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.
KeywordsGuided wave Subducting crust Pacific slab Hokkaido Finite difference method Dehydration Intermediate-depth earthquake
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).
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