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Velocity increase in the uppermost oceanic crust of subducting Philippine Sea plate beneath the Kanto region due to dehydration inferred from high-frequency trapped P waves
© Takemura et al.; licensee Springer. 2015
- Received: 12 September 2014
- Accepted: 26 February 2015
- Published: 18 March 2015
To investigate the detailed structural properties of the oceanic crust of subducting oceanic plate, we analyzed high-frequency (1 to 16 Hz) trapped P waves during earthquakes that occurred near the oceanic crust of the Philippine Sea plate. The distinct trapped P waves observed by the dense seismic network of the Kanto-Tokai region, Japan, did not show any apparent peak delay and frequency-dependent dispersion. These observations suggested that the oceanic crust around the source depths was characterized by a homogeneous velocity structure, rather than an inhomogeneous multiple-layered structure. This interpretation was examined by finite difference method simulations of seismic wave propagation using possible velocity structure models. The simulations demonstrated that a uniform velocity oceanic crust of the subducting Philippine Sea plate, which may result from the velocity increase in this layer at 30 to 40 km depth due to metamorphic-dehydration reactions, effectively trapped seismic energy as a short-distance waveguide and developed distinct pulse-like trapped P waves.
- Philippine Sea plate
- Oceanic crust
- Trapped waves
Many geophysical phenomena in subduction zones, such as the generation of arc volcanism and the occurrence of intraslab earthquakes, are considered to be related to fluid transportation into the mantle by subducting oceanic plates (e.g., Tatsumi 1989; Kirby et al. 1996; Magee and Zoback 1993). Previous studies suggested that the hydrated oceanic crust, which is the uppermost part of a subducting plate, plays an important role in fluid transportation into the mantle (e.g., Iwamori and Zhao 2000; Kawakatsu and Watada 2007). Since hydrous minerals in this layer become unstable with the increasing temperature and pressure due to the subduction, dehydration reactions in the oceanic crust are expected to occur accompanied by the release of fluids to the surrounding structures, resulting in the transformation to an anhydrous eclogitic oceanic crust (e.g., Hacker et al. 2003; Kawakatsu and Watada 2007; Tsuji et al. 2008).
Since hydrous minerals have a lower seismic velocity than anhydrous ones, a subducting oceanic crust at depths where hydrous minerals are stable is characterized by a low-velocity layer compared to the surrounding structures, such as the mantle wedge and the oceanic mantle (e.g., Matsubara et al. 2005; Kawakatsu and Watada 2007; Nakajima et al. 2009). The seismic waves are effectively trapped in this low-velocity layer, and consequently, the trapped seismic energy is often identified as distinct later arrivals during intraslab earthquakes in the subduction zone (e.g., Fukao et al. 1983; Hori et al. 1985; Hori 1990; Abers et al. 2003; Martin and Rietbrock 2006; Miyoshi et al. 2012; Shiina et al. 2014). Trapped P and S waves appear several seconds after the arrival of the relatively weak direct body waves. Previous studies reported that trapped P and S waves are only well observed for a particular geometry between source and receivers for earthquakes occurring in the oceanic crust and that their maximum amplitudes are 5 to 10 times larger than those of the first arrivals (e.g., Fukao et al. 1983; Miyoshi et al. 2012). Furthermore, their apparent velocities are approximately 7 and 4 km/s, respectively, corresponding well with the P- and S-wave velocities in the oceanic crust. Thus, trapped seismic waves are considered as guided waves propagating along the low-velocity oceanic crust. This indicates that the study of trapped seismic waves potentially allows us to investigate the seismological structure relevant to a subducting oceanic crust. Using observations of travel times and dispersion features of trapped waves, previous studies have revealed properties of the oceanic crust, such as its geometry, thickness, seismicity, and depth of eclogitization (e.g., Fukao et al. 1983; Hori 1990; Abers et al. 2003; Miyoshi et al. 2012). However, the detailed features of trapped waves and heterogeneous structure in the oceanic crust still remain uncertain, especially for small-scale velocity heterogeneities that strongly affect high-frequency seismic wave propagation.
In this study, to investigate the velocity change of seismic waves in the subducting crust, we analyzed trapped P waves propagating along the oceanic crust of the Philippine Sea plate beneath the Kanto-Tokai region and conducted finite difference method (FDM) simulations of seismic wave propagation using realistic velocity structure models. A large number of data derived from the dense seismic network Hi-net (Okada et al. 2004) enabled more detailed analyses of the features of high-frequency trapped P waves and of the heterogeneous structure in the crust. Using FDM simulations, we were able to examine not only the travel times but also the amplitude and envelope shape of the filtered trapped P waves for frequencies up to 16 Hz. This allows the investigation of the internal velocity structure of the oceanic crust and of how trapped P waves develop in this layer. Based on the comparison between the observed and simulated seismograms, we demonstrate that the P-wave velocity in the uppermost oceanic crust of the Philippine Sea plate increases up to approximately 7 km/s at depths of 40 to 50 km in the Kanto region. We also show how the trapped P waves are affected by small-scale velocity heterogeneities in the crust and mantle.
Data and analysis method for the observed seismograms
Characteristics of the earthquakes used in the analysis
Origin time (local time)
4 April 2008 19:01
2 April 2011 16:55
26 April 2011 21:12
11 February 2012 10:26
18 May 2012 17:18
16 July 2012 04:31
1 February 2013 01:04
FDM simulation of the seismic wave propagation
To model the observed features of the high-frequency trapped P waves obtained from the waveform analysis, we conducted two-dimensional (2D) FDM simulations of the high-frequency seismic wave propagation along profile A-A′ (Figure 1a), perpendicular to the direction of subduction. We mainly conducted 2D FDM simulations to examine the various possible structure models. The 2D model covered an area of 327.68 × 102.40 km2, in the horizontal and vertical directions, respectively, discretized by a small grid interval of 0.02 km. The propagation of the seismic waves was calculated by solving the equation of motion in a 2D elastic medium based on the staggered-grid FDM technique with fourth- and second-order accuracies in space and time (e.g., Levander 1988; Graves 1996). To effectively conduct the relatively large-scale 2D simulation, we used a parallel FDM simulation code based on a domain partitioning procedure using large number processors through the message passing interface (MPI) (e.g., Furumura and Chen 2004). We applied suitable boundary conditions for the solid-to-air interface at the free surface in the FDM simulation, as developed by Okamoto and Takenaka (2005).
P - and S -wave velocities, densities, and anelastic attenuation of the JIVSM
V P [km/s]
V S [km/s]
ρ [g/cm 3 ]
Philippine Sea plate
Upper oceanic crust
Lower oceanic crust
Oceanic crust layer 2
Oceanic crust layer 3
Each parameter was obtained from Koketsu et al. (2008). V P, V S and ρ are the P-wave and S-wave velocities and density, respectively. Q P and Q S are the quality factors for P and S waves, respectively.
For the 2D FDM simulations, we used the CMT solution derived from the F-net of event 5 in Table 1, which occurred at a depth of 59 km on 18 May 2012. The source depth was slightly adjusted to set the source in the oceanic crust for the assumed velocity structure model, since for earthquakes occurring outside the this layer, large-amplitude trapped seismic waves are not observed (e.g., Fukao et al. 1983; Hori 1990; Miyoshi et al. 2012). The P and SV waves with a source time function represented by a single-cycle Kupper wavelet (Mavroeidis and Papageorgiou 2003) with a dominant frequency of approximately 6 Hz were radiated from the assumed source.
Along profile A-A′ (Figure 1a), the LVA of the mantle wedge was detected just above the oceanic crust of the Philippine Sea plate at depths of 30 to 40 km by tomography studies (e.g., Matsubara et al. 2005). Thus, we introduced the LVA in the simulation model along profile A-A′, where the seismic P- and S-wave velocities were reduced by 10% compared with the original JIVSM. The location of the LVA was referred from tomography results from Matsubara et al. (2008), corresponding to the lower crust and mantle in the JIVSM. In addition, we modified the original JIVSM by introducing the velocity increase for P- and S-wave velocities in the upper oceanic crust of the Philippine Sea plate at depths below the LVA, to reproduce the observed pulse-like trapped P waves. Two constructed models (A1 and A2) had different strength of velocity increases in the lower oceanic crust. We also introduced the small-scale velocity heterogeneities in the crust, mantle, and oceanic plate to examine the effects of seismic scattering on the trapped P waves. The parameters of small-scale velocity heterogeneities were referred from the Table four of Takemura and Yoshimoto (2014).
The 3D effects, especially the 3D geometry of the subducting oceanic crust, were not taken into account in the 2D simulations. Thus, it is difficult to compare directly 2D simulation results with the observations. To confirm the effect of a 3D geometry of the subducting plate on the propagation of the trapped P waves, we also conducted 3D FDM simulations of seismic wave propagation in 3D heterogeneous structure models and examined the limitations of our 2D simulations. Since it is difficult to conduct highly accurate full 3D simulations for frequencies up to 16 Hz, our 3D simulation was localized in the region covering an area of 81.0 × 40.5 × 45.0 km3 around seismic source (dotted rectangle in Figure 1a) discretized by a uniform grid size of 0.03 km, to focus on the propagation of the trapped P waves along the oceanic crust. The 3D simulation provided P wave seismograms for a virtual station in the lower crust located where the trapped energy was released from the oceanic crust. The effect of the 3D structure on the released trapped waves might be weak because of a relatively simple structure of the crust.
Waveform analysis of the observed high-frequency trapped P waves
To understand how trapped P waves of different frequency components propagate along the oceanic crust, we first demonstrate their frequency dependency. As shown in Figure 2, trapped P waves were clearly observed for all frequency bands at epicentral distances of 90 to 150 km, whereas the long-duration and incoherent coda waves were excited by seismic wave scattering in the crust and mantle. This indicates that the trapped energy from P waves might not be strongly affected by the scattering loss due to small-scale heterogeneities in the crust.
Despite the differences in source mechanisms and hypocenter locations, the trapped P waves were clearly observed in the filtered vertical velocity seismograms recorded at the N.NSHH station for all earthquakes used in the analysis (Figure 3). In general, as the frequency increases, the seismic wave scattering due to small-scale velocity heterogeneities along the propagation path becomes more dominant and distorted seismograms characterized by frequency-dependent pulse broadening and peak delay are observed (Sato et al. 2012; Takahashi et al. 2007). However, the pulse-like trapped P waves were observed irrespective of the frequency, while the seismograms became more complicated with increasing frequency. Trapped P waves were observed for frequencies up to 16 Hz and those dominant features were not influenced by slight changes in source location and fault mechanisms.
Figure 4 shows the stacked RMS envelopes of the vertical component recorded at four Hi-net stations with average epicentral distances of 100 to 150 km. The trapped P waves were clearly observed at all stations, showing pulse-like waveforms without significant dispersion and peak delay. The averaged amplitudes of the trapped P waves were 5 to 7 times larger than those of the first arrivals. These characteristics suggest that the trapped P waves might be caused by a specific velocity structure of the subducting oceanic crust, which effectively traps high-frequency P-wave energy.
Simulations of the high-frequency trapped P waves
Dehydration reactions in the uppermost oceanic crust of the Philippine Sea plate
According to the 2D and 3D simulations, the distinct pulse-like trapped P waves were caused by the short-distance waveguide due to a uniform velocity structure of the oceanic crust the Philippine Sea plate at depths below 40 km. We may suggest that the dehydration of upper oceanic crust at depths of 30 to 40 km contributes to the formation of this uniform velocity structure. Seismic velocities and thickness of the uniform oceanic crust around source region well correspond to those estimated by tomography and seismic activities studies (Hori 2006; Kimura et al. 2006; Hirose et al. 2008a, b). In Tokai and southwestern Japan, using travel-time tomography, Hirose et al. (2008b) and Kato et al. (2010, 2014) detected similar velocity changes in the oceanic crust of the Philippine Sea plate at depths beneath 40 km related with the overlying serpentinized fore-arc wedge mantle. Bostock et al. (2002) analyzed scattered teleseismic waves and also reported velocity change at similar depths beneath the Cascadia. Their works could not resolve the detailed velocity changes of the upper oceanic crust of oceanic plate but support our conclusions.
Based on thermodynamic forward modeling, Kuwatani et al. (2011) have showed that dehydration proceeds near the boundary between the greenschist and amphibolite facies, where chlorite is reacted out (e.g., Apted and Liou 1983), at depths of approximately 40 km, under the pressure-temperature condition of the subducting Philippine Sea plate beneath southwestern Japan. They discussed that this metamorphic-dehydration reaction is expected to generate about 2 wt% of water and amphibolite rocks with P-wave velocities of approximately 7 km/s. Our seismological findings derived from the trapped P-wave analysis might be consistent with the results from these geochemical studies. However, the low-velocity layer at the top of the Philippine Sea plate beneath the Kanto region was not strictly identified as either oceanic or arc crusts. There are lateral structural variations of the Philippine Sea plate around Izu-Bonin arc that is located at south of the Kanto region (e.g., Kodaira et al. 2007). Ohmi and Hurukawa (1996) also pointed out that the shallow crustal layer of the Philippine Sea Plate might be the crust of the subducting Izu-Bonin arc. This issue, including our interpretation of velocity increase in the low-velocity layer, is still open question and should be examined in the future.
In the southwestern Japan, the detailed structure of the Philippine Sea plate has been revealed by extensive seismic studies (e.g., Hirose et al. 2008a, b; Nakanishi et al. 2008; Kato et al. 2010, 2014; Nishizawa et al. 2011). Comparing characteristics of seismic wave propagation between the southwestern Japan and Kanto area examined in this study could reveal more precise structure and fluid distribution in the subducting oceanic crust of the Philippine Sea plate.
We analyzed the high-frequency (1 to 16 Hz) trapped P waves propagating along the oceanic crust of the Philippine Sea plate beneath the Kanto-Tokai region, Japan. The trapped P waves were clearly observed at epicentral distances up to 150 km during earthquakes occurring in the oceanic crust of the Philippine Sea plate beneath the central Kanto region, showing pulse-like waveforms without significant dispersion and peak delay. These characteristics suggest that the trapped P waves might be caused by a specific velocity structure of the subducting oceanic crust, which effectively traps high-frequency P-wave energy.
Using FDM simulations, we confirmed that the pulse-like trapped P waves were efficiently developed by trapping of seismic energy in a uniform velocity oceanic crust at the source depths. A uniform velocity oceanic crust at depths of 40 to 50 km could be formed by the velocity increase in the upper oceanic crust at 30 to 40 km depth due to metamorphic-dehydration reactions. Our results and further relating studies may constrain the detailed depth-dependent metamorphic-dehydration reactions in the subducting oceanic crust of the world.
We would like to thank two anonymous reviewers and the editor, A. Nishizawa, for constructive comments that improved an earlier draft of this manuscript. We acknowledge the National Research Institute for Earth Science and Disaster Prevention, Japan (NIED) for providing the Hi-net waveform data and the CMT solutions from the F-net. The computations were conducted on the Earth Simulator at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) under the support of the joint research project ‘Seismic wave propagation and strong ground motion in 3-D heterogeneous structure’ conducted by the Earthquake Research Institute, the University of Tokyo, and the Center of Earth Information Science and Technology, JAMSTEC. ST is grateful for the financial support provided by the Grant-in-Aid for JSPS (the Japan Society for Promotion of Science; No. 24.5704) fellows. All figures in the present study were drawn using the Generic Mapping Tools software package.
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