by stress drops of small earthquakes, as pointed out by some previous studies.

Localized fluid discharge by tensile cracking during the post-seismic period in subduction zones

It is thought that extensional structures (extensional cracks and normal faults) generated during the post-seismic period create fluid pathways that enhance the drainage of the subducting plate interface, thus reducing the pore pressure and increasing fault strength. However, it remains to be elucidated how much pore fluid pressure decreases by the extension crack formation. Here we examined (i) the pore fluid pressure decrease, and (ii) the degree fault strength recovery by the extension crack formation during the post-seismic period by analyzing extension quartz veins exposed around the Nobeoka Thrust, southwestern Japan. The Nobeoka Trust is an on-land analog of the modern splay fault at shallow depths (~ 8 km) in the Nankai Trough. The poro-elastic model of extensional quartz vein formation indicates that the formation of extensional cracks only releases up to ~ 7–8% of the total pore fluid pressure at ~ 8 km depth. The pore pressure around the Nobeoka Thrust was close to lithostatic pressure during the entire seismic cycle. The estimated effective frictional coefficient along the Nobeoka Thrust after this small fluid-loss by the extensional crack formation does not exceed 0.15. Hence, the pore fluid pressure reduction due to the post-seismic extensional cracks contributes little to increase the fault strength of the megasplay fault.

Locating Fully Locked Asperities Along the South America Subduction Megathrust: A New Physical Interseismic Inversion Approach in a Bayesian Framework

AbstractThe largest earthquakes in subduction zones occur where significant interseismic slip deficit has accumulated on the plate interface. Slip deficit accumulates most quickly in mechanically locked regions, and these also cause the regions around them to accumulate slip deficit; therefore, large earthquakes are typically expected to rupture in and around locked areas. The locations and dimensions of these locked zones have been difficult to resolve using standard techniques and available data sets. We develop a new statistical interseismic inversion approach that incorporates the physical interactions between nearby fault areas to directly determine the distribution of locking on the subduction plate interface (simultaneously with rigid forearc motions) from interseismic surface velocities. Because we include physical prior information in the inversion procedure, this approach reduces uncertainties in the rate of slip deficit accumulation, even in locations (such as near the trench) where kinematic inversions of onshore data have relatively low resolution. Applying the inversion to the South America subduction zone, we find that the pattern of locking and corresponding slip deficit rates correlate well with recent and historical large earthquake ruptures. Locked patch dimensions are <40 km and account for no more than 30% of the area of the plate interface. The small size of the imaged locked zones is a natural outcome of our physical assumptions and implies that mechanical locking is caused by correspondingly small geological features. Despite their small dimensions, locked zones generate substantial slip deficit on the surrounding plate interface, consistent with the slip patterns of large megathrust earthquakes.

Characteristic activities of slow earthquakes in Japan.

Slow earthquakes are a recently discovered phenomenon that mainly occur updip and downdip of the seismogenic zones of great earthquakes along the subducting plate interface. The spatiotemporal activity of various slow earthquakes occurring in the Nankai subduction zone is characterized by along-strike heterogeneity and along-dip systematic changes. Various slow earthquakes are horizontally distributed at their own depths and along-strike segments can be observed with respect to this distribution downdip of the locked zone; however, slow and great earthquakes occur in the same depth range near the Nankai Trough and Japan Trench axes. The frequently observed spatiotemporal interactions between different slow earthquakes can be attributed to their sensitivity and the stress transfer of the surrounding areas. This stress transfer is expected to extend to the adjacent sections in the seismogenic zone. Therefore, precise monitoring of slow earthquakes is important for future evaluations of great earthquakes, which requires the long-term maintenance and continuous improvement of the high-quality observation networks.