Transition from collision to subduction and its relation to slab seismicity and plate coupling
© Arai and Iwasaki; licensee Springer. 2015
Received: 15 March 2015
Accepted: 14 May 2015
Published: 27 May 2015
The Philippine Sea plate exhibits an along-trench variation in structure, from an eastern volcanic arc to a typical oceanic crust in the west. The regional difference of intra-slab seismicity implies that this transition occurs around the Itoigawa-Shizuoka Tectonic Line (ISTL). However, the nature of the subducting slab in this region has not been studied in detail. Here, we investigate the structure of the Philippine Sea plate subducting beneath the ISTL. Using active source data, we found reflective portions at depths of 14–18 km. An amplitude evaluation for the reflection phases showed that the reflective zone has a P wave velocity as low as 3.0 km/s. Together with its slightly northwestward-dipping geometry, we interpreted that the low-velocity zone is at the plate boundary and can be attributed to high water content supplied by the slab. This feature is in great contrast to the collision zone further east, where the slab top is less reflective and the dehydration process is inactive. This structural difference also correlates well with the regional distribution of slab seismicity. The reflective zones we found are likely located at the down-dip end of the locked zone, where high slip deficit rates are currently observed. This may suggest that changing fluid pressures and the resulting frictional properties in the down-dip direction control the transition from a coseismic rupture zone to a deeper aseismic zone.
This systematic variation in fluid distribution correlates well with the regional distribution of intra-slab seismicity; Seno and Yamasaki (2003) pointed out that, unlike the Nankai subduction zone, there is no intraslab seismicity beneath the Izu collision zone and indicated that the dehydration process is a controlling factor for the seismogenic process. This along-trench change in seismicity occurs suddenly at the Itoigawa-Shizuoka Tectonic Line (ISTL), a geological boundary between the northeast Japan and southwest Japan arcs, implying that the ISTL coincides spatially with the structural transition of the Philippine Sea plate from arc-like crust to oceanic crust. On the other hand, onshore seismic surveys and bathymetric features indicate that not oceanic crust but arc-like crust (several kilometers thicker than typical oceanic crust) exists just south of the ISTL (Nakanishi et al. 1998; Nishizawa et al. 2006; Kodaira et al. 2008), leaving the open question of the nature of the Philippine Sea plate subducting beneath the ISTL.
Several seismic experiments were carried out around the southernmost part of the ISTL in the last few decades (Research Group for Explosion Seismology 1988, 1992), and these experiments have the potential to constrain the eastern limit of the subduction system in the western Nankai subduction zone (Fig. 1). However, these seismic data were not fully examined in the context of the transition of the Philippine Sea plate mentioned above. In this paper, we reinterpret them in order to determine the nature of the slab and its relation to intraslab seismicity, especially in relation to the dehydration process within the oceanic lithosphere.
The Nankai subduction zone is also known as a seismogenic zone where large megathrust earthquakes have repeatedly occurred (e.g., Ando 1975). Around the southernmost ISTL, a strongly coupled zone outlined by geodetic measurements and seismic profiles used in this paper cross the zone where large slip deficit rates are currently observed (Suito and Ozawa 2009). Spatial variation in frictional properties along plate boundaries often appear as a difference in the seismic reflectivity of active source data; large-amplitude reflections are found in aseismic (conditionally stable) regions with weak plate coupling, while weakly reflective interfaces correspond to locked zones that rupture during a large earthquake (Kodaira et al. 2004; Nakanishi et al. 2004; Mochizuki et al. 2005). We used this correlation between the intensity of seismic reflections and frictional properties to investigate the structure atop the slab. Our seismic data provide insights on the down-dip end of the rupture zone along the plate interface in the easternmost Nankai subduction zone.
Data and methods
Two seismic experiments along the southernmost ISTL carried out by Research Group for Explosion Seismology (RGES) (Fig. 1) are expected to provide important structural constraints on the easternmost part of the area of high intraslab seismicity of the Philippine Sea plate; the eastern profile, laid out almost parallel to the ISTL, was carried out in 1983 (Research Group for Explosion Seismology 1988), and the western profile was performed in 1987 on the western side of the ISTL (Research Group for Explosion Seismology 1990, 1992) (Fig. 1b). Both seismic lines were approximately 60-km-long profiles and 71 (eastern profile) and 77 (western profile) stations were deployed along these lines. As seismic sources, five (eastern profile) and six (western profile) dynamite shots (with a charge of 300–700 kg) were fired. Seismic waves were recorded at a sampling rate of 100 Hz at all stations.
In order to explain these reflection phases, we constructed P wave velocity models for both profiles by forward modeling using a ray tracing method (Iwasaki 1988). The P wave velocity models for the upper 5 km of the crust were constructed from first arrivals. Then, the model was extrapolated to deeper parts using velocity models from other nearby active-source experiments (Arai et al. 2009; Arai and Iwasaki 2014) and regional passive-source tomography (Matsubara et al. 2008). Thereafter, we mapped a reflector using travel times of high-amplitude reflections. Finally, synthetic seismograms were calculated using the code SEIS83 (Červený and Pšenčík 1983) to evaluate the velocity contrast across the reflector.
Results and discussions
A distinct reflector was imaged at 14 km depth in the eastern profile (Fig. 2b) and 18 km depth in the western profile (Fig. 2e). Based on the misfit of travel times (within 0.1 s) and the uncertainty of the velocity model (0.5 km/s), we roughly estimated the error for the depth of the reflector to be less than 2.0 km. To estimate the velocity contrast across the reflective zone, Fig. 2 compares observed seismic records of SP-W1 and SP-E4 with synthetic seismograms calculated by models in which a low-velocity layer with Vp = 2.0–3.0 km/s was assumed at depth. The thickness of the layer was assumed to be 100 m according to previous studies (e.g., Mochizuki et al. 2005). We see that the observed high-amplitude reflections are successfully reproduced by these models. Figure 3 examines other models by assuming different velocity values for the layer. We confirmed that the seismic record of SP-W5 also prefers a model with a low-velocity layer of Vp = 3.0 km/s (Fig. 3a–c). The synthetic seismograms from a model with Vp = 4.0 km/s may also be acceptable (Fig. 3d), implying that the velocity value for the layer includes a large error of up to 1.0 km/s. However, a model with Vp = 6.0 km/s (Fig. 3e) and no low-velocity zone (Fig. 3f) cannot explain the reduced amplitude of observed first arrivals.
Although the depth of the reflector in this study is 5–10 km shallower than the slab depth proposed by passive-source seismic data (e.g., Nakajima et al. 2009), the geometry (gently dipping northwestward) is consistent. This discrepancy may arise from the large uncertainty of the slab location in previous models because of its distorted geometry in this region and lack of deep seismicity in the Izu collision zone to the east. Together with the necessity for a low-velocity zone along the reflector as explained above, this suggests that the reflector is the top of the subducting Philippine Sea slab, forming a thin low-velocity layer. We also examined other interpretations for the reflector, such as the Moho occurring beneath the island arc and a mid-crustal reflector, and excluded both possibilities for the following reasons. First, the reflector we observed is located at 14–18 km depths, which is more than 15 km shallower than the Moho depth estimated in the study area (Kato et al. 2010; Katsumata 2010). Second, the velocity jump across the reflector needs to be more than 2.0 km/s. On the other hand, typical velocity contrasts at a mid-crustal reflector and Moho are less than 1.0 km/s (Arai et al. 2009). Thus, it is unlikely that the reflector is either of these.
The estimated velocity values for the reflector (2.0–4.0 km/s) are consistent with those proposed by other seismic experiments in the Nankai subduction zone (Kodaira et al. 2002; Iidaka et al. 2004). According to Kodaira et al. (2002), a thin low-velocity layer lying between the subducting oceanic plate and the overriding landward plate has a high water content, expelled from the downgoing crust and/or serpentinized mantle. Although the seismic data in this study cannot constrain the internal structure of the slab, we speculate from the similarity in the reflection phases that the interpretation by Kodaira et al. (2002) is also true for the easternmost Nankai trough. Our interpretation of the reflective zone as having a high fluid content is supported by studies of electrical resistivity; a resistivity model by Aizawa et al. (2004) showed that a conductive zone exists just above the subducting Philippine Sea slab on the western side of Mt. Fuji, close to our study area, while such a body is not imaged on the eastern side. Matsubara et al. (2008) pointed out a similar contrast from regional seismic tomography, namely, the slab in the Nankai subduction zone, imaged as a zone with low P wave and S wave velocities and high Vp/Vs ratios, indicates a high water content, while the subducting portion beneath the Izu collision zone is imaged as a high-velocity anomaly.
Our seismic data may provide another important constraint on the frictional property along the plate interface, since the seismic structure is closely correlated to interplate coupling (e.g., Fujie et al. 2013). In the easternmost Nankai subduction zone, a strongly coupled zone is outlined by geodetic measurements (Sagiya 1999; Suito and Ozawa 2009). The estimated slip deficit rate of >20 mm/year, almost equivalent to the convergence rate between the overriding and subducting plates, implies that strain is being steadily accumulated at the contact, and will probably lead to coseismic rupture during a megathrust earthquake (Ando 1975). Our seismic lines in the north–south direction cross the transitional boundary between a strongly coupled zone and a weakly coupled zone along the plate boundary. The reflective zones we found are likely located close to the down-dip end of this locked zone (Fig. 4b). This spatial correlation may suggest that the reflective zone delimits the down-dip end of the coseismic rupture zone. Similar spatial patterns of seismic reflectivity (a shallow coseismic region with low reflectivity and a deeper aseismic region with relatively high reflectivity) are also observed in other areas of the Nankai subduction zone (Kodaira et al. 2002, 2004; Iidaka et al. 2004). In the Tokai region further west of this study, Kodaira et al. (2004) correlated this reflective zone with the distribution of slow slips, and suggested that high-pressure fluids supplied by hydrous minerals within the slab effectively extend a region of stable slips, and consequently generate the slow slip. Changing fluid pressures and the resulting frictional properties in the down-dip direction probably control the transition from a coseismic rupture zone to a deeper aseismic zone.
The reflective zone we observed is 10–15 km wide in the down-dip direction, which is well constrained by the fact that high-amplitude reflections are not observed in shot records other than those shown in Figs. 2 and 3. In the western part of the Tokai region where the slow slips occur, the reflective zone is located at depths of approximately 30 km and is extended over a much broader region in the down-dip direction than in this study (Iidaka et al. 2004). The difference in slab depth must produce the difference in the nature of dehydration from the subducting plate and the frictional property along the plate boundary.
We found a reflective zone at 14–18 km depths at the eastern end of the Nankai subduction zone. An amplitude evaluation of the reflections showed that the layer has a P wave velocity as low as 3.0 km/s, which we interpreted as a low-velocity layer along the plate boundary that is probably enriched in fluids supplied by the subducting slab. The high-amplitude reflection phases are similar to those observed in other regions of the Nankai subduction zone and are in a great contrast to the Izu collision zone further east where the slab top is less reflective and the dehydration process is inactive. This structural difference also agrees well with the sudden increase in slab seismicity across the ISTL. Therefore, we suggest that, similar to the western Nankai subduction zone, the fluid-rich crust is subducting beneath the ISTL, and dehydrated fluids are enhancing the intraslab seismicity. The reflective zones we found are likely located at the down-dip end of the locked zone where high slip deficit rates are currently observed, which may suggest that the reflective zone with high water content delimits the down-dip end of the coseismic rupture zone.
We are grateful to the members of the Research Group for Explosion Seismology for the data acquisition. We also thank the editor and two anonymous reviewers for useful comments and the Japan Meteorological Agency for the hypocentral data. We used the GMT software (Wessel and Smith 1998) to draw the figures.
- Aizawa K, Yoshimura R, Oshiman N (2004) Splitting of the Philippine Sea Plate and a magma chamber beneath Mt. Fuji Geophys Res Lett 31:L09603. doi:10.1029/2004GL019477Google Scholar
- Ando M (1975) Possibility of a major earthquake in the Tokai district, Japan and its pre-estimated seismotectonic effects. Tectonophysics 25:69–85View ArticleGoogle Scholar
- Aoike K (1999) Tectonic evolution of the Izu collision zone. Res Rep Kanagawa prefect Mus Nat His 9:111–151Google Scholar
- Arai R, Iwasaki T (2014) Crustal structure in the northwestern part of the Izu collision zone in central Japan. Earth Planets Space 66:21View ArticleGoogle Scholar
- Arai R, Iwasaki T, Sato H, Abe S, Hirata N (2009) Collision and subduction structure of the Izu-Bonin arc, central Japan, revealed by refraction/wide-angle reflection analysis. Tectonophysics 475:438–453View ArticleGoogle Scholar
- Arai R, Iwasaki T, Sato H, Abe S, Hirata N (2013) Crustal structure of the Izu collision zone in central Japan from seismic refraction data. J Geophys Res Solid Earth 118:6258–6268. doi:10.1002/2013JB010532View ArticleGoogle Scholar
- Arai R, Iwasaki T, Sato H, Abe S, Hirata N (2014) Contrasting subduction structures within the Philippine Sea plate: Hydrous oceanic crust and anhydrous volcanic arc crust. Geochem Geophys Geosyst 15:1977–1990. doi:10.1002/2014GC005321View ArticleGoogle Scholar
- Červený V, Pšenčík I (1983) Program package SEIS83. Charles University, PragueGoogle Scholar
- Fujie G, Miura S, Kodaira S, Kaneda Y, Shinohara M, Mochizuki K, Kanazawa T, Murai Y, Hino R, Sato T, Uehira K (2013) Along-trench structural variation and seismic coupling in the northern Japan subduction zone. Earth Planets Space 65:75–83View ArticleGoogle Scholar
- Iidaka T, Takeda T, Kurashimo E, Kawamura T, Kaneda Y, Iwasaki T (2004) Configuration of subducting Philippine Sea plate and crustal structure in the central Japan region. Tectonophysics 388:7–20View ArticleGoogle Scholar
- Iwasaki T (1988) Ray-tracing program for study of velocity structure by ocean bottom seismographic profiling. J Seismol Soc Jpn 41:263–266Google Scholar
- Kato A, Iidaka T, Ikuta R, Yoshida Y, Katsumata K, Iwasaki T, Sakai S, Thurber C, Tsumura N, Yamaoka K, Watanabe T, Kunitomo T, Yamazaki F, Okubo M, Suzuki S, Hirata N (2010) Variations of fluid pressure within the subducting oceanic crust and slow earthquakes. Geophys Res Lett 37:L14310. doi:10.1029/2010GL043723Google Scholar
- Katsumata A (2010) Depth of the Moho discontinuity beneath the Japanese islands estimated by traveltime analysis. J Geophys Res 115:B04303. doi:10.1029/2008JB005864Google Scholar
- Kodaira S, Kurashimo E, Park JO, Takahashi N, Nakanishi A, Miura S, Iwasaki T, Hirata N, Ito K, Kaneda Y (2002) Structural factors controlling the rupture process of a megathrust earthquake at the Nankai trough seismogenic zone. Geophys J Int 149:815–835View ArticleGoogle Scholar
- Kodaira S, Iidaka T, Kato A, Park J, Iwasaki T, Kaneda Y (2004) High pore fluid pressure may cause silent slip in the Nankai trough. Science 304:1295–1298View ArticleGoogle Scholar
- Kodaira S, Sato T, Takahashi N, Ito A, Tamura Y, Tatsumi Y, Kaneda Y (2007) Seismological evidence for variable growth of crust along the Izu intraoceanic arc. J Geophys Res 112:B05104. doi:10.1029/2006JB004593Google Scholar
- Kodaira S, Sato T, Takahashi N, Yamashita M, No T, Kaneda Y (2008) Seismic imaging of a possible paleoarc in the Izu-Bonin intraoceanic arc and its implications for arc evolution processes. Geochem Geophys Geosyst 9:Q10X01. doi:10.1029/2008GC002073View ArticleGoogle Scholar
- Kurashimo E, Tokunaga M, Hirata N, Iwasaki T, Kodaira S, Kaneda Y, Ito K, Nishida R, Kimura S, Ikawa T (2002) Geometry of the subducting Philippine Sea Plate and the crustal and upper mantle structure beneath eastern Shikoku Island revealed by seismic refraction/wide-angle reflection profiling. Zisin 54:489–505Google Scholar
- Matsubara M, Obara K, Kasahara K (2008) Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography. Tectonophysics 454:86–103View ArticleGoogle Scholar
- Mochizuki K, Nakamura M, Kasahara J, Hino R, Nishino M, Kuwano A, Nakamura Y, Yamada T, Shinohara M, Sato T, Moghaddam PP, Kanazawa T (2005) Intense PP reflection beneath the aseismic forearc slope of the Japan Trench subduction zone and its implication of aseismic slip subduction. J Geophys Res 110:B01302. doi:10.1029/2003JB002892Google Scholar
- Nakajima J, Hirose F, Hasegawa A (2009) Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity. J Geophys Res 114:B08309. doi:10.1029/2008JB006101Google Scholar
- Nakanishi A, Shiobara H, Hino R, Kodaira S, Kanazawa T, Shimamura H (1998) Detailed subduction structure across the eastern Nankai Trough obtained from ocean bottom seismograph profiles. J Geophys Res 103:27151–27168View ArticleGoogle Scholar
- Nakanishi A, Smith AJ, Miura S, Tsuru T, Kodaira S, Obana K, Takahashi N, Cummins PR, Kaneda Y (2004) Structural factors controlling the coseismic rupture zone of the 1973 Nemuro-Oki earthquake, the southern Kuril Trench seismogenic zone. J Geophys Res 109:B05305. doi:10.1029/2003JB002574Google Scholar
- Nakanishi A, Kodaira S, Miura S, Ito A, Sato T, Park JO, Kido Y, Kaneda Y (2008) Detailed structural image around splay-fault branching in the Nankai subduction seismogenic zone: Results from a high-density ocean bottom seismic survey. J Geophys Res 113:B03105. doi:10.1029/2007JB004974Google Scholar
- Nishizawa A, Kaneda K, Nakanishi A, Takahashi N, Kodaira S (2006) Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin. Earth Planets Space 58:e33–e36View ArticleGoogle Scholar
- Okino K, Ohara Y, Kasuga S, Kato Y (1999) The Philippine Sea: New survey results reveal the structure and the history of the marginal basins. Geophys Res Lett 26:2287–2290View ArticleGoogle Scholar
- Research Group for Explosion Seismology (1988) Explosion seismic observations in Yamanashi and Shizuoka Prefecture, Central Japan: Kushigata-Shimizu profile. Bull Earthq Res Inst Univ Tokyo 63:1–22Google Scholar
- Research Group for Explosion Seismology (1990) Explosion seismic observations on Hayakawa-Shizuoka profile. Abstr Programs Seismol Soc Jpn 1:E11–07Google Scholar
- Research Group for Explosion Seismology (1992) Explosion seismic observations along the southern part of the Itoigawa-Shizuoka Tectonic Line, Hayakawa-Shizuoka profile. Bull Earthq Res Inst Univ Tokyo 67:303–323Google Scholar
- Sagiya T (1999) Interplate coupling in the Tokai District, Central Japan, deduced from continuous GPS data. Geophys Res Lett 26:2315–2318View ArticleGoogle Scholar
- Sato H, Hirata N, Koketsu K, Okaya D, Abe S, Kobayashi R, Matsubara M, Iwasaki T, Ito T, Ikawa T, Kawanaka T, Kasahara K, Harder S (2005) Earthquake source fault beneath Tokyo. Science 309:462–464View ArticleGoogle Scholar
- Seno T, Yamasaki T (2003) Low-frequency tremors, intraslab and interplate earthquakes in southwest Japan—from a view point of slab dehydration. Geophys Res Lett 30:2171. doi:10.1029/2003GL018349View ArticleGoogle Scholar
- Seno T, Stein S, Gripp AE (1993) A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J Geophys Res 98:17941–17948. doi:10.1029/93JB00782View ArticleGoogle Scholar
- Stern RJ, Fouch MJ, Klemperer SL (2003) An overview of the Izu-Bonin-Mariana subduction factory. In: Eiler J (ed) Inside the subduction factory. American Geophysical Union, WashingtonGoogle Scholar
- Suito H, Ozawa T (2009) Transient crustal deformation in the Tokai district. J Seismol Soc Jpn 2(61):113–135Google Scholar
- Wessel P, Smith WHF (1998) New improved version of the generic mapping tools released. Eos Trans AGU 79:579View ArticleGoogle Scholar
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