LETTER

Earth Planets Space, 55, 59–64, 2003

High resolution receiver function imaging of the seismic velocity discontinuities
in the crust and the uppermost mantle beneath southwest Japan
Makiko Yamauchi1 , Kazuro Hirahara1 , and Takuo Shibutani2
1 Graduate School of Environment, Nagoya University, Nagoya 464-8602, Japan
2 Disaster Prevention Research Institute, Kyoto University, Kyoto 611-0011, Japan(Received December 4, 2002; Revised January 8, 2003; Accepted January 20, 2003)

Clear images of the seismic velocity discontinuities in the crust and the uppermost mantle beneath southwest
Japan have been obtained with Receiver Function (RF) analyses of teleseismic waveforms observed at extraordinary
dense short-period and broadband stations. For imaging, we construct several 2-D profiles of depth-transformed
RFs to delineate the fine crustal structure and the 3-D configuration of the Philippine Sea plate (PHS). The Mohois clearly seen and locally depressed just beneath the Chugoku Mountains. The Conrad, the velocity discontinuity
within the crust, in the northern Kinki region is recognized and depressed just beneath the Chugoku Mountains
as well as the Moho. RF analyses also clearly elucidate the largely contorted configuration of the PHS including
aseismic portions. The PHS subducts aseismically under the western Chugoku region with a high dip angle of 35
degrees at least to a depth of 65 km. In the western Kii Peninsula, it descends steeply to a depth of 90 km withsome curvature, while it subducts with a low dip of 20 degrees to a depth of 60 km from the eastern Kii Peninsula
to the Lake Biwa.

1.

Introductionsity is realized and clear teleseismic waveforms have been
observed.
This paper, using such high-density and high-quality
waveform data, reveals the detailed structure of the seismic
velocity discontinuities in the crust and the uppermost mantle beneath southwest Japan with RF analyses. Although RF
imaging beneath the Japan islands has been already executedby Li et al. (2000), they aim mainly the imaging of the upper mantle down to a depth of 1000 km with only broadband
data. In this study we use a larger amount of data including
short-period data observed at incomparably dense stations,which dramatically raises the spatial resolution of imaging
structure beneath the whole southwest Japan. Shiomi (2002)
has executed the detailed RF analyses using more events thanour study. His detailed analyses have, however, been confined in the Shikoku and the Chugoku regions using the data
observed only at Hi-net stations. Our analyses extend to the
Kinki region and the Kii Peninsula using the data observedat three network stations.

It is of great significance to grasp the detailed structure of
the crust and the upper mantle including seismic velocity discontinuities beneath southwest Japan (see Fig. 1) in view ofunderstanding the mechanism of great interplate earthquakes
repeatedly occurring along the Nankai trough and large inland ones in active faults, which have brought out great disasters. So far, the complex structure beneath southwest Japan
has been investigated for the past a few decades based onseismic activity, travel time tomography and the reflection
or refraction surveys using artificial explosion sources. The
studies based on seismicity, however, cannot detect any possible aseismic slab. Travel time tomography studies, assuming the gradual change of the structure, have not provided
clear image of seismic velocity discontinuity. Reflection or
refraction surveys have the restrict limitation of the study areas and the deeper portions of the subducting slab cannot be
investigated.
There have recently been deployed three nation-wide seismic networks over the Japan Islands, short-period Hi-net(634 stations), broadband F-net (65 stations) and shortperiod and broadband J-array (269 stations). The former two
networks are operated by NIED (National Research Institute for Earth Science and Disaster Prevention) and the latterby the Japanese universities and JMA (Japan Meteorological
Agency), and the number of stations in the former two networks is increasing still at present. These networks realize
the high density of seismic stations in the whole Japan Islands. Especially in southwest Japan, the highest station den-

2.

Data and Method

In this study, we use a large number of teleseismic waveforms observed at 117 stations of Hi-net, 96 stations of Jarray and 4 stations of F-net in southwest Japan, whose station spacing reaches a few kilometers, and the distribution isshown in Fig. 1. Figure 2 and Table 1 summarize the event
distribution and the number of events for networks of Hi-net,
J-array and F-net used in our analyses.
RFs effectively pick up the P-S waves converted atseismic velocity discontinuities just beneath a station among
the whole observed phases. We employ waveforms witha duration of 95 seconds from 5 seconds before to 90 seconds

c The Society of Geomagnetism and Earth, Planetary and Space Sciences
Copy right
(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan;
The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.

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Table 1. Number of the stations and events used in this study for respective networks. There is also included type of seismograms, and the period andmagnitude range for events.
Hi-netJ-array

F-net

Networks

Number of stations
Type of seismograms

117
short-period

96
short-period broadband

4
broadband

Event data

PeriodM>
Number of events

00/10–02/06
5.5
72

96/11–00/01
6.0
91

96/11–00/01
6.0
94

Fig. 1. Location of the stations and the 2-D RF profiles, and the contour map of the upper boundary of the PHS estimated in this study. Each symbol
represents the network to which the station belongs. The filled or open symbols indicate the stations where the depth of the upper boundary of the PHS
is estimated or not, respectively. The solid lines indicate the locations of 11 lines along which the 2-D RF profiles are constructed, and 4 lines with thelabels of a-d show the ones along which the results of 2-D RF profiles are displayed in this paper. In the inserted figure, are given the region names usedin this study.

Fig. 2. Equidistant distribution of the events used in this study. Each symbol represents the network whose stations observe the event.

M. YAMAUCHI et al.: RECEIVER FUNCTION IMAGING BENEATH SOUTHWEST JAPAN61

Fig. 3. Obtained 2-D RF profiles. (a)–(d) correspond to RFs along the lines of a-d shown in Fig. 1. The color bar from red to blue indicates the positive tonegative normalized by respective first large amplitude.

after the onset of P-waves to construct RFs. The water level
method (Langston, 1977, 1979), where the value of the water level is 0.01, is applied to original waveforms in the frequency domain. We also apply Gaussian filter for high-cut
filtering of 1 Hz and 1.6 Hz for short period data of Hi-net
and J-array and broadband data of J-array and F-net, respectively.
Assuming that each later phase in a RF is the wave converted from P to S at a depth, we transform the time domain
RF into the depth domain one along each ray path in a ref-erence velocity model. IASP91 model is used in this study.
The differences of the depth of each phase by transformation in possible different velocity models may be a measureof the estimation error of absolute depth of each discontinuity. Therefore, at ISI, a station in Fig. 1, we compare thedepth domain RF using IASP91 to that using two test models. One is the resultant velocity model of detailed RF analysis (Shibutani, 2001). This velocity model has some sediments at the surface, the 5 percent faster upper crust, thelower crust with the same velocity as that of IASP91 and a

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6 percent lower velocity zone from 40 to 45 km. The otheris JMA model, in which the velocity gradually increases instead of some clear discontinuities. In the resultant depth
domain RFs by the first and second test models, a phase converted at the possible Moho in the descending PHS sifts 1 km
more shallowly and 2 km more deeply, respectively. Hence
we recognize the estimation error for the absolute values ofthe depths of each discontinuity is about 3 km.
In the RFs arranged in this way, we can find some coherent
phases. Those phases are further more enhanced by applying
Singular Value Decomposition filter (Chevrot and Girardin,
2000). This filter removes the incoherent noises among the
waveforms by a truncated sum over first few eigenimages.
We reconstruct RFs with the first 6 eigenimages.
We set 11 lines, as displayed in Fig. 1, to construct 2-D
RF profiles. The RFs at the stations within a width of 25 kmfrom each cross section are selected. Then the amplitudes ofthe depth domain RFs are projected in the 1 km by 1 km cellson the plane along each line, and then averaged.

3.

Results and Discussion

In Figs. 3(a)–3(d), we show the results only along therepresentative 4 lines that have remarkable features. Eachfigure displays the depth domain RF profile along each line
‘a’ to ‘d’ in Fig. 1 at depths of 0–100 km. And Figs. 4(a),4(b), 4(c) and 4(d) show the figures in which seismic activity(JMA, 1995–2002, M > 1) in each area and some ourinterpretations are added to Figs. 3(a), 3(b), 3(c) and 3(d),respectively. We mainly pay attention to the results down toa depth of 70 km, because the RF results below the depth
are contaminated by multiple P-S phases at discontinuities
existing in the upper portion.
3.1 Profile a
In Fig. 3(a), three portions of consecutive positive amplitudes indicated by red in RFs can be clearly traced andwe can draw three lines in Fig. 4(a). These positive amplitude portions are interpreted as P-S converted phases at velocity discontinuities corresponding to the Conrad (line C),
the Moho (line M) and the Moho in the descending PHS(line P) by referring to seismicity. The inland Moho is notseen beneath the southern Shikoku region. But it appears beneath the northern Shikoku, and steeply goes down just under the southern Chugoku Mountains, and then it is getting
shallower towards the Sea of Japan. The velocity discontinuity within the crust, which is usually called the Conrad, is
not always found beneath the Japan islands. We recognize,
however, the positive RF amplitudes located at depths corresponding to the Conrad along most of lines in southwest
Japan, and we simply call the Conrad for the corresponding
discontinuity in this study. The Conrad lies at depths between 15 and 20 km throughout the profile. On the other
hand, the PHS with a low dip angle of 10 degrees, which canbe also traced by seismic activity, suddenly changes its dip to
35 degrees just under the southern Chugoku Mountains, andsubducts deeper as an aseismic slab to at least 65 km depth.
In this region, Nakanishi (1980) points out the existence ofaseismic slab and recent tomographic studies (e.g. Yamane etal., 2000) have shown the same structure. Our RF analyses
provide far clearer image of the dipping PHS.
In this paper we regard the portions indicated by red withpositive RF amplitudes produced by the P-S phases converted at the high to low seismic velocity discontinuity as
the upper boundary of the PHS. Existence of the low velocity zone at the top of the PHS, which is the oceanic crust, issuggested by explosion studies (e.g. Kurashimo et al., 2002)
and in a detailed RF study (Shibutani, 2001). Accordingly
the red portions with positive RF amplitudes in figures indicate the lower boundary of the oceanic crust (i.e., the Moho)
within the PHS, but not the top of the PHS. However, we
can trace clearly in figures only the positive amplitude (red)portions instead of the negative amplitude (blue) ones. Thus,
we adopt the positive RFs as indicators of the location of the
PHS although they do not correspond to the plate boundary
but the Moho in the PHS.
3.2 Profile b
In Fig. 3(b), we also find three consecutive positive RF
portions and those can be indicated by lines of C, M and
P in Fig. 4(b). According to Ito (1999) that states inland
earthquakes occurs in the upper crust, comparing with theupper crustal seismicity, we regard the line C as the Conrad.
The Conrad is seen beneath the Chugoku region at depths of
12–20 km. Line M1 seen beneath the northern Shikoku at
depths of 20–30 km and line M2 seen beneath the northern
Chugoku at depths of 30–40 km indicate the Moho, which
are depressed beneath the central Chugoku region. And thenwe identify the line P that dips northwards with a very low
dip angle from the Nankai trough side as the configuration of
the PHS. Even we take account of the estimation error of theabsolute depth as stated in Section 2, line P seems to be located above the deeper seismic activity beneath the Shikoku
region. Therefore earthquakes in this profile seem to occur
within the mantle of the PHS, which is consistent with Baba
et al. (2002). Shiomi (2002) has recently executed more detailed RF analyses in the Shikoku and Chugoku regions andobtained the results on the PHS configuration in these regions, which are similar to our study.
3.3 Profile c
The most remarkable features in Fig. 3(c) are the clear
positive RF traces in the Kinki region and the steeply dipping ones beneath the Kii Peninsula. The clear line M in
Fig. 4(c), which is interpreted to be the Moho, is located at
depths of 35 km beneath the Kinki region and has the deepest in the central Kinki region. It is also located at a depthof 40 km beneath the northwestern Kii Peninsula, though it
is not clearer than in the Kinki region. It is noted that the
Moho is depressed beneath the Osaka Bay and there seems
to exist a gap of 5 km in the Moho depth between the inland
Kinki region and the Osaka Bay. Line C drawn in Fig. 4(c)is regarded as the Conrad by referring to the upper crustal
seismicity, which also goes down beneath the central Kinki
region as well as the Moho. According to Ito (1999), the inland large earthquakes are likely to occur in the region where
the lower cut off depth of the crustal seismicity and/or theConrad depth are abruptly changed laterally. The discontinuity corresponds to the Conrad as the cutoff depth is usually
shallower than the depth of the Conrad, which can be seen inthis figure, but both are related to the top of the ductile lower
crust. Therefore, the detailed investigation of the undulationof the Conrad enables us to know the areas where large inland events are likely to occur. RF method has proved to be

M. YAMAUCHI et al.: RECEIVER FUNCTION IMAGING BENEATH SOUTHWEST JAPAN

63

Fig. 4. 2-D RF profiles with seismic activity of JMA. The positive RF traces with lines denoted by C, M and P are interpreted to correspond to the Conrad,
the Moho and the upper boundary of the PHS.

a useful way to know the detailed undulation of the Conrad.
Positive RF traces denoted by the line P in Fig. 4(c), whichclosely correspond to the deeper seismic activity down to adepth of 60 km, shows the dipping PHS with high dip angles.
The line is located at about 30 km beneath the southern tip ofthe Kii Peninsula, and is steeply dipping to at least a depth of
90 km. Furthermore, this line seems to have some curvaturein the deep aseismic portion.

3.4 Profile d
It can be found in Fig. 3(d) that profile d has less spatial
resolution than the other profiles due to the sparse distribution of stations. We draw line C in Fig. 4(d) and it is seen beneath the Kinki region and lying around 5–10 km under thecrustal seismicity. Therefore we define line C as the Conrad.
A group of lines M1 to M3 in Fig. 4(d), which appears at
the northern root of the Kii Peninsula and dips to the Sea ofJapan with a low dip of 10 degrees, can be regarded as the
Moho. In this profile the Moho beneath the Kinki region is

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about 10 km deeper than that in profile c of Fig. 4(c), which isconsistent with the observed Bougure gravity anomaly (e.g.
Hagiwara, 1967). Furthermore, beneath the Lake Biwa line
M4 is seen under line M2. The line P drawn in Fig. 4(d),which dips from the Nankai trough side to the Sea of Japan
in parallel to the Moho, is seen as the dipping PHS. It isclearly recognized that this line P extends down to a depthof 60 km beneath the Lake Biwa. Aseismic Philippine Sea
slab, which is diving into the region beneath the Lake Biwa
with a relatively low dip angle, is also pointed out by Hori etal. (1985).
3.5 Contour map of the upper boundary of the PHS
We produce the contour map of the upper boundary of
the PHS by interpolating the visually inspected depths oflines P in Figs. 4(a)–4(d) along the respective 11 profiles.
The resulting contours are drawn by blue dashed curves inFig. 1. The stations where the PHS depth can be visuallyobtained are shown by filled symbols. Therefore the contour
is relievable in the regions where the stations with filled
symbols are densely distributed.
In the Kii Peninsula, our contour shape, which shows gentle and steep dips in the eastern and western Kii Peninsula
respectively, is consistent with that in the past study basedon seismic activity and analyses of later phases (e.g. Nakamura et al., 1997; Hori et al., 1985). On the other hand, in
the Shikoku region our study shows an interesting configuration. In the western Shikoku, the PHS is somehow concaved
toward the Nankai trough side comparing to those in the surrounding regions. To the contrary, in the eastern Shikoku andthe southern tip of the eastern Chugoku region, the dip angle of the PHS is lower, that is, the PHS swells beneath thisregion.
We can draw the contour depth of the aseismic portion of
the PHS down to a depth of 60 km, which cannot be detectedbased on seismic activity in the Chugoku region and aroundthe Lake Biwa. This is a great progress in the understanding
seismic tectonics in these regions.
As indicated by the contour map in Fig. 1, the PHS including the aseismic portion down to a depth of 60 km or greater
is largely contorted with the concave portions in the western
Kii Peninsula and the western Shikoku regions. The features
found beneath the Shikoku and the Chugoku regions are also
found in more detailed analyses of Shiomi (2002). We newly
reveal the PHS configuration beneath the Kii Peninsula andthe Kinki regions, in addition to these regions.

4.

Conclusion

over the Conrad in the Kinki region is also depressed justbeneath the Chugoku Mountains. On the other hand, theaseismic PHS is detected beneath the Chugoku region witha high dip of 35 degrees and beneath the Lake Biwa with alow dip of 20 degrees. In the Kii Peninsula we obtain the extremely clear image of the dipping PHS with some curvature
subducting to at least a depth of 90 km beneath the northern
Kii Peninsula or the Osaka Bay.
Acknowledgments. We are so grateful to K. Obara and K. Shiomi
in NIED who kindly help us to use the Hi-net data and give us helpful comments. We gratefully acknowledge fruitful discussions with
T. Matsuzawa on several points in the paper. Our gratitude is also
due to the Japanese university group, JMA and NIED for the use of
the data of J-array and F-net. We use the unified hypocentral datathat was collected by JMA and MEXT. This study is financially
supported by Grad-in-Aid for Scientific Research (B) (1434013) of
MEXT.

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We analyze a large amount of teleseismic waveforms observed at dense stations of Hi-net, J-array and F-net using RF
method, to reveal the detailed structure of the crust and thedipping PHS beneath southwest Japan. By constructing 2-D
profiles of SVD filtered depth domain RFs, we have obtained
far clearer image of the velocity discontinuities corresponding to the Conrad and the Moho and the configuration of thedipping PHS. The notable feature of the Moho configuration M. Yamauchi (e-mail: yamauti@eps.nagoya-u.ac.jp), K. Hirahara, and T.
is that it goes down just beneath the Chugoku Mountains in Shibutani
the western Chugoku and the northern Kinki regions. More-