Kuril Subduction Zone Research Articles

Joint inversion of ocean-bottom pressure and GNSS data from the 2003 Tokachi-oki earthquake

When the 2003 Tokachi-oki earthquake generated a large tsunami off eastern Hokkaido, Japan, it was detected by cabled ocean-bottom pressure gauges (OBPs) installed within the seismic source region. The OBP records included characteristic features of tsunami waveforms in the source region, indicating significant offset due to the permanent sea-bottom displacement, which can provide critical information about the earthquake source. However, no studies have considered the OBP tsunami data observed in the focal region for the source inversion of the Tokachi-oki earthquake. Here, in order to better understand the 2003 Tokachi-oki earthquake source, we estimate the spatial distribution of fault slip by conducting a joint inversion using two different datasets: tsunami data from four OBPs, located in the focal region and about 300 km away to the south, as well as Global Navigation Satellite System (GNSS) data obtained throughout Hokkaido and northern Honshu. Through our joint inversion, we found that the most dominant slip peak is located 40 km northwest from the hypocenter with an abnormally large slip of sim15 m. This large slip may result from releasing a high amount of slip deficit accumulated by a stick–slip patch surrounded by stable slip regions on the southernmost Kuril subduction zone. By comparing the OBP observations and the synthetic tsunami waveforms derived from our fault model and other fault models, we conclude that the OBP tsunami data recorded within the focal region better constrained the magnitude and the location of the slip compared to other geophysical data.Graphical

Across‐Slab Propagation and Low Stress Drops of Deep Earthquakes in the Kuril Subduction Zone

AbstractDeep earthquakes occur down to 700 km depth where pressure is up to two orders of magnitude greater than in the crust. Rupture characteristics and propagation mechanisms under those extreme conditions are still poorly constrained. We invert seismic waveforms for the spatial dimensions, duration and stress drop of deep‐focus earthquakes (Mw6.7–7.7) in the Kuril subduction zone. We find stress drops of ∼1–10 MPa and rigidity‐corrected spatial dimensions and durations similar to crustal earthquakes. Radiated efficiency >1 is observed, suggesting that undershooting is prevalent in deep earthquakes, consistent with laboratory derived weakening mechanisms. Comparisons with subduction models suggest across‐slab propagation within regions with temperaturesT < ∼ 1,000°C, similar to shallow events. Hence, despite different triggering mechanisms, the same physics seems to control the rupture propagation of both shallow and deep earthquakes.

Геодинамические GNSS наблюдения на Курильских островах

The network of geodynamic GNSS observations was deployed in 2006 throughout the Kuril island arc from Japan to Kamchatka. The network includes 11 stations of continuous and periodic registration. The article provides information on the organization of the network and its current status. The creation of the GNSS network provided extensive material for studying the modern geodynamic processes in the Kuril segment of the subduction zone of the North American (Okhotsk) and Pacific lithospheric plates. The performed observations made it possible to obtain the first information on the modern geodynamics of the region. The article presents an overview of the results of the previous years obtained by the authors together with other researchers. The source models of the largest seismic events are constructed on the basis of the instrumental data: the 2006 Mw 8.3 and 2007 Mw 8.1 Simushir earthquake doublet and the 2013 Mw 8.3 deep-focus Okhotsk earthquake. At the initial stage of the post-seismic process in the epicentral zone of the Simushir earthquakes, the dependence of the asthenosphere viscosity on the observed post-seismic displacement velocity of the Earth’s surface was found. The results obtained earlier were supplemented by new data on the changes in the geodynamic setting in the subduction zone. The dynamics of the transient decaying post-seismic process in the central part of the island arc is studied. Stress relaxation in the Earth’s crust at various stages of this process could be the trigger of powerful volcanic eruptions occurred in 2009–2019 on the central Kuril Islands. The seismic potential of various segments of the Kuril subduction zone has been clarified on the basis of the modeling of current mechanical coupling of lithospheric plates. It contributes to a more accurate assessment of the seismic hazard of the region together with other methods. The continuation of the GNSS observations on the Kuril Islands in the future will allow us to study in detail the features of the modern geodynamics of the region.

Constraining the 410-km discontinuity and slab structure in the Kuril subduction zone with triplication waveforms

SUMMARYThe detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e. triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analysed the trade-off between model parameters in triplication studies using synthetic examples. Specifically, we illustrated the necessity of using array-normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained. We have observed triplications due to both the 410-km discontinuity and the slab upper surface. And, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths for the northern and southern regions are at 420$\pm $15 and 425$\pm $15 km, respectively, with no observable uplift. The slab upper surface is inverted to be located about 50–70 km below the 410-km discontinuity. This location is between the depths of the 1 and 2 per cent P-wave speed perturbation contours of a regional 3-D full-waveform inversion (FWI) model, but we found twice the wave speed perturbation amplitude. A wave speed increase of 3.9–4.6 per cent within the slab, compared to 2.0–2.4 per cent from the 3-D FWI model, is necessary to fit the waveforms with the shortest period of 2 s, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.

New Megathrust Locking Model for the Southern Kurile Subduction Zone Incorporating Viscoelastic Relaxation and Non‐Uniform Compliance of Upper Plate

AbstractDense 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.

Similarities and Differences in the Rupture Processes of the 1952 and 2003 Tokachi‐Oki Earthquakes

AbstractWe investigated the similarities and differences between the rupture processes of the 1952 and 2003 Tokachi‐oki earthquakes (Mw ∼ 8) that occurred 51 years apart in the southernmost region of the Kuril subduction zone. This event pair is rare because seismic waveforms of both events were instrumentally recorded. We performed joint source inversion analyses of both earthquakes to better understand the rupture processes of this earthquake sequence. The dataset for the 1952 earthquake consists of teleseismic and strong motion data which were obtained by digitizing copies of analog seismograms. Two datasets were made for the 2003 earthquake to examine the effect of data limitation and to facilitate a comparison of the inversion results of the two earthquakes. One dataset consists of a large amount of teleseismic, strong motion, and geodetic data. The other dataset is similar to that of the 1952 earthquake. Our results show that the rupture propagation, slip areas, and maximum slip amounts were similar for the 1952 and 2003 earthquakes in the Tokachi‐oki region. However, there are two important differences: the 1952 earthquake was initiated by an Mw 6.1 subevent in the Tokachi‐oki region, and its rupture extended to the adjacent Akkeshi‐oki region. The similarity in the rupture pattern of the M8‐class earthquake pair suggests the persistence of asperities on the plate interface, while the differences imply a degree of variability in the initiation and termination of the cascading rupture process.

Patterns of the Seismic Cycle in the Kuril Island Arc from GPS Observations

The 2006–2007 Simushir earthquakes started a new episode in the rupture history of the Kuril-Kamchatka subduction zone. This earthquake doublet terminated the seismic silence in the Central Kurils, which had lasted for almost a century. The analysis of seismologic, geologic and GPS data over the Kuril Islands region allows us to reveal the patterns of deformation of the lithosphere in the Kuril subduction zone, in particular, the peculiarities of accumulation and relaxation of elastic stresses during the seismic cycle. We analyzed > 9 years of continuous GPS observations on the Kuril Islands to determine the surface displacements caused by the geodynamic processes in the subduction zone. We used available GPS data to model the coseismic stress release and postseismic stress relaxation related to the 2006–2007 Simushir earthquakes. We showed that the duration of postseismic relaxation after the 2006 earthquake exceeds 10 years, preventing transition of the central Kurils to the interseismic stage and thus delaying the beginning of the new seismic cycle. The value of Maxwell viscosity of the asthenosphere found in this study (3 × 1017 Pa s) in the Kuril Island arc is substantially lower than the values determined for several other subduction zones. This contrast in viscosity agrees with Kogan et al. (J Geophys Res 118:3691–3706, 2013), who used shorter GPS time series than in our study. The results of our analysis show that multidirectional motions along the Kurils Islands are explained by the fact that segments of the island arc are at different stages of the seismic cycle.

Evidence of giant earthquakes and tsunamis of the seventeenth-century type along the southern Kuril subduction zone, eastern Hokkaido, northern Japan: a review

Abstract Large earthquakes of magnitude 7–8 occur at intervals of about 70–100 years in the southern Kuril Trench off eastern Hokkaido, northern Japan. However, especially large giant earthquakes (M w c. 8.8) have recurred at 300–500 year intervals, and the most recent of these was in the early seventeenth century. Surveys on the Pacific coast of eastern Hokkaido have documented field evidence that includes sharp contacts of peat and mud layers, indicative of coseismic coastal uplift, and large tsunami sand layers. Several proposed fault models of the seventeenth-century giant earthquake have been tested with numerical tsunami simulations. The best of these models incorporates a multi-segment interplate earthquake (M w 8.8) with large slip near the trench axis. The seventeenth-century giant earthquake and the 1611 Keicho Sanriku tsunami appear to have had different sources because varve counts from Lake Harutori suggest that the seventeenth-century giant earthquake occurred at c. 1637. Moreover, a consideration of regional tectonics suggests that convergence of the Kuril forearc sliver controls the current topography of eastern Hokkaido. Further progress in understanding the seismicity of this region will benefit from the combined insights of both geologists and seismologists.

Mechanism of subsidence of the Northeast Japan forearc during the late period of a gigantic earthquake cycle

The forearc in Northeast Japan subsided (3–4 mm/year) in the interseismic ~100 years before the 2011 Tohoku earthquake (MW9.1) just like it did during this event. This study attempts to understand the mechanism of the vertical displacement of the forearc during gigantic earthquake cycles via numerical modeling. The results suggest that the interseismic subsidence rate in the forearc increases with the duration of the locking of the asperity of the gigantic earthquake over several hundred years, due to the increasing slip deficit rate on the deeper parts of the plate interface. The increasing slip deficit rate is caused by both the decreasing the shear stress in the shear zone owing to the continuous locking of the asperity and the increasing the mobility of the continental lithosphere owing to the viscoelastic relaxation in the mantle wedge. The deep slip deficit rate extending to ~100 km depth of the plate interface is necessary to explain the observed interseismic forearc subsidence rate. The results also suggest hundreds of years of continuous locking of the asperities of a gigantic earthquake in the western Kuril subduction zone, where fast forearc subsidence has been observed as well.

Giant earthquake tsunami traces and related regional crustal movements recognized around the Kuril subduction zone, eastern Hokkaido, northern Japan

Giant earthquake tsunami traces and related regional crustal movements recognized around the Kuril subduction zone, eastern Hokkaido, northern Japan

Recent geodynamics of the Kuril subduction zone

The collected GPS/GLONASS data allow us to reveal new information on the recent geodynamics of the Kuril Island arc. The maximum deformation stress accumulates in the southern and northern parts of the study area, while a long fading transition process of postseismic motions is observed in the central segment of the Kuril arc as a result of the 2006–2007 great Simushir earthquakes of Mw = 8.3 and Mw = 8.1. We have succeeded in revealing the recent interplate coupling geometry of the Pacific and the North American lithospheric plates and also in estimating the seismic potential of different segments of the Kuril subduction zone.

Postseismic gravity change after the 2006-2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands.

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and ~1018 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

The distribution of split shear-wave parameters in the backarc region of the South Kuril subduction zone

This paper reports a study in the distribution of parameters of split shear waves excited by deep earthquakes in the Sakhalin and Hokkaido area, with the orientation of the axes of symmetry in the earth being estimated assuming a viscoelastic anisotropic model for the mantle. A mantle flow along NW 310° ± 20° has been identified beneath the Japan Sea. The flow is inclined at an angle of 20–30° relative to the horizontal plane and is consistent with the motion of the Pacific plate. Beneath the southern Sea of Okhotsk the fast a-axis [100] of olivine is oriented NE 30 ± 15°, nearly parallel to the trend of the Kuril arc, while the c-axis [001] is inclined at ∼35° relative to the horizontal plane. Bearing in mind the increased heat flow in the region, we assume the development of a mantle flow along the olivine c-axis at NW 300° ± 20° and with the shear plane [010] in the conditions of partial mantle melting (the B-type LPO). The lowest anisotropy (1–2.5%) was identified beneath Sakhalin and the greatest (3–5%) beneath the Japan Sea. An increasing degree of anisotropy is also noted to occur with an increasing depth of focus (down to 350 km).

Rapid postseismic relaxation after the great 2006–2007 Kuril earthquakes from GPS observations in 2007–2011

AbstractThe 2006–2007 doublet of Mw > 8 earthquakes in the Kuril subduction zone caused postseismic transient motion in the asthenosphere, which we observed on the Kuril GPS Array in 2007–2011. Here we show that the Maxwell asthenospheric viscosity that best fits the geodetic data increased by nearly an order of magnitude over the interval of 4 years, from 2 × 1017 to 1 × 1018 Pa s. These effective values of viscosity can be explained by a power law rheology for which strain rate is proportional to stress raised to a power n > 1. The apparent change in viscosity can also be caused by other factors such as coupling between afterslip and viscoelastic flow. The open and intriguing question in connection with postseismic data after the Kuril earthquake doublet is the magnitude of the long‐term asthenospheric viscosity, which shall be revealed by continued observations. An asthenosphere with viscosity of about 1 × 1019 Pa s is favored by the postseismic deformation still observed several decades after the 1960 Chile and 1964 Alaska Mw ~9 earthquakes. However, postseismic deformation associated with the 1952 southern Kamchatka Mw ~9 earthquake currently is not observed in the northern Kurils, an indication that the long‐term asthenospheric viscosity in the Kurils is lower than that in Chile and Alaska.

Identification and possible recurrence of an oversized tsunami on the Pacific coast of northern Japan

Geological evidence of severe tsunami inundation has been discovered in northern Japan. In the dune fields of Shimokita, in northernmost Tohoku, we have found two distinctive sand layers that are tsunami deposits. The run-up height of >20 m and inland inundation of at least 1.4 km are notably larger than any known historical case in Japan. The tsunami-genic earthquake that resulted in these deposits is thought to have taken place in the Kuril Forearc-Trench system nearly 700 years ago. The recurrence interval of major tsunamis originating in the Kuril subduction zone is about 400 years. Given that the most recent unusually large earthquake took place in AD 1611 (corresponding to the Keicho earthquake tsunami), the findings presented here increase the potential and hazard for an outsized tsunami striking the Pacific coast of northern Japan.

Subducted lithosphere beneath the Kuriles from migration of PP precursors

We seismically image both thermal and chemical heterogeneity of the mantle beneath the Kurile subduction zone using P-wave energy reflected from the underside of discontinuities, arriving as precursors to the seismic phase PP. We take advantage of new broadband seismic data provided by the High Lava Plains Seismic Experiment and EarthScope's USArray, collecting a dataset of 31 high-quality Sumatran earthquakes sampling beneath the Kuriles. We employ high-resolution array analysis techniques, including migration and vespagrams, to identify precursory arrivals and study lateral variations in discontinuity depth, sharpness, and impedance of the mantle transition zone. We find the 410 km boundary is at 395 km near the subducting Kurile slab, though the boundary is 410–425 km deep elsewhere. In regions away from subduction, we do not detect a laterally continuous underside reflection of P-waves from the 660 km discontinuity. However, in the vicinity of the subducting Kurile slab, we detect robust P660P reflections from interfaces near 620–670 km depth, signifying an increase in the impedance contrast at 660 km depth. We also detect deeper reflectors, down to 720 km depth, beneath the Kurile slab in a localized area. Cold, aluminum-depleted harzburgitic lithosphere residing at the base of the transition zone best explains the local enhancement of the 660 km discontinuity P-wave impedance contrast. Our new discontinuity measurements support the hypothesis of cold, depleted lithosphere stagnating at the 660 km discontinuity beneath the Kuriles subduction zone, and imply the 660 km boundary can locally impede mantle flow and produce chemical heterogeneity within the transition zone.

The mechanism of postseismic deformation triggered by the 2006-2007 great Kuril earthquakes

[1] In 2006–2007, a doublet of great earthquakes (Mw > 8) struck in the center of the Kuril subduction zone, a thrust event followed by an extensional event. Our observations of the Kuril GPS Array in 2006–2009 outline a broad zone of postseismic deformation with initial horizontal velocities to 90 mm/a, and postseismic uplift. We show that most of the postseismic signal after the great Kuril doublet is caused by the viscoelastic relaxation of shear stresses in the weak asthenosphere with the best-fitting Maxwell viscosity in the range of (5–10) × 1017 Pa s, an order of magnitude smaller than was estimated for several subduction zones. We predict that the postseismic deformation will die out in about a decade after the earthquake doublet. Our results suggest large variations among subduction zones in the asthenospheric viscosity, one of the most important rheological parameters.

On the deformation cycle of a strongly coupled plate interface: The triple earthquakes of 16 March 1963, 15 November 2006, and 13 January 2007 along the Kurile subduction zone

The great plate interface Kurile earthquake of 15 November 2006 (Mw = 8.3) was immediately followed by normal aftershocks in the outer rise of the central Kurile subduction zone. Two months later at a distance <50 km from this event the 13 January 2007 (Mw = 8.1) earthquake with a pure normal mechanism occurred in the outer rise of the central Kurile. The 15 November 2006 earthquake ruptured an area of 280 × 150 km2 with maximum slip of almost 7 m, while the 13 January 2007 earthquake ruptured an area of 240 × 40 km2 with a maximum slip of almost 14 m. These two significant events were preceded by a major reverse earthquake on 16 March 1963 (Mw = 7.7) that took place close to the location of the 13 January 2007 earthquake. These very different earthquakes demonstrate the different stages of the deformation cycle along strongly coupled plates at the central Kurile arc and present a rare case of stress regime reversal over a short time span. In order to understand the details of these three earthquakes, we inverted for the source rupture processes of the earthquakes and delineated the location and dimension of asperities. The role of gravity and topography anomalies and their correlation to the asperity distribution as well as the long‐term seismicity is studied in detail. Using trench‐parallel Bouguer anomaly, we have identified the differences in the asperities and the location of larger normal outer rise aftershocks as well as the location of 13 January 2007 earthquake. We also show that the long‐term seismicity of the central Kurile arc is confined to the intraslab and not the plate interface. The static stress transfer for these earthquakes showed that the stress on optimally oriented thrust faults is increased in the northeastern part of the rupture area of the 15 November 2006 earthquake. Absence of major and great earthquakes to the northeastern immediate vicinity of the rupture area of the 15 November 2006 earthquake supports the hypothesis that this area is mature for the next strong earthquake along the Kurile arc.

Validation and Joint Inversion of Teleseismic Waveforms for Earthquake Source Models Using Deep Ocean Bottom Pressure Records: A Case Study of the 2006 Kuril Megathrust Earthquake

The importance of accurate tsunami simulation has increased since the 2004 Sumatra-Andaman earthquake and the Indian Ocean tsunami that followed it, because it is an important tool for inundation mapping and, potentially, tsunami warning. An important source of uncertainty in tsunami simulations is the source model, which is often estimated from some combination of seismic, geodetic or geological data. A magnitude 8.3 earthquake that occurred in the Kuril subduction zone on 15 November, 2006 resulted in the first teletsunami to be widely recorded by bottom pressure recorders deployed in the northern Pacific Ocean. Because these recordings were unaffected by shallow complicated bathymetry near the coast, this provides a unique opportunity to investigate whether seismic rupture models can be inferred from teleseismic waves with sufficient accuracy to be used to forecast teletsunami. In this study, we estimated the rupture model of the 2006 Kuril earthquake by inverting the teleseimic waves and used that to model the tsunami source. The tsunami propagation was then calculated by solving the linear long-wave equations. We found that the simulated 2006 Kuril tsunami compared very well to the ocean bottom recordings when simultaneously using P and long-period surface waves in the earthquake source process inversion.

Geochemistry of volcanic and hydrothermal fluids and volatile budget of the Kamchatka–Kuril subduction zone

A data base for the composition and emission rates of more than 100 thermal manifestations including boiling geothermal systems and 23 volcanoes along the 1900 km long Kamchatka–Kuril (KK) arc is presented. These results were used to estimate mean fluxes of volatiles from the KK arc. The fluxes from the KK arc are compared with the fluxes from the best studied Central American (CA) arc and with the compiled literature data on global fluxes. The error ranges and the OUT/IN (in)balance calculations are also discussed. The estimated fluxes of volatiles from volcanic fumaroles and the observed, normalized to the Cl content, fluxes from hydrothermal systems are very close, with the higher hydrothermal flux from Kuril Islands due to a larger number of the acidic Cl–SO 4 springs on the Islands and their outflow rates. The total volcanic SO 2 flux from the whole KK arc is estimated to be higher than 3000 t/d. The measured S and C fluxes from hydrothermal systems are much lower than the volcanic output due to the loss of these components in the upper crust (mineral precipitation). The Cl/ 3He ratio is inferred to be a stable indicator of the arc setting for hydrothermal and volcanic fluids with a mean value of (2 ± 4) × 10 9. Comparison of the obtained volcano–hydrothermal fluxes with fluxes calculated from the erupted solid volcanic products at Kamchatka and Kurils during Holocene time reveals that the total estimated volatile output from the KK arc is compatible with the total magmatic output if the intruded to erupted ratio is close to 7, i.e. almost the same as assumed for the Central American arc. Calculated fluxes as well as the ratios for OUT/IN fluxes (volcanic + hydrothermal output/slab + mantle input) for CO 2, S, H 2O, Cl, N 2, 4He and 3He from the KK arc normalized to the arc length are in general close to the global estimates. The fractions of CO 2 and S in the total volatile output at KK arc derived directly from the mantle wedge are 18% and 16% (mole basis), respectively. Fractions of mantle derived H 2O, N 2 and Cl are much lower, less that 5% of their output.