Earth Planets Space, 58, 1007–1012, 2006

Upper mantle imaging beneath the Japan Islands by Hi-net tiltmeterrecordings
Takashi Tonegawa1 , Kazuro Hirahara2 , Takuo Shibutani3 , and Katsuhiko Shiomi4
1 Department of Earth and Environmental Sciences, Graduate School of Environmental Studies,

Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
2 Kyoto University, Kyoto, Japan
3 DPRI, Kyoto University, Uji, Japan
4 NIED, Tsukuba, Japan(Received January 19, 2006; Revised April 5, 2006; Accepted April 25, 2006; Online published September 16, 2006)

We present a novel receiver-function image of the upper mantle structure around the Japan subduction zone.
To increase the amount of available waveform data containing the relatively lower frequency component, we
examined whether the Hi-net tiltmeter recordings are usable for imaging the upper mantle discontinuities bycomparing them with broadband seismograms in different frequency bands. We found that the two are comparable
at a frequency band between 0.02 and 0.16 Hz. To make receiver functions from tiltmeter data, stacked verticalcomponents of broadband seismograms were used as source-time functions. Since such source-time functions
may include biases from local structure, we also produced regional stacked source-time functions. The receiver
function with the above frequency band does not seem to be affected by local structure. In the images derived
from the receiver-function gathers, we were able to visualize both the oceanic Moho and the lower slab boundary,which could be traced down to depths of 400 km and 600 km, respectively. These images also show an uplift of
the 410-km discontinuity and a depression of the 660-km discontinuity in the regions that are probably affectedby the cold subducting Pacific slab.
Key words: Receiver function, upper mantle, subducting slab, tiltmeter.

1.

Introduction

Slab-induced depth variation of the seismic discontinuities in the upper mantle beneath the Japanese Islands has
been well mapped by receiver function (RF) analysis basedon broadband seismograms (see Li et al., 2000; Tonegawa
et al., 2005). However, the density of broadband stations
throughout the Japanese Islands is still too low (140 stationsin Japan) to investigate the relation between the slab and the
410 km and 660 km upper mantle discontinuities (hereafter
called the ‘410’ and ‘660’, respectively). These two discontinuities are observed globally with velocity jumps ofseveral percentages, and are usually interpreted as mineralphase changes (Ringwood, 1975). In addition to the broadband stations, a high-density short-period network (Hi-net),
which provides a highly sensitive observation of groundmotion, has been deployed in the Japanese Islands by the
National Research Institute for Earth Science and Disaster Prevention (NIED). Ramesh et al. (2005) found thatthe short-period recordings of the Hi-net stations include
Ps phases from the mantle, and they successfully detected
the 410 and the 660 beneath the Japanese Islands. However, Ps phases from the mantle are probably dominated bylower frequency signals of less than 1 Hz. Since the dominant frequency of the Hi-net short-period station is aroundc The Society of Geomagnetism and Earth, Planetary and Space SciCopyright ences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

1 Hz, it might be difficult to image the upper mantle discontinuities using these short-period data. The ideal setting
for providing high-resolution receiver function images is adense network that can record relatively lower frequency
signals.
Cosited with the short-period sensors, NIED also deployed a high sensitivity accelerometer (hereafter called atiltmeter) network for monitoring crustal deformation. BothHori (2002) and Shiomi et al. (2003) suggested that therecordings of these tiltmeters could be used as broadband
seismometers to disclose the structure of the upper mantle.
If this were to be true, it may be possible to dramatically
improve our knowledge of the dynamics of the upper mantle because we would be able to obtain a large quantity ofavailable data that includes a sufficient number of lower frequency signals.
In this study, we examine whether these recordings areapplicable for imaging the seismic discontinuities in the upper mantle. To calibrate tiltmeter recordings we first chosea closely located broadband station. We then compared theradial component waveforms (e.g. the amplitudes and thetimes of the peaks of phases following the direct P wave)
recorded by the two types of sensors from the same eventin different frequency bands. Through this comparison wewere able to identify a proper frequency band in which thetwo types of recordings show the greatest similarity. We
then used the filtered tiltmeter data and constructed RF images of the upper mantle discontinuities.

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Fig. 2. (a) Comparison of the radial component velocity waveforms observed at the tiltmeter and broadband stations. Band limit is 0.02–0.16
Hz. Red and black lines show tiltmeter recordings observed at NKOH
and a broadband recording observed at YZK [Fig. 1(b)], respectively,
for an event that occurred at 21:21 (UT), May 13, 2003, with the
hypocenter at 17.3◦ S, 167.7◦ E, at a depth of 33.0 km, Vanuatu Islands.
(b) Amplitude spectra corresponding to Fig. 2(a); tiltmeter (red) andbroadband seismogram (black).

Fig. 1. (a) Triangles represent the Hi-net tiltmeter stations used in thisstudy. Contours (in km) denote the depths of the slab surface of the Pacific plate obtained from the distribution of deep and intermediate-deepearthquakes by Zhao et al. (1994). Line A-A indicates the location of
the profile, and the line at the right end of Line A-A shows the widthof the cross section, ±40 km. (b) Two triangles indicate the YZK ofthe F-net broadband network and the NKOH of the Hi-net tiltmeter network. (c) As for Fig. 1(b), but for the NAA of the F-net and ASHH ofthe tiltmeter. (d) As for Fig. 1(b), but for the SGN of the F-net and theTU2H of the tiltmeter.

2.

Data

We examined waveforms from a total of 124 teleseismic
events with Mw ≥ 5.5 that occurred during the period from
April 2003 to May 2004. A total of 675 tiltmeter stations
were used in this study (Fig. 1), and more than 46,000 RFs
with a good SN ratio were obtained over the network. The
original tiltmeter data provided by NIED was in tilt angles(in radians) for the NS and EW directions, with a sampling
rate of 20 Hz. We produced horizontal accelerations bymultiplying the tilt angles by −g (g: gravitational constantand 9.8 m/s2 ).

3.

Method

3.1 Comparison of the waveforms
To use a tiltmeter recording as a seismogram, we needto examine whether the tiltmeter recording is similar to thatof the broadband in the same frequency band. Because thetiltmeter recording is provided as an acceleration seismo-gram, we first integrated it to generate a velocity seismogram. Basically, tiltmeter recordings have a flat response,even at an extremely lower frequency in acceleration. Iflower frequency signals are included in the tiltmeter recordings, the integration from acceleration to velocity would
become unstable. Therefore, before the integration, we applied a high-pass filter of 0.02 Hz to the tiltmeter recordings.
For broadband seismograms, we used recordings obtained
at a nearby F-net station operated by NIED. In Fig. 1(b),we show the location of the two stations, tiltmeter station
NKOH and broadband station YZK, which we chose in ourcalibration. The distance between the two stations is approximately 4 km (Shiomi et al., 2003). Different frequency
bands (0.02–1.0 Hz, 0.02–0.3 Hz, 0.02–0.16 Hz and 0.02–
0.1 Hz) were used to investigate the best bands that show
the greatest similarity. We found very similar waveforms
in velocity between the two if we used the last two bands,
0.02–0.16 Hz and 0.02–0.1 Hz. As an example, for 0.02–
0.16 Hz band, we show the comparisons of radial component velocity waveforms in Fig. 2(a), and the corresponding spectra in Fig. 2(b). These comparisons indicate thattiltmeter and broadband recordings have similar instrument
responses at the frequency band, thereby implying that instrument response for the tiltmeter can be removed approximately through deconvolution by the stacked vertical component, including instrument response for the broadband.
3.2 Construction of the source-time function
Since a tiltmeter records only the horizontal motions, weneed to use other types of records to generate source-timefunctions for deconvolution. To this end we constructedsource-time functions for each teleseismic event by stacking all of vertical components obtained by the F-net broadband stations in the Japanese Islands; that is, we consider
this stacked waveform as the source-time function for the
event (Langston and Hammer, 2001). Because we usedT. TONEGAWA et al.: UPPER MANTLE IMAGING BENEATH THE JAPAN ISLANDS BY HI-NET TILTMETER RECORDINGS

Fig. 3. (a) A vertical broadband seismogram at YZK [Fig. 1(b)] for thesame event as that in Fig. 2. (b) A source-time function calculated bystacking 56 vertical broadband recordings. (c) Regional stacked (RS)source-time functions calculated by stacking 17 vertical recordings thatare observed at broadband stations within a 300-km radius of NKOH.

the 0.16-Hz low-pass filtered RF, the source-time functions
stacked with all vertical components in the Japanese Islands
might not be affected by biases induced by local structure.
However, it is necessary to examine whether we need to use
regional stacked source-time functions to produce 0.16-Hz
low-pass filtered RF. Moreover, tiltmeter recordings in thehigher frequency band may also be available if we deconvolve with regional stacked source-time functions. Therefore, we also constructed regional stacked source-time functions by stacking vertical seismograms that were observedat broadband stations located within a 300-km radius ofa tiltmeter station. When stacking the broadband seismograms, we took the cross correlation to account for moveout

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across the network. An example of the vertical component
observed at YZK, the stacked source-time function, and regional stacked source-time function are shown in Fig. 3(a)–(c).
3.3 Calculation of the receiver function
Based on the comparisons described in the previous section, we decided to set the low-pass filter to 0.16 Hz when
producing the RFs from the tiltmeter recordings. In addition, we produced 0.3-Hz low-pass filtered RFs deconvolved by regional stacked source-time functions. RFs were
calculated through frequency domain division of tiltmeterrecordings by stacked source-time functions with a waterlevel of 0.05 and low-pass Gaussian filter. Under these
conditions, the stacked source-time functions for frequencies lower than 0.01 Hz tend to be suppressed, as shownin Fig. 3, because the pulsation is likely to be suppressedby stacking the waveforms obtained in the Japanese Islands. Therefore, we need to use a large value for thewater level compared to those applied in previous studies(e.g. Langston, 1979). For comparison, we show four examples of receiver function for low-pass filtering of 0.3 Hz[Figs. 4(a)–(d)] and 0.16 Hz [Figs. 4(e)–(h)]. We also confirmed whether we can apply a low-pass filter of 0.16 Hz
to waveforms obtained at the other stations in complex tectonic regions and with the other events by comparing RFs(Fig. 5).
3.4 Stacked RF image in cross-section
Assuming all later phases of the direct P wave are the Ps
phases converted at depth, we migrated the time domain RF
to the depth domain RF using the IASP91 velocity model(Kennett and Engdahl, 1991). To make a transect, we followed the methodology of previous RF studies with CCP(common conversion point) stacking. We used a cell size of
1×1×10 km and two widths of ±40 km and ±100 km fromthe line of the cross-section. See Yamauchi et al. (2003)
and Tonegawa et al. (2005) for a detailed explanation of theabove analysis.

Fig. 4. Four types of 0.3-Hz low-pass filtered RF for: (a) the radial component at YZK deconvolved by the vertical component at YZK, (b) the radial
component at YZK by the stacked source-time function, (c) the radial component at NKOH (tiltmeter) by the stacked source-time function, (d) theradial component at NKOH (tiltmeter) by the regional stacked (RS) source-time function. (e)–(h) Same as (a)–(d), but for the 0.16-Hz low-passfiltered RF. These waveforms are calculated for the same event as described in Fig. 2.

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Fig. 5. (a) 0.16-Hz low-pass filtered RFs obtained at ASHH (red) andNAA (black) [Fig. 1(c)]. The distance between these stations is approximately 4 km. (b) 0.16-Hz low-pass filtered RFs obtained at ASHH
(red) and NAA (black) [Fig. 1(c)], but for the other event that occurred
at 20:26 (UT), November 22, 2004 (hypocenter, 46.6◦ S, 164.7◦ E; 10.0
km in depth). (c) 0.16-Hz low-pass filtered RFs obtained at TU2H (red)and SGN (black) [Fig. 1(d)] for the same event as in Fig. 5(b). Distance
between these stations is approximately 2 km. (Red and black show RFs
for tiltmeter and broadband stations, respectively).

4.

Results

Figure 6 shows the stacked RFs applied 0.16-Hz lowpass filter for the profile A-A shown in Fig. 1. The redcolor represents a positive RF amplitude, indicating velocity increase with depth (e.g. continental and oceanic Moho),
while blue represents a negative RF one, indicating velocity decrease downwards (e.g. top slab surface, lower slab
boundary). The size of the RF image is 1,600×900 km.
Despite the depth of 900 km and the narrow width of ±40
km from the line of the RF image [Fig. 6(a)], there areenough ray paths to image the upper mantle, and the image
is greatly improved in comparison to that of ±200 km usedin the broadband analysis of Tonegawa et al. (2005). Here,in the 0.16-Hz low-pass filtered RF, the detectable thicknessof seismic discontinuity is approximately 15 km; it is 7 km
in a 0.3-Hz low-pass filtered RF described in a following
section.
In the Japan subduction zone, the Pacific slab extends
from the surface to around or below the 660, as suggested
by a recent tomographic study (Fukao et al., 2001). Our RF
image suggests that the oceanic Moho within the subducting slab can be seen to depth of 400 km by positive RF amplitude, and the lower slab boundary is also detected down
to a depth of 600 km by negative dipping RF amplitudes[Figs. 6(a) and (b)]. Based on such a depth difference in appearance, negative amplitudes are not due to side lobes ofthe bandpass filter used in the seismic processing. Although
the imaging of the Pacific plate down to a depth of 200 km
has been reported in a previous study using broadband data
(Tonegawa et al., 2005), those of the oceanic Moho and thelower slab boundary reaching the transition zone have neverbeen reported and are presented for the first time here. The
410 and the 660 are also detected and show undulation inpositive RF amplitude. The uplifted and the depressed partof them affected by the descending slab can also be seenFig. 6. Stacked images, 1,600×900 km, at A-A for 0.16-Hz low-pass
Gaussian filtered RFs. (a) RF image with a width of ±40 km with earthquake hypocenters. (b) RF image with a width of ±100 km and with its
interpretation: OMH, the oceanic Moho inside the Pacific slab; LBP, thelower boundary of the Pacific slab; 410, the 410 km discontinuity; 660,
the 660 km discontinuity.

in the cross-sections [Figs. 6(a) and (b)] and topographicalmaps [Figs. 7(a) and (b)] that were calculated by following Tonegawa et al. (2005), as reported by previous studies focusing on the Japan subduction zone (e.g. Collier andHelffrich, 1997; Tonegawa et al., 2005; Tono et al., 2005).

5.

Discussion

5.1 Tiltmeter recordings as broadband seismograms
Figures 4(e)–(h) show that the four types of RFs are almost the same in the waveform shape, implying that tiltmeter recordings can be substituted for broadband seismograms in this frequency band (lower than 0.16 Hz). For thehigher frequency bands (e.g. lower than 0.3 or 1.0 Hz), although the waveforms were similar with respect to both thelocations of the peaks (e.g. Ps phases) and amplitudes, especially for frequencies lower than 0.3 Hz [Figs. 4(a)–(d)],the waveform in Fig. 4(c) was slightly different from thatin Fig. 4(d). This indicates that the 0.3-Hz low-pass filtered RF deconvolved by the source-time function, which is
obtained by stacking all vertical seismograms over the network, seems to be affected by local structure while the 0.16Hz low-pass filtered RF is presumably not affected. Indeed,
remarkable traces of Ps phase shown in Fig. 6 are also
clearly seen in the imaging with 0.3-Hz low-pass filteredRFs (Fig. 8), which are deconvolved by regional stacked
source-time functions.
However, the continental Moho cannot be seen even with
the 0.3-Hz low-pass filtered RF because large negative amplitudes appear following P-arrival in RF calculated with

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Fig. 7. Topographical contour maps of the 410 and the 660 calculated for 0.16-Hz low-pass filtered RFs. (a) Depth variations of 410 for 0.16-Hz
low-pass filtered RFs. Colors indicate the depth differences from 410 km. Red to yellow indicates the elevation, and pale-blue to blue one indicatesdepression. The red circle shows the uplift portion of the 410. (b) Same as in Fig. 7(a), but for the depth variations of the 660 km discontinuity from
670 km.

Fig. 8. Stacked images, 1,600×900 km, at A-A with a width of ±40 km
for 0.3-Hz low-pass Gaussian filtered RFs. These RFs are produced bydeconvolving with regional stacked source-time functions.

tiltmeter recording [Fig. 4(d) and (g)]. There are two possible reasons for such negative phases. One is water level andthe other is high-pass filter before integration. For the waterlevel, however, Fig. 9(a) shows that different values of water level would not greatly affect waveform shape, implyingthat those negative phases are not produced by a large valueof water level. For high-pass filtering, as shown in Fig. 9(b),we found such negative amplitudes disappear if we maintain the lower frequencies up to 0.005 Hz before integratingfrom acceleration to velocity, indicating that they are presumably produced by a lack of lower frequencies. In thisstudy, however, we choose a high-pass filter of 0.02 Hz forthe following two reasons: (1) if we apply a high-pass filter of 0.005 Hz to the recordings from the other events toremove negative amplitudes, some of these recordings will
be unstable in integration; (2) it seems that a lack of lower
frequencies affects waveform shape, but only just aroundthe direct P wave [Fig. 9(b)]. This defection should be improved if we investigate the structure in the crust and theuppermost mantle.

Fig. 9. (a) Three 0.16-Hz low-pass filtered RFs with different waterlevels: blue line, 0.10; black line, 0.05 [same as Fig. 4(g)]; red line,
0.01. (b) Two 0.16-Hz low-pass filtered RFs calculated by tiltmeterrecordings with different frequency bands: black line, 0.02–0.16 Hz[same as Fig. 4(g)]; red line, 0.005–0.16 Hz.

5.2

Upper mantle discontinuities and the subducting
slab
Why does the positive RF amplitude of the oceanic Mohodisappear in the transition zone? As seen in Fig. 5, theoceanic Moho can be traced down to a depth of 400 km,
terminating below the 410, while the lower slab boundary
can be imaged by remarkable negative RF amplitudes down
to a depth of 600 km. The dipping oceanic Moho shouldbe traced in the transition zone if a sharp velocity contrast
is retained there because the lower slab boundary with thealmost same dip angle as that of the oceanic Moho can beimaged there. The possible reason for such a termination isthat the slab surface is a complex structure by the stagnatingprocess of the slab above the 660. This suggestion is ingood agreement with the results of the tomographic survey
around the Japanese Islands (Fukao et al., 2001).
Moreover, which discontinuity for the subducting slab
does the dipping positive RF amplitude in our result indicate? Because the uppermost portion of the Pacific plate
consists of the oceanic crust, which has a low velocity com-

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pared to both the overriding mantle and the oceanic mantle,
the downward velocity jump corresponding to the slab surface is described by negative RF amplitude. On the other
hand, basalt in the oceanic crust would be metamorphosed
to eclogite at depths of 80–130 km (Yuan et al., 2000; Ferris
et al., 2003; Hacker et al., 2003b), which cannot be seismically distinguishable with respect to mantle peridotite.
Therefore, the negative phase would disappear below thedepth of its transformation, and hence the dipping positive phase corresponding to the oceanic Moho would be
isolated. In this study, the oceanic Moho can be detectedby positive phases down to a depth of 400 km, while theoceanic crust above its transformation depth cannot be detected because of the low resolution of the RF. However,
the oceanic Moho and the lower slab boundary are consistent with the hypocenter distribution in Fig. 6(a). Moreover,since the seismic velocity contrast among overriding mantle
peridotite, eclogite of the oceanic crust and underlying peridotite in the upper mantle possibly vary based on whether
those phases are metamorphosed before or during the subducting process (Hacker et al., 2003a), it is possible that thepositive phase corresponds to the slab surface. To be able
to discuss in more detail to which discontinuity the positive
RF amplitude in our result corresponds, we need to await
the results of future seismological, theoretical and experimental investigations showing the subduction process down
to a depth of at least 410 km.
The 660 is also depressed down to a depth of 700 km in
the region where the lower slab boundary does not reachthe 660 (700–800 km in horizontal axis in Figs. 6(a) and(b)), and its downward returns up to 680 km depth (1,000–
1,500 km in horizontal axis in Figs. 6(a) and (b)). The cold
thermal anomaly associated with the subducting slab seems
to expand relatively large area and affect the distortion of
the 660, even if the subducting slab does not reach the 660.

6.

Conclusion

We report that Hi-net tiltmeter recordings are available
for imaging the upper mantle discontinuities in the Japansubduction zone. The tiltmeter recordings are very similar to the broadband seismograms at a frequency band between 0.02 and 0.16 Hz. If we use regional stacked sourcetime functions for deconvolution, the tiltmeter recordings
at a frequency band between 0.02 and 0.3 Hz can also be
available to the image upper mantle structure. The tiltmeter
RF image shows that the oceanic Moho and the lower slab
boundary can be detected down to a depth of 400 and 600
km, thus realizing one of the highest quality images for subducting slab. Moreover, the upward and downward undulation of the 410 and the 660 affected by the cold interior ofthe descending slab are also traced much more clearly thanhas been possible in previous RF studies.
Acknowledgments. We thank X. Yuan, H. Gilbert and one
anonymous reviewer for reviewing our manuscript. We are also
grateful to F. Niu and Editor E. Fukuyama for their helpful com-ments that greatly improved the manuscript. We used a software
package, GMT, to draw maps (Wessel and Smith, 1999). This
study was funded by a Research Fellowship of the Japan Society
for the Promotion of Science (JSPS) for Young Scientists and byGrand-in-Aid for Scientific Research (B) (17340130) of MEXT.

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