Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan
© Shiina et al.; licensee Springer. 2014
Received: 14 December 2013
Accepted: 30 June 2014
Published: 11 July 2014
At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.
The subducting crust at the uppermost part of the oceanic lithosphere is considered to play important roles in fluid circulation in subduction zones because the crust contains a large amount of water in the form of hydrous minerals (e.g., Hacker et al. 2003). Aqueous fluids and volatiles released by dehydration of hydrous minerals contribute to the genesis of intraslab earthquakes (e.g., Kirby et al. 1996) and arc magmatism (e.g., Nakajima et al. 2013).
In cold subduction zones, the subducting crust has been imaged as a low-velocity and high-V p/V s layer at depths of <100 km in which the seismic velocity increases at greater depths (e.g., Ferris et al. 2003; Kawakatsu and Watada 2007; Nakajima et al. 2009a). The increase in velocity in the crust appears to be correlated with an abrupt decrease in seismic activity beyond a depth range of the upper plane seismic belt that is defined as a concentrated crustal seismicity at depths of 70 to 90 km (Kita et al. 2006). This phenomenon suggests that earthquakes in the crust are facilitated as a result of substantial pore fluid generated by dehydration reactions to eclogite from hydrous minerals (e.g., Abers et al. 2013). Therefore, investigations of the locations of which hydrous minerals are hosted and dehydration reaction occurs are important for understanding ongoing metamorphism and the resultant processes in subduction zones.
Observations of prominent later phases
Seismograms recorded in and around the Hidaka Mountain range in the middle of Hokkaido show different features from those observed in its western and eastern sides in terms of amplitude and frequency components of the initial P and S waves (e.g., Furumura and Moriya 1990). In this region, the arc-arc collision between the Kuril and northeastern Japan arcs is ongoing (e.g., Kimura 1996), and the structure is highly complex (e.g., Iwasaki et al. 2004; Kita et al. 2012).
Shimizu and Maeda (1980) reported later phases that have characteristics similar to those of the X phase, and they concluded that the later phases were generated by a P-to-P reflection at an inclined reflector beneath the Hidaka Mountain range. As a benefit of the nationwide dense seismograph network in Japan, we can observe the X phase at stations distributed in a wider area than that in Shimizu and Maeda (1980). The X phase is difficult to interpret as a P-to-P reflection wave at the reflector proposed by Shimizu and Maeda (1980), but it is attributable to highly heterogeneous structures in the Pacific slab.One possible origin of the X phase is a mode-converted wave at velocity discontinuities between the source and receiver. If we assume the X phase to be an S-to-P converted wave (SP wave) at the Pacific slab interface, the time difference between SP and P waves is 1 to 4 s, which increases with epicentral distance (gray-shaded area in Figure 3). These time differences are too small to explain the characteristics of the X phase, such as characteristics 1 and 4 listed above. Therefore, we excluded the SP wave from the Pacific slab interface in the origin of the X phase. If we assume that the X phase is an SP wave from the continental Conrad or Moho, the phase should be observed only at stations near the epicenter because the incident angle to the discontinuities must be less than the critical angle for SP conversion. This contradicts the observation that the X phase appears only at stations with large epicentral distances (Figure 1); hence, the SP wave at either the Conrad or Moho is not a plausible candidate for the origin of the X phase.
Guided waves are known to be generated in the low-velocity subducting crust when earthquakes occur in or immediately below the crust (e.g., Martin et al. 2003; Miyoshi et al. 2012). Time differences between guided and initial waves increase with propagation distance in the subducting crust (e.g., Ohkura 2000). Because the seismic energy is efficiently trapped in the crust when an earthquake is located in the low-velocity crust, the guided P wave shows a larger amplitude than the P wave and dominates in the vertical component (e.g., Martin and Rietbrock 2006). Therefore, guided P waves in the subducting crust can explain observed characteristics 2, 3, and 4. Amplitudes observed for the X phase are dominant in frequencies of 2 to 4 Hz; this frequency range is comparable to that observed for guided waves of the subducting crust in other subduction zones (e.g., Martine et al. 2003). The existence of a serpentinized layer in the Pacific slab mantle (Garth and Rietbrock 2014) is a plausible candidate for the origin of the X phase when earthquakes occur in the mantle. However, the earthquakes for which we observe guided waves are mostly located at the uppermost part of the Pacific slab (Figure 4 and characteristic 5). On the basis of these observations, we consider that guided P waves generated in the low-velocity subducting crust create the X phase.
Model parameters used in numerical modeling
P-wave velocity [km/s]
We evaluated wave fields in a sub-parallel profile to the trench axis because Shimizu and Maeda (1980) concluded that both P and X phases propagate in the same great circle. Because the geometries of the Pacific slab and seismic velocities vary in the subduction direction perpendicular to the assumed profile, an energy leak to a third dimension likely occurs. However, in this study, we focused on only the relative amplitude and time difference of the initial P and guided waves and do not evaluate the absolute amplitude because the effects of this three-dimensional (3D) structure would be small. However, waveform modeling for a realistic 3D model is an important subject for future study.
Results and discussion
In the waveform calculations, we considered two velocity models: a standard model with only the low-velocity subducting crust and a model that additionally involves thick low-velocity materials overlying the Pacific slab based on the result of Kita et al. (2010, 2012).
The numerical simulation for the velocity model with the deepened low-velocity zone indicated that guided waves arrive at stations 2 to 4 s after P waves with apparent velocities similar to P waves. These results explain characteristic 1. Additionally, our results suggest that the contact of the subducting crust with the overlying low-velocity material significantly contributes to release the energy trapped in the subducting crust. Marked X phases observed in a wide area of the western side of the Hidaka Mountain range, as summarized in characteristic 5, are explained by a wide extent of the contact zone.
We interpret the X phase as a guided P wave generated in the low-velocity subducting crust because the characteristics of the X phase can be explained by the propagation of guided waves in the crust. Our interpretations support the results of seismic tomography by Kita et al. (2010, 2012) and provide important and independent evidence for the existence of the low-velocity material overlying the subducting crust beneath the Hidaka Mountain range.
P-wave velocity in the subducting crust
where V p is the P-wave velocity in the subducting crust, L is the inter-event distance, and ΔtX is the travel time difference of X phases at common stations. We assume that all earthquakes analyzed in this study are located in the subducting crust. Because errors in picking X phases and in the origin time are both 0.1 ~ 0.3 s and errors in hypocenters are generally 2 to 4 km, we used only earthquake pairs with inter-event distances >100 km and back azimuthal differences <10° to make the effect of possible errors on the estimates of P-wave velocity as small as possible. Hence, possible errors included in observations are equivalent to errors of travel times <1 s, which results in 5% estimation error of seismic velocity.
The observed velocity may have been affected by anisotropy due to normal faulting formed at the trench outer slope (e.g., Faccenda et al. 2008) because guided waves tend to propagate sub-parallel to strikes of faults. This effect would yield an apparent high velocity of the subducting crust; however, it is difficult to evaluate the effect of anisotropy at present because the geometries and densities of faults are poorly understood. A sedimentary layer located at the top of the subducting slab (e.g., Horleston and Hellfrich 2012) would result in an apparent low velocity of the crust because the layer has a low velocity than that of MORB and gabbro. However, guided waves have dominant frequencies of 2 to 4 Hz. Therefore, they likely represent subducting crust with thicknesses of about 7 km rather than sedimentary layers with thicknesses of 0.5 km (e.g., Martin et al. 2003; Abers 2005).
Shiina et al. (2013) showed the existence of free water in the crust at depths of 60 to 90 km beneath northeastern Japan, which is consistent with depths of dehydration reactions of hydrous minerals (Abers et al. 2013) and concentrated seismicity in the crust (Kita et al. 2006). Although the obtained velocity at the eastern part of Hokkaido was slightly higher than that in northeastern Japan (Figure 7b), which may be associated with the apparent high velocity due to fault-induced anisotropy, the value is still lower than that expected for hydrated compositions of the crust even with anisotropy of 2% to 3% (Fujimoto et al. 2010). Therefore, we consider that free water co-exists with hydrous minerals in the crust in eastern Hokkaido. The observed P-wave velocity is reduced by an average of 7% from the MORB model, and the S-wave velocity reduction from the MORB model (4.1 km/s; Hacker et al. 2003) is calculated to be approximately 15%, assuming that V p/V s in the subducting crust is 1.90 (e.g., Tsuji et al. 2008). Based on the research of Takei (2002), the obtained P-wave velocity can be explained by fluid fractions of <1 vol.% in the crust with equivalent aspect ratios of 0.01.
A clear later phase was observed in seismograms in the western side of the Hidaka Mountain range, the origin of which we determined through numerical modeling. From the obtained observations and results of numerical simulations, we interpreted the later phase as a guided P wave generated in the low-velocity subducting crust. The efficient energy leakage from the crust provides important evidence for the subducting crust being in contact with the overlying low-velocity material beneath the Hidaka Mountain range. The average P-wave velocities estimated from the travel times of the guided waves were 6.8 to 7.5 km/s at depths of 50 to 80 km, suggesting that hydrous minerals are involved in the subducting crust at depths of at least 80 km beneath the eastern part of Hokkaido.
We thank the editor Bruno Reynard and two anonymous reviewers for the thoughtful reviews. We used waveform data observed at a nationwide seismograph network; the arrival time, data, and hypocenter were obtained from the unified catalog of the Japan Meteorological Agency. We thank S. Kita for the fruitful discussions. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions. All of the figures in this paper were generated by using the GMT software of Wessel and Smith (1998).
- Abers GA: Seismic low-velocity layer at the top of subducting slabs: observations, predictions, and systematics. Phys Earth Planet Inter 2005, 149: 7–29. doi:10.1016/j.pepi.2004.10.002View ArticleGoogle Scholar
- Abers GA, Plank T, Hacker BR: The wet Nicaraguan slab. Geophys Res Lett 2003, 30(2):1098. doi:10.1029/2002GRL015649View ArticleGoogle Scholar
- Abers GA, Nakajima J, van Keken PE, Kita S, Hacker BR: Thermal–petrological controls on the location of earthquakes within subducting plates. Earth Planet Sci Lett 2013, 369–370: 178–187. doi:10.1016/j.epsl.2013.03.022View ArticleGoogle Scholar
- Faccenda M, Burlini L, Gerya TV, Maniprice D: Fault-induced seismic anisotropy by hydration in subducting oceanic crust. Nature 2008, 455: 1097–1100.View ArticleGoogle Scholar
- Ferris A, Abers GA, Christensen DH, Veenstra E: High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50–150 km depth. Earth Planet Sci Lett 2003, 214: 576–588. doi:10.1016/S0012-821X(03)00403-5View ArticleGoogle Scholar
- Fujimoto Y, Kono Y, Hirajima T, Kanagawa K, Ishikawa M, Arima M: P-wave velocity and anisotropy of lawsonite and epidote blueschists: constraints on water transportation along subducting oceanic crust. Phys Earth Planet Inter 2010, 183: 219–228. doi:10.1016/j.pepi.2010.09.003View ArticleGoogle Scholar
- Furumura T, Kennett BLN: Subduction zone guided waves and the heterogeneity structure of the subducted plate: intensity anomalies in northern Japan. J Geophys Res 2005., 110: B10302 doi:10.1029/2004JB003486Google Scholar
- Furumura T, Moriya T: Three-dimensional Q structure in and around Hidaka Mountains, Hokkaido, Japan. Zishin II 1990, 43: 121–132. (in Japanese with English abstract) (in Japanese with English abstract)Google Scholar
- Garth T, Rietbrock A: Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati–Benioff zone. Geology 2014, G34730: 1. doi:10.1130/G34730.1Google Scholar
- Hacker BR, Abers GA, Peacock SM: Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents. J Geophys Res 2003, 108(B1):2029.View ArticleGoogle Scholar
- Hori S, Inoue H, Fukao Y, Ukawa M: Seismic detection of the untransformed ‘basaltic’ oceanic crust subducting into the mantle. Geophys J R Astron Soc 1985, 83: 169–197.View ArticleGoogle Scholar
- Horleston AC, Helffrich GR: Constraining sediment subduction: a converted phase study of the Aleutians and Marianas. Earth Planet Sci Lett 2012, 359–360: 141–151. doi:10.1016/j.epsl.2012.10.019View ArticleGoogle Scholar
- Iwasaki T, Adachi K, Moriya T, Miyamachi H, Matsushima T, Miyashita K, Takeda T, Taira T, Yamada T, Ohtake K: Upper and middle crustal deformation of an arc-arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments. Tectonophysics 2004, 388: 59–73.View ArticleGoogle Scholar
- Katsumata A: Depth of the Moho discontinuity beneath the Japanese islands estimated by travel time analysis. J Geophys Res 2010., 115: B04303 doi:10.1029/2008JB005864Google Scholar
- Kawakatsu H, Watada S: Seismic evidence for deep-water transportation in the mantle. Science 2007, 316: 1468–1471. doi:10.1126/science.1140855View ArticleGoogle Scholar
- Kimura G: Collision orogeny at arc-arc junctions in the Japanese Islands. Isl Arc 1996, 5: 262–275. doi:10.1111/j.1440-1738.1996.tb00031.xView ArticleGoogle Scholar
- Kirby S, Engdahl ER, Denlinger R: Intermediate-depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs. Geophys Monogr 1996, 96: 195–214.Google Scholar
- Kita S, Okada T, Nakajima J, Matsuzawa T, Hasegawa A: Existence of a seismic belt in the upper plane of the double seismic zone extending in the along-arc direction at depths of 70–100 km beneath NE Japan. Geophys Res Lett 2006, 33: L24310. doi:10.1029/2006GL028239View ArticleGoogle Scholar
- Kita S, Okada T, Hasegawa A, Nakajima J, Matsuzawa T: Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: possible evidence for thermal shielding caused by subducted forearc crust materials. Earth Planet Sci Lett 2010, 290: 415–426. doi:10.1016/j.epsl.2009.12.038View ArticleGoogle Scholar
- Kita S, Hasegawa A, Nakajima J, Okada T, Matsuzawa T, Katsumata K: High-resolution seismic velocity structure beneath the Hokkaido corner, northern Japan: arc-arc collision and origins of the 1970 M 6.7 Hidaka and 1982 M 7.1 Urakawa-oki earthquakes. J Geophys Res 2012, 171: B12301.View ArticleGoogle Scholar
- Martin S, Rietbrock A: Guided waves at subduction zones: dependencies on slab geometry, receiver locations and earthquakes sources. Geophys J Int 2006, 167: 693–704. doi:10.1111/j.1365-246X.2006.02963.xView ArticleGoogle Scholar
- Martin S, Rietbrock A, Haberland C, Asch G: Guided waves propagating in subducted oceanic crust. J Geophys Res 2003, 108(B11):2536. doi:10.1029/2003JB002450View ArticleGoogle Scholar
- Martin S, Haberland C, Rietbrock A: Forearc decoupling of guided waves in the Chile-Peru subduction zone. Geophys Res Lett 2005., 32: L23309 doi:10.1029/2005GL024183Google Scholar
- Matsuzawa T, Ummino N, Hasegawa A, Takagi A: Upper mantle velocity structure estimated from PS-converted wave beneath the north-eastern Japan arc. Geophys J R Astron Soc 1986, 86: 767–787. doi:10.1111/j.1365-246X.1986.tb000659.XView ArticleGoogle Scholar
- Miyoshi T, Saito T, Shiomi K: Waveguide effects within the Philippine Sea slab beneath southwest Japan inferred from guided SP converted wave. Geophys J Int 2012, 189: 1075–1084. doi:10.1111/j.1365-246X.2012.05409.XView ArticleGoogle Scholar
- Nakajima J, Tsuji Y, Hasegawa A: Seismic evidence for thermally-controlled dehydration reaction in subducting oceanic crust. Geophys Res Lett 2009., 36: L03303 doi:10.1029/2008GL036865Google Scholar
- Nakajima J, Hirose F, Hasegawa A: Seismotectonics beneath the Tokyo metropolitan area, Japan: effect of slab-slab contact and overlap on seismicity. J Geophys Res 2009., 114: B08309 doi:10.1029/2008JB006101Google Scholar
- Nakajima J, Hada S, Hayami E, Uchida N, Hasegawa A, Yoshioka T, Matsuzawa T, Umino N: Seismic attenuation beneath northeastern Japan: constraints on mantle dynamics and arc magmatism. J Geophys Res Solid Earth 2013. doi:10.1002/2013JB010388Google Scholar
- Ohkura T: Structure of the upper part of the Philippine Sea plate estimated by later phases of upper mantle earthquakes in and around Shikoku, Japan. Tectonophysics 2000, 321: 17–36.View ArticleGoogle Scholar
- Reynard B, Bass JD: Elasticity of lawsonite and seismological signature of metamorphism and water cycling in the subducting oceanic crust. J Metamorphic Geol 2014, 32: 479–487. doi:10.1111/jmg.12072View ArticleGoogle Scholar
- Shiina T, Nakajima J, Matsuzawa T: Seismic evidence for high pore pressures in the oceanic crust: implications for fluid-related embrittlement. Geophys Res Lett 2013, 40: 2006–2010. doi:10.1002/grl.50468View ArticleGoogle Scholar
- Shimizu N, Maeda I: Analysis of seismic waves with the remarkable phases observed at station KMU. Zishin II 1980, 33: 141–155. (in Japanese with English abstract) (in Japanese with English abstract)Google Scholar
- Takei Y: Effect of pore geometry on Vp/Vs: from equilibrium geometry to crack. J Geophys Res 2002, 170(B2):2043. doi:10.1029/2001JB000522View ArticleGoogle Scholar
- Tsuji Y, Nakajima J, Hasegawa A: Tomographic evidence for hydrated oceanic crust of the Pacific slab beneath northeastern Japan: implications for water transportation in subduction zones. Geophys Res Lett 2008., 35: L14308 doi:10.1029/2008GL034461Google Scholar
- Ueno H, Hatakeyama S, Aketagawa T, Funasaki J, Hamada N: Improvement of hypocenter determination procedures in the Japan Meteorological Agency. Q J Seismol 2002, 65: 123–134. (in Japanese) (in Japanese)Google Scholar
- Virieux J: P-SV wave propagation in heterogeneous media: velocity-stress finite difference method. Geophys J Int 1986, 152: 649–668.Google Scholar
- Wessel P, Smith WHF: New, improved version of generic mapping tools released. EOS Trans Am Geophys Un 1998, 79: 579.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.