Abstract
Fluids are thought to play an important role in controlling episodic tremor and slow slip (ETS) in subduction zones. Therefore, constraining the along-dip distribution of fluids is necessary to better understand source mechanism of ETS, and particularly the role played by fluids in ETS generation. Here, we report clear observations of coherent ScSp phases with a dense seismic array in western Shikoku, Japan, where ETS has been most active over the past decade. Using numerical simulations of elastic-wave propagation to reproduce the observed ScSp phases, we demonstrate that, relative to shallower depths, either the Vp/Vs ratio or the thickness of a low-velocity zone (LVZ) within the subducting oceanic crust increases with depth beneath the mantle wedge corner where ETS has been observed. Based on these depth dependences of the structural elements, a wide semi-ductile shear zone appears to be lubricated by high-pressurized fluid in the subducting oceanic crust at ETS source depths, and to be a key factor regulating ETS activity.
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Introduction
Among worldwide subduction zones, ETS commonly occurs on the deep extension of major tectonic boundaries that host megathrust earthquake ruptures1, 2. Increasing numbers of studies of seismic structure in ETS zones, including seismic tomography and receiver function analysis, indicate that high-pressurized fluids, characterized by low seismic velocities with high Poisson’s ratios, are trapped within the subducting oceanic crust3,4,5,6. In addition, the strong sensitivity of tremors to tidal stress changes supports the presence of high-pressurized fluids in ETS zones7,8,9. Thus, fluids trapped within subducting oceanic crust could play an important role in controlling ETS behavior10, 11. However, the along-dip distribution of fluids is poorly constrained, and it is unknown whether there are unique structural properties of the oceanic crust that may be related to the generation of ETS along plate boundary faults12.
To investigate spatial variations in structural elements within the subduction complex, we deployed a dense linear seismic array in western Shikoku, Japan, above a region extending from the locked zone to the ETS zone (Fig. 1), where ETS has been most active over the past decade13, 14. The array consisted of 45 portable seismic stations aligned subparallel to the direction of subduction, with a total length of ~60 km.
Here, we focus on identifying seismic phase conversions at the top of the subducting slab, for near-vertically incident ScS waves originating from a large, deep earthquake. The ScSp (ScS-to-P conversion) phase represents an S wave reflected at the core-mantle boundary and then converted to a P waves within the subduction zone. Previous studies using ScSp phases have investigated the geometry and physical properties of the subducting Philippine Sea (PHS) Plate15,16,17. We observed coherent ScSp phases on the dense seismic array, which appear to be associated with the subducting PHS Plate. To reproduce these observations, we performed numerical simulations of elastic-wave propagation using a finite difference method (FDM)18 that incorporated a three-dimensional structural model. The combination of coherent ScSp phases and numerical simulations allows us to investigate the depth dependence of Poisson’s ratios or thickness of the LVZ, and to infer the potential presence of fluids confined to the ETS zone.
Results
Based on comparisons of transverse and vertical component waveform data (Fig. 2a and b), we found clear, coherent signals arriving before ScS on the vertical components of most stations in the array. Figure 2c and d shows particle motions in the vertical and horizontal planes, respectively, for a time window that includes the ScS precursor observed at station EH03. Vertical motion is dominant during the precursory phase arrival. Particle motion in the horizontal plane is polarized along the orientation of the subducting plate; i.e., NW–SE. These particle motions satisfy the criteria proposed by Okada15 for identification of the ScSp phase. Therefore, these coherent precursor signals recorded by the seismic array are interpreted to be ScSp phases. Interestingly, travel time differences between ScS and ScSp increase from 3.0 s to 4.5 s along-dip in the direction of subduction. This means that the ScS-to-ScSp conversion point deepens to the northwest, indicating in turn that the converted waveforms propagate from the top of the subducting PHS Plate15, 16. The depth variations of these conversion points (28–43 km) are roughly consistent with the depth of the plate boundary interface estimated previously19, 20.
We simulated the propagation of synthetic ScSp waveforms using the JIVSM model (Fig. 3a). The simulated ScSp phases were generated at multiple velocity discontinuities within the subducting oceanic crust; i.e., the top and bottom of the LVZ, and the oceanic Moho. We then aligned the simulated ScS phases using a procedure similar to that employed to align the observed waveform data, and compared the arrival times of simulated ScSp phases with our observations. However, the calculated ScS-ScSp travel time differences were systematically lower than predicted by our observations (Supplementary Figure S1). Although we replaced the plate interface model with that proposed by Hirose et al.19, the goodness of fit between the observed and calculated waveforms decreased. This is because the depth of the oceanic crust reported by Hirose et al.19 is ~3 km shallower than that of the JIVSM model, resulting in further reduction of the simulated ScS-ScSp time difference.
The predicted difference in travel times between the ScS and ScSp phases appears to be too low compared to observations. To increase the goodness of fit, we propose here that the S-wave velocity within the LVZ in the oceanic crust could be slower than the original values assumed in the JIVSM model, or the thickness of the LVZ could be thicker than the original JIVSM model. These features would delay only the ScS arrival, while keeping the ScSp arrival almost constant. In addition to adjusting our models, because the travel time difference between the observed and simulated waveforms was larger at the northern stations, toward which the LVZ is subducting (Figure S1), we partitioned the LVZ into shallower and deeper parts around the upper corner of the mantle wedge, taking into account the depth dependence of the physical properties. The S-wave velocity and the layer thickness in the partitioned LVZ are labeled Vs u, h u in the shallower part of the LVZ, and Vs d, h d in the deeper part, respectively (Fig. 3b).
First, we incorporated the depth dependence of S-wave velocities, assuming a constant layer thickness (h u = h d = h); here, we call this the constant layer thickness model. S-wave velocities were linearly interpolated between the shallower (Vs u) and deeper (Vs d) portions to ensure that no new discontinuities were introduced to the structural model (Fig. 3b). To obtain the best possible fit between simulated and observed waveforms, we conducted a grid search over the three-parameter space defined by Vs u, Vs d, and the layer thickness h of the LVZ (Fig. 4). To quantify the fit, we averaged the cross-correlation coefficients between observed and simulated ScSp phases centered in a 7 s time window over all stations with identifiable ScSp phase onsets (EH01–EH35), since ScSp phases at the northern stations are emergent. We then used a contour plot to search for the best-fit parameters.
As an alternative model, we assumed a depth-dependent LVZ thickness around the upper corner of the mantle wedge with constant seismic velocities (i.e., Vs u = Vs d = Vs with a constant Vp/Vs ratio within the LVZ.) Hereafter, we call this the constant velocity model. To obtain the best possible fit between simulated and observed waveforms, we conducted another set of grid searches over the three-parameter space defined by h u, h d, and Vs within the LVZ (Fig. 5).
From these grid search results, within reasonable ranges of the parameters (Figs 4 and 5), the best-fit model has Vs u = 2.9 km/s, Vs d = 2.2 km/s, and h = 6 km for the constant layer thickness model, or, h u = 3 km, h d = 5 km, and Vs = 2.1 km/s for the constant velocity model. The averaged cross-correlation coefficients reached 0.8 for each best-fit model. Given the trade-off between the depth dependence of S-wave velocities and LVZ thickness, the difference in arrival times of the ScSp and ScS waves means that each best-fit model is able to adequately explain the observed waveforms. Figure 3c and d compare the observed and simulated ScSp and ScS waves, calculated using the best-fit parameters for the constant layer thickness model (black star in Fig. 4d). Along the seismic array, from south (EH01) to north (EH35), the simulated ScSp arrivals correlate well with observed waveforms (Fig. 3c). In summary, these modeling results suggest that either the S-wave velocities within the LVZ decrease or the thickness of the LVZ increases at depths greater than around the continental Moho; i.e., at ETS source depths.
Discussion
Based on comparison of transverse and vertical waveform data (Fig. 2a and b), we identified clear, coherent ScSp phase arrivals along a seismic array in western Shikoku, Japan, which converted near the PHS Plate boundary. Although some previous studies have detected ScSp phases associated with the PHS Plate16, 17, it was difficult to precisely illustrate the depth variations of the plate boundary, due to the sparseness of the seismic network at the time of these previous works. The present study is the first to obtain a spatially continuous image of the PHS Plate boundary using ScSp phases with fine spatial resolution.
We performed numerical simulations of elastic wave propagation to reproduce observed ScSp phases, using the JIVSM velocity model as an initial (reference) model. However, the calculated ScS-ScSp travel time differences were systematically smaller than those predicted by our observations. To improve the goodness of fit, we conducted an extensive parameter search of shear wave speeds (Vs u, Vs d) and LVZ thicknesses (h u, h d) within the oceanic crust (Figs 4 and 5). However, it could be claimed that the interpretations are nonunique, because spatial variations of the other parameters in the velocity model, especially within the overriding plate, can also explain the features of the observed ScSp phases. To address this possibility, we conducted additional numerical simulations while changing other model parameters with the physical properties of the LVZ held constant.
As a first case, we considered serpentinization of the mantle wedge above the ETS source. We simulated the ScSp phases by changing both Vp and Vs in the mantle wedge while assuming several degrees of serpentinization, from dunite to antigorite serpentinite (0%, 50%, and 100% serpentinization)21, as well as from dunite to lizardite serpentinite (30%, 60%, and 100% serpentinization)21. However, the simulated waveforms did not fit the observed waveforms well for the two species of serpentinite (Supplementary Figures S2 and S3), because serpentinization in the mantle wedge corner reduces the ScS-ScSp travel time difference along the linear seismic array. In addition, there is no significant variation of crustal seismic structure in the overlying plate in the studied area, based on regional tomographic images22 revealed by a dense seismic network in Japan (Hi-net). Therefore, heterogeneous structure in the mantle wedge or in the crust of the overlying plate is not sufficient to explain the observed features of ScS-ScSp travel time difference along the dense seismic array.
In the present analysis, we assumed a constant P-wave velocity of 5 km/s in the LVZ for all waveform simulations. To investigate the sensitivity of the results to the assumed Vp value, we conducted a grid search over a Vp range of 4–6 km/s in the LVZ, following the procedure described above. For the constant layer thickness model (h = 6 km), the S-wave velocities of the LVZ at greater depths are lower than those at shallower depths (Supplementary Figure S4), regardless of the assumed Vp value. Therefore, we consider the present conclusions to be robust with respect to the assumed Vp within the LVZ. Presumably this robustness arises because we focus on the ScS-ScSp travel time difference, which has only a weak dependence on the absolute value of the velocity.
From an extensive parameter search of shear wave speeds (Vs u, Vs d) and LVZ thicknesses (h u = h d = h) within the oceanic crust (Fig. 4), the best-fit parameters for a constant layer thickness model were estimated to be Vs u = 2.9 km/s, Vs d = 2.2 km/s, and h u = h d = 6 km. The resulting Vp/Vs ratio has a high value (~2.3) at depths of >~30 km. Interestingly, at depths greater than the mantle wedge, the shear wave velocity within the LVZ is slower than that of the shallower portion of the LVZ for any reasonable LVZ thickness (Fig. 4). This result indicates that the reduction in shear wave speeds at greater depths is a plausible and robust feature within a reasonable range of layer thicknesses. The reduction in shear wave velocity leads to an increase in Vp/Vs ratio (Poisson’s ratio) within the LVZ beneath the mantle wedge corner where ETS has been observed (Fig. 6a), because we assume Vp is constant within the LVZ. The incremental change in the Vp/Vs ratio in the deep portion of the LVZ is around 0.5 with the parameters that yield the best-fit constant layer thickness model.
With a constant LVZ thickness, the active ETS zone roughly overlaps the high-Vp/Vs layer within the oceanic crust beneath the mantle wedge corner (Fig. 6a). This feature matches well with previous studies of seismic velocity models in SW Japan, which have shown that ETS takes place above low-velocity layers of oceanic crust with high Vp/Vs ratios, as well as beneath the mantle wedge corner3, 5, 23. Because this Vp/Vs ratio is significantly higher than expected for a fully hydrated (about 5 wt%) MORB24, the high-Vp/Vs zone implies the presence of high-pressurized fluids released by progressive metamorphic dehydration reactions in the oceanic crust around the ETS zone3,4,5,6.
Alternatively, the best-fit model with a constant S-wave velocity has h u = 3 km, h d = 5 km, and Vs u = Vs d = 2.1 km/s; i.e., Vp/Vs ratio = 2.4 (Fig. 6b). The Vp/Vs ratio is higher than that derived from the constant layer thickness model. This also indicates that fluids released by dehydration reactions could be trapped in the LVZ from shallow to deep depths. Regardless of the absolute value of the assumed LVZ S-wave velocity, the layer thickness increases below the depth of the mantle wedge corner (Fig. 5). The incremental change in LVZ thickness is 2–3 km for the good-fit models, which is almost double the thickness of the shallow LVZ. A similar increase in layer thickness of the LVZ near ETS source depths has been suggested to explain the results of wide-angle seismic reflection surveys in the Nankai subduction zone25, 26. In addition to Nankai, in the Cascadia, Chile, Alaska, and Mexico subduction zones27,28,29,30,31 there seems to be a positive correlation between the thickness of the reflection band near the plate boundary and the degree of unlocking between the overlying and subducting plates. The locked region along the plate boundary fault shows a thin, sharp reflection, whereas the deeper part, characterized by aseismic slip, exhibits a thick reflection band26, 29, 30. Based on geological studies of plate boundary faults in exhumed subduction-related rock assemblages32, the increased thickness of the LVZ near ETS source depths is attributed to a well-developed ductile shear zone, consisting mainly of trench-fill sediments. The thicker shear zone may accommodate shear strain by distributed flow deformation, lubricated by fluids trapped in the LVZ32, 33.
Although it is difficult to distinguish between these different models, the structures within the oceanic crust at ETS source depths are commonly characterized by a relatively thick layer (4~6 km) with high Vp/Vs ratio (>~2.3) (Fig. 6). This feature indicates that a wide semi-ductile shear zone is lubricated by high-pressurized fluid 31in the subducting oceanic crust at ETS source depths. These down-dip variations in physical properties might play an important role in regulating ETS activity.
Methods
We deployed a dense linear seismic array from October 2011 to April 2013 on western Shikoku Island, SW Japan (Fig. 1). The array consisted of 45 three-component seismometers recording in continuous acquisition mode: 43 instruments with a natural frequency of 1 Hz, and 2 with a natural frequency of 0.2 Hz. We also used permanent stations near the array from the Hi-net network, operated by the National Research Institute for Earth Science and Disaster Prevention (NIED; Okada et al.34, and stations of the Japan Meteorological Agency (JMA). During the deployment period, we visually inspected seismograms of Mw ≥ 6 deep earthquakes with focal depths greater than 90 km and epicentral distances Δ < 25°; within this range, no other phases are expected to arrive that could be misidentified as ScSp 17. Using this procedure, we found a clear ScS precursor following an Mw 6.9 earthquake that occurred on 8 November 2011 in the backarc of the Ryukyu Trench, at a focal depth of 224.8 km (Fig. 1). We corrected the frequency response of the seismometers using a recursive time domain filter35, then applied a 0.2–4.0 Hz band-pass filter to the corrected seismograms. Using the transverse components of rotated seismograms from the array, we shifted the ScS phases relative to the arrival at station EH42 (Fig. 1) by cross-correlating the ScS phase from EH42 with other ScS waveforms to achieve the maximum correlations. In other words, we aligned the seismograms as if the ScS phase arrived simultaneously. The vertical component data at each station were then time shifted by the corresponding time lags, relative to EH42 (Figure 2a).
We performed numerical simulations of elastic-wave propagation to reproduce the observed ScSp phases using the three-dimensional FDM code of Maeda et al.18. Figure 3a shows a vertical cross-section of the Japan Integrated Velocity Structure Model (JIVSM20) used in the numerical simulations, where P- and S-wave velocities (Vp, Vs) in each layer are labeled. This model was constructed by integrating seismic structures deduced from extensive refraction/reflection experiments, gravity surveys, surface geology, borehole logging data, microtremor surveys, and earthquake ground motion records. The thickness of the LVZ within the subducting oceanic crust was assumed to be 3 km in the JIVSM model. The target area comprised a zone 150 × 150 km in horizontal extent, and 85 km in depth, which was discretized in grid intervals of 0.1 km in each direction. As an incident wave, we used an SH plane wave with 20 km wavelength, propagating vertically toward the seismic array, mimicking the ScS wave, whose incidence angle is nearly vertical36. The polarization azimuth of the synthetic SH-wave was set to N29°W, which roughly corresponds to the polarizations of the ScS phases observed in this study (Fig. 2d).
References
Rogers, G. & Dragert, H. Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip. Science 300, 1942–1943, doi:10.1126/science.1084783 (2003).
Schwartz, Y. S. & Rokosky, M. J. Slow slip events and seismic tremor at circum-Pacific subduction zones. Rev. Geophys. 45, RG3004, doi:10.1029/2006RG000208 (2007).
Shelly, R. D., Beroza, C. G. & Ide, S. Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature 442, 188–191, doi:10.1038/nature04931 (2006).
Audet, P., Bostock, G. M., Christensen, I. N. & Peacock, M. S. Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing. Nature 457, 76–78, doi:10.1038/nature07650 (2009).
Kato, A. et al. Variations of fluid pressure within the subducting oceanic crust and slow earthquakes. Geophys. Res. Lett. 37, L14310, doi:10.1029/2010GL043723 (2010).
Hansen, T. R., Bostock, G. M. & Christensen, I. N. Nature of the low velocity zone in Cascadia from receiver function waveform inversion. Earth Planet. Sci. Lett. 337–338, 25–38, doi:10.1016/j.epsl.2012.05.031 (2012).
Nakata, R., Suda, N. & Tsuruoka, H. Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events. Nature Geosci. 1, 676–678, doi:10.1038/ngeo288 (2008).
Rubinstein, L. J. et al. Tidal modulation of nonvolcanic tremor. Science 319, 186–189, doi:10.1126/science.1150558 (2008).
Houston, H. Low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress. Nature Geosci. 8, 409–415, doi:10.1038/ngeo2419 (2015).
Audet, P. & Kim, Y. Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review. Tectonophysics 670, 1–15 (2016).
Hyndman, R. D. et al. Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition. J. Geophys. Res. 120, 4344–4358 (2015).
Janiszewski, H. A. & Abers, A. G. Imaging the Plate Interface in the Cascadia Seismogenic Zone: New Constraints from Offshore Receiver Functions. Seismol. Res. Lett. 86(5), 1261–1269, doi:10.1785/0220150104 (2015).
Hirose, H. & Obara, K. Short-term slow slip and correlated tremor episodes in the Tokai region, central Japan. Geophys. Res. Lett. 33, L17311, doi:10.1029/2006GL026579 (2006).
Nishimura, T., Matsuzawa, T. & Obara, K. Detection of short-term slow slip events along the Nankai Trough, southwest Japan, using GNSS data. J. Geophys. Res. Solid Earth 118, 3112–3125, doi:10.1002/jgrb.50222 (2013).
Okada, H. Forerunners of ScS waves from nearby deep earthquakes and upper mantle structure in Hokkaido. Zisin 24, 288–239 (in Japanese with English abstract) (1971).
Nakanishi, I. Precursors to ScS phases and dipping interface in the upper mantle beneath southwestern Japan. Tectonophysics 69, 1–35, doi:10.1016/0040-1951(80)90125-0 (1980).
Helffrich, G. & Stein, S. Study of the structure of the slab-mantle interface using reflected and converted seismic waves. Geophys. J. Int. 115, 14–40, doi:10.1111/j.1365-246X.1993.tb05586.x (1993).
Maeda, T. et al. Seismic- and tsunami-wave propagation of the 2011 Off the Pacific Coast of Tohoku Earthquake as inferred from the tsunami-coupled finite-difference simulation. Bull. Seism. Soc. Am. 103(2B), 1456–1472, doi:10.1785/0120120118 (2013).
Hirose, F., Nakajima, J. & Hasegawa, A. Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double-difference tomography. J. Geophys. Res. 113, B09315, doi:10.1029/2007JB005274 (2008).
Koketsu, K., Miyake, H. & Suzuki, H. Japan Integrated Velocity Structure Model version 1, Proceedings of the 15th World Conference on Earthquake Engineering, Paper No. 1773 (2012).
Christensen, N. Serpentinites, peridotites, and seismology. Int. Geol. Rev. 46(9), 795–816, doi:10.2747/0020-6814.46.9.795 (2004).
Matsubara, M., Obara, K. & Kasahara, K. High-VP/VS zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan. Tectonophysics 472, 6–17 (2009).
Kato, A. et al. Non-volcanic seismic swarm and fluid transportation driven by subduction of the Philippine Sea slab beneath the Kii Peninsula, Japan. Earth, Planets and Space 66, 88, doi:10.1186/1880-5981-66-86 (2014).
Hacker, B. R., Peacock, M. S., Abers, A. G. & Holloway, D. S. Subduction factory, 2, Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions? J. Geophys. Res. 108(B1), 2030, doi:10.1029/2001JB001129 (2003).
Kodaira, S. et al. High pore fluid pressure may cause silent slip in the Nankai Trough. Science 304, 1295–1298, doi:10.1126/science.1096535 (2004).
Kurashimo, E. et al. Along-strike structural changes controlled by dehydrationrelated fluids within the Philippine Sea plate around the segment boundary of a megathrust earthquake beneath the Kii peninsula, southwest Japan. Geophys. Res. Lett. 40, doi:10.1002/grl.50939 (2013).
Fisher, M. A. et al. Seismic reflection images of the crust of the northern part of the Chugach terrane, Alaska: Results of a survey for the Trans-Alaska Crustal Transect (TACT). J. Geophys. Res. 94, 4424–4440 (1989).
Buske, S. et al. Broad depth range seismic and seismological imaging of the subduction processing North Chile. Tectonophysics 350, 273–282 (2002).
Nedimović, M. R., Hyndman, D. R., Ramachandran, K. & Spence, D. G. Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface. Nature 424, 416–420 (2003).
Bostock, G. M. The Moho in subduction zones. Tectonophysics 609, 547–557, doi:10.1016/j.tecto.2012.07.007 (2012).
Song, T.-R. A. & Kim, Y. Localized seismic anisotropy associated with long-term slow-slip events beneath southern Mexico. Geophys. Res. Lett. 39, L09308, doi:10.1029/2012GL051324 (2012).
Fagereng, Å. & Sibson, H. R. Mélange rheology and seismic style. Geology 38, 751–754, doi:10.1130/G30868.1 (2010).
Abers, G. A. et al. Imaging the source region of Cascadia tremor and intermediate-depth earthquakes. Geology 37, 1119–1122 (2009).
Okada, Y. et al. Recent progress of seismic observation networks in Japan–Hi-net, F-net, K-NET and KiK-net. Earth Planets Space 56, xv–xxviii (2004).
Maeda, T., Obara, K., Furumura, T. & Saito, T. Interference of long-period seismic wavefield observed by the dense Hi-net array in Japan. J. Geophys. Res. 116, B10303, doi:10.1029/2011JB008464 (2011).
Maeda, T., Furumura, T. & Obara, K. Scattering of teleseismic P-waves by the Japan Trench: A significant effect of reverberation in the seawater column. Earth Planet. Sci. Lett. 397(1), 101–110, doi:10.1016/j.epsl.2014.04.037 (2014).
Wessel, P. & Smith, W. H. F. New, improved version of Generic Mapping Tools released. Eos Trans. Am. Geophys. Union 79, 579 (1998).
Acknowledgements
We thank the NIED and JMA for allowing us to use waveform data collected from their permanent stations. We used the EIC computer system, hosted by the Earthquake Research Institute at the University of Tokyo, for numerical simulations. The data to support this article are available at the Earthquake Research Institute in the University of Tokyo and the Data Management Center of NIED. This work was supported by JSPS KAKENHI Grant Numbers 23244091 and 25287113.
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M.T. and A.K. conceived the study, analysed seismic waveform data, conducted numerical simulations, and interpreted the results. M.T., A.K. and T.M. wrote the manuscript. T.M. supported to conduct the numerical simulations. A.K., K.O. and T.T. performed the seismic observations and helped in data processing. M.T., A.K. and K.Y. discussed the results and obtained the conclusions.
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Toya, M., Kato, A., Maeda, T. et al. Down-dip variations in a subducting low-velocity zone linked to episodic tremor and slip: a new constraint from ScSp waves. Sci Rep 7, 2868 (2017). https://doi.org/10.1038/s41598-017-03048-6
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DOI: https://doi.org/10.1038/s41598-017-03048-6
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