P-wave velocity structure in the southernmost source region of the 2011 Tohoku earthquakes, off the Boso Peninsula, deduced by an ocean bottom seismographic survey
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2012
Received: 5 February 2012
Accepted: 17 June 2012
Published: 28 January 2013
We present the result of a seismic experiment conducted using ocean bottom seismometers and controlled sources in the region off Ibaraki and the Boso Peninsula. This region is the southern edge of the rupture zone of the 2011 off the Pacific coast of Tohoku Earthquake. We estimated the P-wave seismic velocity structure beneath the profile using a 2-D ray-tracing method. The crustal structure in the southern area is more heterogeneous than that of the northern area. This heterogeneity is thought to be related with subducting the Philippine Sea plate (PHS). The plate boundary between the landward plate and the Pacific plate (PAC) is positioned at depths of 20 km at a distance of 170 km from the southern end of the profile. The subducting PHS is imaged on the southern part of the profile. However, we could not obtain a distinct image of the contact zone of PHS and PAC. The contact zone of PHS and PAC is estimated to have a large heterogeneity resulting from strong deformation due to the collision of the two plates. We infer that the termination of the rupture, and the large afterslip in the collision region, are caused by this strong heterogeneity.
In the southernmost part of the JT, there is a trench-trench-trench-type triple junction (Mckenzie and Morgan, 1969). The Philippine Sea plate (PHS) is subducting northwestward from the Sagami Trough and the PAC is subducting westward from the Japan and Izu-Ogasawara trenches (e.g. Seno, 1977; Ishida, 1992). The subduction of the PHS below the landward plate is also estimated from the existence of low-angle thrust-fault-type earthquakes which occur below this region. The thrust-type earthquakes can be grouped by slip vector direction. The earthquakes associated with the subduction of the PAC have a P-axis in the direction west or west-northwest, which is consistent with the relative plate motion of the PAC. Earthquakes occurring between the PHS and NEJ arc have a relatively shallow depth and are mainly observed below the Kanto-region. The direction of the P-axis is northwest, which is consistent with the direction of the subduction of the PHS. The PHS is suggested to overlay on the PAC in the Kanto-region including the Boso-peninsula. From the study for repeating earthquakes which have thrust-type mechanisms, it is inferred that the PHS is in contact with the PAC below the Boso-peninsula (Uchida et al., 2009). However, it is difficult to determine the exact position of the northern limit of the PHS off the Boso-peninsula due to a lack of seismicity in the marine area.
Various seismic events have occurred as a result of the subduction of the PAC and PHS beneath the NEJ arc. Slow slip events and devastating earthquakes such as the 1923 Kanto earthquake associated with the subduction of the PHS were known in the region off Boso-peninsula (e.g. Sato et al., 2005; Ozawa et al., 2007). In order to analyze these seismic events, it is necessary to know the crustal structure of the off-Boso region. Several seismic reflection surveys (e.g. Takeda et al., 2007; Kimura et al., 2009, 2010) and a refraction survey using OBSs (Hirata et al., 1992) have been conducted. These studies revealed a shallow structure in the off-Boso region, and the subduction of the PHS below the landward plate from the Sagami trough is imaged in the shallow region. However, the deep seismic structure of this region has not been obtained. In addition, little seismicity is observed at the plate boundary between the PHS and the landward plate below the off-Boso region. Therefore, there has been a controversy about the precise position of the tip of the PHS below the landward plate.
Moreover, the 2011 off the Pacific coast of Tohoku earthquake (hereafter, the 2011 Tohoku earthquake) (MJMA = 9.0) occurred in the forearc region of the JT on March 11, 2011 (Hirose et al., 2011). The source region of this earthquake is considered to spread to the off-Ibaraki and Boso region. The largest aftershock (M = 7.7) occurred thirty minutes after the mainshock in this region (Nishimura et al., 2011). Furthermore, the afterslip area of the mainshock reaches the off-Boso region (Ozawa et al., 2011). Determination of the detailed seismic structure of the region off Ibaraki and Boso-peninsula is important for considering the spread of the source region of the 2011 Tohoku earthquake. We investigate, therefore, the precise deep seismologi-cal structure beneath the landward slope off Ibaraki and the Boso Peninsula by a seismic survey using OBSs and explosives as seismic sources. In this paper, we present a two-dimensional (2-D) P-wave velocity model along a 400-km-long profile on the southernmost part of the source region of the 2011 Tohoku earthquake.
2. Experiment and Data Analysis
The root-mean-square error for the final model.
First and later arrivals from entire model
First arrivals from entire model
First and later arrivals from island arc crust
P n arrivals from Pacific plate
RMS error (ms)
3. Results and Discussion
3.1 Detailed P-wave velocity structure and its implications
The shallow part of the structure can be divided into several layers based on velocity changes and vertical velocity gradients (Fig. 4). The sedimentary section varies in thickness from 4 km to 9 km. The sedimentary layer which consists of three layers is present throughout the entire. The thickest sedimentary layer is found around a distance of 50 km from the southern end of the profile. The uppermost sedimentary layer and the second layer have a P-wave velocity of 1.6–1.8 km/s and 2.3–2.7 km/s, respectively. These layers are a common feature among the JT forearc region and correspond to Tertiary/Quaternary sediments (e.g. Miura et al., 2005). The third sedimentary layer has a P-wave velocity of 3.8–4.6 km/s. This layer is thought to be a Pre-Oligocene layer (e.g. Miura et al., 2005). A layer with P-wave velocities of 5.4–5.9 km/s underlies the sedimentary sections. The thickness of this layer varies from 2 to 9 km. The depth of the top of this layer becomes shallow at a distance of approximately 110 km from the southern end of the profile. The 5.4-km/s layer corresponds to the upper crust of the landward plate. A layer with P-wave velocities of 6.1–6.5 km/s underlies the upper crust in the landward plate. The thickness of this layer ranges from 6 to 10 km. The 5.4-km/s layer and the 6.1-km/s layer are interpreted as the island arc crust because similar velocities are widely observed not only below inland areas, but also along the forearc region of the JT (e.g. Iwasaki et al., 2002; Miura et al., 2003; Ito et al., 2004). Compared with the southern part of the island arc crust, the northern part of crust has less lateral heterogeneity. This laterally homogeneous crustal structure in the northern part is consistent with previous studies (Miura et al., 2003, 2005).
Strong reflection waves were observed and were estimated to be reflected at a deep interface (Fig. 3). This significant deep interface is interpreted as the top of the subducting PAC. The subducting oceanic crust of the PAC is imaged beneath the island arc crust (Fig. 4). The plate boundary between the landward plate and the PAC is positioned at depths of 17 km at the northern end of the profile, and 20 km at a distance 170 km from the southern end of the profile. The PAC deepens towards the south from a distance of 200 km on the profile. The subducting PAC is in contact with the island arc crust. According to several studies, the contact zone between the island arc crust and the oceanic crust can be the rupture areas of a great earthquake with magnitude >7 (e.g. Nakanishi et al., 2004; Takahashi et al., 2004). Depths of the Moho in the oceanic crust of the PAC is 24–26 km below sea level in the northern part of the profile. The P-wave velocity of the PAC oceanic upper mantle is 7.7–7.9 km/s. The depth and P-wave velocity of the uppermost mantle (P n velocity) of the PAC correspond with a previous study located at the landward slope along the JT (Miura et al., 2003). In contrast, the P n velocity of the PAC is slightly different from the result of Nishizawa et al. (2009). Their survey line was located perpendicular to our survey line. It is believed that the uppermost mantle in the Northwestern Pacific Basin exhibits a seismic anisotropy (e.g. Shinohara et al., 2008). The azimuthal anisotropy in the oceanic upper mantle appears to be aligned with the direction of spreading at the time of crustal generation rather than in the direction of present plate motions (e.g. Christensen, 1984). The fast direction of the P n velocity appears to be perpendicular to the lin-eation of magnetic anomalies. The magnetic lineation of PAC in the off Boso region is oblique to the JT (Nakanishi et al., 1989). There is a possibility that this difference in the velocity is affected by seismic anisotropy, which formed at the time of crustal generation.
In the southern part of the profile, we also observed reflected waves from a shallow interface (Fig. 2). This interface deepens toward the north, and is interpreted as the top of the subducting PHS. Position of the top of the PHS is consistent with the result of a previous seismic reflection survey in this area (Takeda et al., 2007). The crustal structure in the southern part of the landward plate is more heterogeneous than that in the northern part. The heterogeneity in the southern part is thought to be influenced by the subducting PHS plate.
Because the plate boundary between the PHS and the landward plate dips towards the north, an extension of the plate boundary reaches the top of the PAC. Uchida et al. (2010) investigated small repeating earthquakes which occur on the plate boundary between the PHS and the PAC, and estimated positions of the northeastern limit of the PHS. According to their results, the PHS is in contact with the PAC at distances from 0 km to 130 km on our profile. However, we could not resolve the structure of the contact region between the subducting PHS and the subducting PAC, since the reflection and refraction waves passing through this region could not be identified clearly from the OBS records. A heterogeneity structure, due to the collision of the two plates, causes an indistinct seismic signal. We infer that the PHS collides with the PAC in this region.
3.2 P-wave velocity structure and the 2011 Tohoku earthquake
After the occurrence of the 2011 Tohoku earthquake, post-seismic deformations were detected by the GPS network of Japan. Ozawa et al. (2011) estimated the afterslip distribution in and around the source region of the main-shock using GPS data. The Geospatial Information Authority of Japan continues the monitoring of crustal deformation on land using the GPS network and estimates the distribution of the afterslip. A large afterslip is estimated to overlap the coseismic slip region and extends to the surrounding region of the mainshock. A relatively large afterslip is estimated in the south of the source region. The large afterslip region in the south corresponds to the collision region between the PHS and the PAC (Figs. 6 and 7). The rupture of the mainshock sequence was terminated at the collision region of the PHS and the PAC, and the afterslip occurs in the collision zone. We infer that the termination of the rupture and the large afterslip in the collision region are caused by the strong heterogeneity resulting from the large deformation due to the collision.
We have conducted a seismic experiment using ocean bottom seismometers and controlled sources in the region off Ibaraki and the Boso Peninsula in 2008. We modeled the P-wave seismic velocity structure beneath the profile. The characteristics of the seismic velocity structure are as follows. (1) The crustal structure in the southern part is more heterogeneous than that of the northern part. (2) The northward dipping structure presents the subduction of the Philippine Sea plate in the southern part. (3) The subducting oceanic crust of the Pacific plate is in contact with the island arc crust at a depth of about 17 km in the northern region and a depth of 20 km at a distance of 170 km from the southern end of the profile. The geometry of the top of the PAC changes at a distance of about 200 km of the profile. (4) The contact zone of the Philippine Sea plate and the Pacific plate is not imaged clearly. It seems that the contact zone is characterized by a strong heterogeneity resulting from the large deformation.
The southern end of the seismically active region of the aftershocks, and the co-seismic slip, of the 2011 Tohoku earthquake correspond to the contact region of the Philippine Sea plate and the Pacific plate. The large afterslip region coincides with the collision region of the two plates. The termination of the mainshock rupture, and the large af-terslip, in the collision region are thought to be caused by the collision of the two oceanic plates.
We would like to thank the officers and crew of M/V Kaiko-maru No. 5, and R/V Hakuho-maru. We also thank Messrs. S. Hashimoto, T. Yagi, and S. Suzuki for preparation of the OBS observation. We also thank Drs. A. Kuwano, T. Takanami, R. Arai, R. Azuma, R. Miura, T. Shinbo, Y. Yamamoto, Mses. H. Mizukami, M. Mizuno, Messrs. S. Amamiya, K. Ichi-jyo, K. Suzuki, and U. Yamashita, for the OBS survey and fruitful discussions. We wish to acknowledge the useful comments and suggestions from the editor, Dr. T. Okada, the reviewer, Dr. F. Klingelhoefer, and an anonymous reviewer. This study is partly supported by the Special Coordination Funds for the Promotion of Science and Technology (MEXT, Japan). Most of the figures were created using GMT (Wessel and Smith, 1999).
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