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2‐D diagrams of the geological structure of the study area. (a) Horizontal cross section under the Kanto Basin (KB) at depth of ∼10 km. Geological structure is based on Hayashi et al. (2006). (b) 2‐D schematic diagram of the geological structure along line XX′ shown in (a). The cross section includes the Kanto Basin (KB), which is the thick sediments covering the study area. The red wedge sandwiched between the PHS and KB corresponds to the primary Neogene accretionary complex. The horizontal dashed line indicates the location of the horizontal cross section shown in (a). The vertical axis is not scaled greater than 30 km, because the geometry of the bottom of the PHS slab is not well constrained. PAC, Pacific; PHS, Philippine Sea.
Source publication
The Tokyo metropolitan area, in the Kanto region of Japan, lies on the Okhotsk plate. Historically, the city has suffered severe damage as a result of megathrust ruptures and intra‐plate earthquakes (M ≥ 7) caused by the dual subduction of the Philippine Sea (PHS) slab and the Pacific slab. To understand the unique seismotectonics of the region, we...
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Citations
... According to Ishise et al. (2021), strong anisotropy of up to almost 10% exists in the crust of the analysis region. Because we converted the RFs into the depth domain without considering the effects of anisotropy on the ray-path and travel time, there may be errors in the depth estimation of the discontinuities. ...
... When a particular anisotropy presents in a large volume in the study region, the effect of the anisotropy cannot be ignored. However, the anisotropy structure model of Ishise et al. (2021) shows depth variation in anisotropic features. Therefore, the error in our estimates due to anisotropy is expected to be only a few percent. ...
... We compared the distribution of the discontinuity with P-wave velocity models recently determined through seismic tomography (Ishise et al., 2021). The velocity model was based on the travel time data observed by densely distributed stations containing those of the MeSO-net. ...
Plain Language Summary
The Philippine Sea Plate subducts beneath the continental plates along its entire length; however, it collides with the Izu collision zone at one point. Little information is available on the three‐dimensional (3D) configuration of the rock layers in and around the collision zone or the mechanisms by which they accommodate relative plate motion. To understand this, we investigated the subsurface structure using seismic waveform data. Based on the results and previously determined structural properties, the geometry of the crust‐mantle boundary (termed the “Moho”) of the Philippine Sea Plate was determined. We estimated the crustal thickness of the plate by considering the previously determined geometry of its upper surface. The crust is >35 km thick beneath the Izu Peninsula and the collision zone; however, the crust becomes thinner with increasing distance. Thus, the thick crust collides, and the thin crust subducts. Although the thin crust subducts through the unstable sliding of its upper surface, the upper surface of the thick crust is fixed. Instead, a stable slip was previously assumed to occur at a 15–20 km depth beneath the Izu Peninsula. The results of this study indicate that a stable slip surface exists within the crust of the Philippine Sea Plate.
... The Philippine Sea (PHS) plate and the Pacific (PAC) plate are subducting beneath the Okhotsk (OKH) plate ( Fig. 1), and many earthquakes related to dual plate subduction occur. Various subducting slab models have been developed by previous studies based on controlled-source seismic surveys (e.g., Sato et al. 2005;Kimura et al. 2010) and earthquakebased analyses, such as seismic tomography, receiver functions, and repeating earthquakes (e.g., Kimura et al. 2006;Nakajima and Hasegawa 2007;Hirose et al. 2008;Toda et al. 2008;Nakajima et al. 2009;Igarashi 2009;Uchida et al. 2010;Ito et al. 2019;Ishise et al. 2021). Our reflection imaging results add geophysical constraints to the crustal structure models that are deeper than those obtained from controlled-source surveys while also providing higher resolution profiles than those obtained by earthquake-based seismic tomography. ...
... The upper reflector (red line) is located a few km shallower than the previously suggested PHS surface (Fig. 5, magenta line) in the western to middle part, and they converge on the eastern side, where most of the interpretations are similar around a 35 km depth. The possibility of a shallower depth of the PHS surface than that of Nakajima et al. (2009) was also suggested by Ishise et al. (2021) based on their anisotropic seismic tomography model. The depth of the upper reflector is approximately consistent with those of the PHS surface around the intersecting points in previous studies that use complementary techniques and data sets (Sato et al. 2005;Ishise et al. 2021;Kimura et al. 2006) (Fig. 5b). ...
... The possibility of a shallower depth of the PHS surface than that of Nakajima et al. (2009) was also suggested by Ishise et al. (2021) based on their anisotropic seismic tomography model. The depth of the upper reflector is approximately consistent with those of the PHS surface around the intersecting points in previous studies that use complementary techniques and data sets (Sato et al. 2005;Ishise et al. 2021;Kimura et al. 2006) (Fig. 5b). Consequently, the upper eastwarddipping reflector (red line) is likely the top of the PHS slab. ...
We applied a novel method of passive seismic reflection imaging to actual local earthquake data collected by a dense seismic network in the Kanto region, Japan. This method, which implements reverse time migration (RTM), is based on the cross-correlation of wavefields that are extrapolated forward and backward in time from receiver locations using passively observed seismic records. Using multiple reflections between the Earth’s surface and subsurface boundaries, internal structures are imaged using many earthquakes without well-defined source information. The objective of this case study is to evaluate the possibility of acquiring seismic reflection images of the deep crustal structure by applying the RTM-based method using P-wave reflections in the earthquake data collected by a dense seismic network. The P-wave reflection profile along a 191-km-long pseudo-survey line down to a 100 km depth is obtained using the seismic records of hundreds of local earthquakes observed at 72 receiver stations. A P-wave velocity model for RTM imaging is extracted from an existing 3D model obtained by seismic tomography in a previous study. The resulting image shows several continuous reflectors at depths of 15–70 km. These reflectors correspond to the spatially variable velocity and suggest deep structures related to dual plate subduction in this region. Two eastward-dipping reflectors imaged at depths of 15–50 km are likely the top and bottom surfaces of the crust of the Philippine Sea slab, and the westward-dipping reflector at depths of 50–70 km implies the top surface of the Pacific slab. The en-échelon reflectors at depths of 15–20 km may be reflective boundaries between the upper and lower crust in the overlying Okhotsk plate. Our case study results confirm the possibility of obtaining profiles at higher resolutions than are typically obtained by earthquake-based seismic tomography and of imaging at depths beyond the limits of artificially controlled-source seismic surveys. Further implementation of the RTM-based imaging method will improve its potential use for subsurface imaging and monitoring from dense passive seismic data.
Graphical Abstract
... In the past one and a half decades, a number of researchers have used the AAN tomographic methods to study the 3-D Vp structure and dynamics of many subduction zones, including New Zealand (Eberhart-Phillips and Henderson 2004; Eberhart-Phillips and Reyners 2009), NE Japan (Wang and Zhao 2008;Cheng et al. 2011;Ishise et al. 2018Ishise et al. , 2021, SW Japan (Ishise and Oda 2008;Zhao 2012, 2013), South Kuril (Koulakov et al. 2015a;Niu et al. 2016;Gou et al. 2019b;Liu et al. 2013), NW Pacific (Wei et al. 2015;Ma et al. 2019;Liang et al. 2022), Taiwan-Philippine (Koulakov et al. 2015b;Zhao 2019, 2021a), SE Asia (Koulakov et al. 2009;Hua et al. 2022;Wei et al. 2022), Alaska (Tian and Zhao 2012;Gou et al. 2019a), Cascadia (Zhao and Hua 2021), Central America (Rabbel et al. 2011), Chile (Huang et al. 2019), the Eastern Mediterranean and Middle East (Wei et al. 2019), and Alps (Hua et al. 2017;Rappisi et al. 2022). Figure 2 shows a recent example of subduction-zone AAN tomography, which was determined by a joint inversion of local and teleseismic travel-time data recorded by dense seismic networks deployed on the Cascadia margin (Zhao and Hua 2021). ...
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D variations of both structural heterogeneity and seismic anisotropy, providing unique constraints on geodynamic processes in the Earth’s crust and mantle. Here we introduce recent advances in the theory and application of seismic anisotropy tomography, thanks to abundant and high-quality data sets recorded by dense seismic networks deployed in many regions in the past decades. Applications of the novel techniques led to new discoveries in the 3-D structure and dynamics of subduction zones and continental regions. The most significant findings are constraints on seismic anisotropy in the subducting slabs. Fast-velocity directions (FVDs) of azimuthal anisotropy in the slabs are generally trench-parallel, reflecting fossil lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation due to transform faults produced at the mid-ocean ridge and intraslab hydrated faults formed at the outer-rise area near the oceanic trench. The slab deformation may play an important role in both mantle flow and intraslab fabric. Trench-parallel anisotropy in the forearc has been widely observed by shear-wave splitting measurements, which may result, at least partly, from the intraslab deformation due to outer-rise yielding of the incoming oceanic plate. In the mantle wedge beneath the volcanic front and back-arc areas, FVDs are trench-normal, reflecting subduction-driven corner flows. Trench-normal FVDs are also revealed in the subslab mantle, which may reflect asthenospheric shear deformation caused by the overlying slab subduction. Toroidal mantle flow is observed in and around a slab edge or slab window. Significant azimuthal and radial anisotropies occur in the big mantle wedge beneath East Asia, reflecting hot and wet upwelling flows as well as horizontal flows associated with deep subduction of the western Pacific plate and its stagnation in the mantle transition zone. The geodynamic processes in the big mantle wedge have caused craton destruction, backarc spreading, and intraplate seismic and volcanic activities. Ductile flow in the middle-lower crust is clearly revealed as prominent seismic anisotropy beneath the Tibetan Plateau, which affects the generation of large crustal earthquakes and mountain buildings.
... These results agree with our estimations that the tremor signals radiated from the deep part beneath the Hakone volcano, and propagated with the S-wave velocity. Ishise et al. (2021) estimated that the depth of Moho discontinuity beneath the Hakone volcano is approximately 40 km using the anisotropic tomography method. These results suggest that the tremor signal radiated around the depth level of the uppermost mantle or lower crust. ...
The feeding system of magmatic fluid from the volcanic root to a shallow magma reservoir remains a poorly understood issue. Seismic events, including volcanic tremors and low-frequency earthquakes, in a deep part beneath volcanos are key observations for understanding the feeding system at the depth. Although deep low-frequency (DLF) earthquakes beneath volcanos have been recognized universally through dense seismic observations, volcanic tremors with harmonic frequency components originating at volcanic roots have rarely been observed. Here, we report the observation of a harmonic volcanic tremor event that occurred beneath the Hakone volcano on May 26, 2019. The tremor signal continued for approximately 10 min and was recognized at seismic stations 90 km away from the Hakone volcano. The apparent velocity of the tremor wave train is 5 km/s, corresponding to the S-wave velocity of the lower crust beneath the Hakone volcano. The frequency components varied with time. In the initial part of the tremor signal, a spectrum had a broad peak of around 1.2 Hz, whereas the tremor became harmonic with a sharp fundamental peak at 0.98 Hz in the latter part, increasing its amplitude. We estimated the source location of the volcanic tremor using the relative arrival times of the waveform envelope. The optimal source locations were estimated at a deep extension of the hypocenter distribution of the DLF earthquakes beneath the Hakone volcano, around the depth level of Moho discontinuity. The DLF earthquakes were activated immediately before the onset time of the volcanic tremor and continued for several months. The harmonic volcanic tremor may have been generated by the migration of magmatic fluid in the volcano’s deep region.
Graphical Abstract
The Kanto region, Japan, has often been hit by large earthquakes, such as the 1855 Ansei-Edo Earthquake and the 1923 Kanto Earthquake, and has been damaged by strong seismic ground motion. The distribution pattern of strong ground motion, or seismic intensity, in this region depends on the location of the hypocenter because the three-dimensional seismic wave attenuation structure is complex due to the dual subduction system by the Philippine Sea and the Pacific slabs. Since the S-wave attenuation structure (QS) particularly affects the amount of the ground motion and seismic intensity, the Qs structure is essential for seismic hazards in and around the Kanto. In this study, therefore, we examined the three-dimensional Qs structure in and around the Kanto using the short-period (1-10 Hz) seismic motions. The resultant detailed Qs structure reconfirmed the presence of strong attenuation (low-Qs) zone at the depths of 20-40 km, which lay east to west beneath the border of Ibaraki and Chiba prefectures. Another low-Qs zone was detected beneath the Boso peninsula, reflecting the serpentinization of the easternmost part of the subducting Philippine Sea slab mantle. The three-dimensional Qs structure obtained in this study will improve seismic intensity predictions for earthquakes that occur in and around the Kanto region and will contribute to investigations of hypocenter locations of historical earthquakes.
Metropolitan Tokyo is subject to significant seismic hazards because it is located near a triple junction of tectonic plates and because it sits atop the soft and thick sediments of the Kanto sedimentary Basin. Numerical simulations of earthquake ground motions rely on accurate velocity models of the basin elastic and anelastic structure, which remains to be improved. Here, we leverage the density of permanent stations of the Metropolitan Seismic Observation network seismic network to construct a high‐resolution radially anisotropic (VSV ≠ VSH) shear wave velocity model of the Kanto basin. We construct surface waves using ambient seismic noise cross correlations. In sedimentary structures, it is common for several surface‐wave modes to arise and to couple in time, so we use local sub‐arrays to extract the phase velocity of the fundamental mode and first overtone of Rayleigh and Love waves at short periods (1–7 s). We find that a strong and negative shear‐wave anisotropy (−5%) is confined to the upper 300 m depth range, and a strong and positive anisotropy (+5%) at greater depths. We interpret them as a result of the vertical cracks from repeated earthquake damage and sediment stratigraphy in the Kanto Basin. Because anisotropy models are not routinely used in physics‐based ground motion prediction, our study motivates the consideration of seismic anisotropy in sedimentary basins for the ground motion of future earthquakes.
