東京大学地震研究所付属
海半球観測研究センター

自己点検報告書

２００６年２月
東京大学地震研究所
海半球観測研究センター
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8,Q,    NIU Fenling (Rice U., USA), D. S. RAMESH (NGRI, India),         PALSHIN Nick A. (RAS, Russia),         `;r5U\3y

Center member:
Professor:
UTADA Hisashi, KAWAKATSU Hitoshi (Director),
KANAZAWA Toshihiko (joint appointment)
Associate Professor:
SHIOBARA Hajime, YAMANO Makoto
Research Associate:
ICHIHARA Mie, SHIMIZU Hisayoshi, TAKEUCHI Nozomu,
BABA Kiyoshi, WATADA Shingo
Secretarial Staff:
MATSUSHIMA Nobuyo
Research Fellow:
SHITO Azusa, HARCOUET Virginie, SHI Xue-Ming,
Technical Support Staff: YOKOYAMA Keiichi
Adjoint Researcher:
ICHIKI Masahiro
Graduate Student:
ASARI Seiki, HAMAMOTO Hideki, TAKAHASHI Yuji,
OHKI Satoko, KAWAKAMI Yoshitaka, BABA Yuta
Visiting Professor:
Fenling NIU (Rice U., USA), D. S. RAMESH (NGRI , India),
PALSHIN Nick A. (RAS, Russia),
FUKAO Yoshio (JAMSTEC), SUETSUGU Daisuke (JAMSTEC)
Research group (faculty member):Global Seismology:
Global Geomagnetism:
Geothermics:
Ocean Bottom Seismology:
Ocean Bottom Geomagnetism:
Physics of Multi-phase Systems:
Data Center:

H. Kawakatsu, S. Watada, N. Takeuchi
H. Utada, H. Shimizu
M. Yamano
H. Shiobara, T. Kanazawa
H. Utada, K. Baba
H. Kawakatsu, S. Watada, M. Ichihara
N. Takeuchi, H. Shimizu

4

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 Research Activities
2-1. Execution and Management of Big Projects
2-1-1. Ocean Hemisphere Network Project (OHP)The Ocean Hemisphere Network Project (OHP), supported by a Grant-in-Aid for Creative
Scientific Research (New Program) from the Ministry of Education, Science, Sports and Culture(Monbusho), was carried out during 1996–2001 to obtain new observational findings that will provide
a new view of the Earth’s interior. For this purpose, a network of multidisciplinary geophysicalobservatories was constructed and long-term observations were carried out in the Ocean Hemisphere,
especially in the western Pacific region, which has long been recognized as the largest spatial gap interms of global coverage of geophysical data. The OHP began in 1996, involving the collaboration of
more than 50 scientists from the University of Tokyo and other universities and national institutions.
The total budget was approximately 1,700m yen for the 6 years of the project. The Ocean Hemisphere
Research Center (OHRC) (involving both scientists and support staff) functioned as the core of theentire project both in terms of research activity and management. The project ended in 2001 with thecompletion of the OHP network (Fig. 2.1.1) of 14 onland broadband seismic observatories (most sitedon Pacific Islands), three seafloor borehole seismic observatories, 10 onland geomagnetic
observatories, one seafloor geomagnetic observatory, seven submarine cables used to measuregeo-electric field variations, 19 GPS stations, five stations for superconducting gravimetry, and a data
center (see also Sections 2-5 and 2-6). Analysis of data from this multidisciplinary geophysicalobservation network, as well as data from temporary observations and existing observatories, has led
to the publication of significant scientific results. The major outcomes of the project are summarized
below.

Kamenskoe

Yakutsuk
KamenskoeOkha Magadan
Khaborowsk
Vladivostok
Ulaanbaatar
TaejeonOkino-Torishima
IshigakiMarcus
Mania
l
Chuuk

DH SF
SF
SF
DH
IshigakiChichijimaMidway
HawaiiManial
DH Marcus SF
SF
PalauPonapeTarawa
Kiritimati
ParapatFunafuti Canton Nukuhiva
Penrhyn
PortMoresby
Vava'u Niue
VanauLevu
Tongatapu
Pitcairn
Norfolk Raoul
TaejeonKota-Kinabalu Palau
Kwajalein
Parapat
Port Moresby

GEODESYSEISMOLOGY

ChangChun

SF
SF

SF

Marcus
GP

Md
i way

HAW1,2

TPC2

TPC1

CO
MP
AC

PonapeTarawaNorfolk

Kiritimati

Huancayo

Pitcairn
EasterFig. 2.1.1. Geodetic, Broadband Seismic
and Geo-electromagnetic components of the
Ocean Hemisphere Project network.

TASM
AN

GEOMAGNETISM(1) Discovery of the Earth’s background free oscillations
A surprising phenomenon was discovered, in that free oscillation of the solid Earth occurs
continuously even in the absence of excitation by major earthquakes. The first discovery of thisphenomenon was made from a time-series analysis of data from superconducting gravimetric stationsof the OHP network (e.g., Suda et al., 1998). As shown in Section 2-4, scientists in the OHRC haveconsistently led this new area of study from its initial discovery to demonstrated proof of its global
nature and subsequent development of the research field.
(2) Whole mantle seismic tomography
A new method of whole mantle tomography has been developed that is optimized to image aparticular part of the mantle–the mantle beneath the western Pacific region where the OHP network
was deployed in the present case–with the highest resolution. This method has been termed the'non-uniform grid method', and has been used in a number of studies. This method provides a clear
image of underlying high-velocity material in the transition zone that indicates stagnation of the slabmaterial supplied from the western Pacific subduction zone (Fig. 2.1.2). The concept of a 'stagnant
slab' was first proposed on the basis of these results. This concept is contradictory to the concept ofslab penetration proposed by the MIT group, and therefore caused great controversy, related in part tothe problem of whether mantle convection is single- or double-layered. Later improvements in theamount and quality of data (Obayashi et al., 2003), as well as inversion schemes and tomographic
studies at much larger scales, have revealed the presence of stagnant slabs in transition zones withinvarious circum-Pacific subduction zones (Fig. 2.1.3), thus demonstrating its global nature (Fukao et al.,2001). Thus, the OHP tomography group made a significant contribution to understanding the physics
of mantle convection.

Fig. 2.1.2. Cross-section of P-wave
velocity perturbation along the great
circle shown by the red line on the map.
A stagnant slab is imaged as a large
high-velocity anomaly in the transitionzone beneath southwest Japan and East
China.



Fig. 2.1.3. Stagnant slab images from the southwest Pacific Ocean(upper), Central America (middle) and Southeast Asia (lower).

(3) Study of fine structures of the mantle transition zone
Seismic investigations were performed in the western Pacific region to reveal detailed structures ofthe mantle transition zone, which are considered to be one of the major factors controlling convection
patterns in the mantle. Two important results were obtained from this project. The first is the discoveryof new seismic discontinuities/reflectors in the uppermost part of the lower mantle at depths of
900–1200 km, immediately below the stagnant slab (Niu and Kawakatsu, 1997; Vinnik et al., 1998;
Fig. 2.1.4). This newly discovered discontinuity/reflector is considered to be strongly related to an
abrupt change in the tomographic image at approximately 1200 km depth. The second main result isthe successful estimation of the density distribution in the transition zone beneath Japan (Kato and
Kawakatsu, 2001). This was the first case in the world of an in situ estimate of density distribution,
which is considered to be the most difficult challenge in seismology. This study clearly showed that it
is possible to determine the density distribution within the transition zone, which is one of the sourcesof the driving force of mantle convection, by applying the method introduced in this study to data
analysis from different regions.

Fig. 2.1.4. Seismic discontinuities/reflectors
beneath the Indonesia arc. Upper: map view.
Middle: location of discontinuities/reflectors.
Lower: tomographic image of the region(after Vinniki et al., 1998).

Fig. 2.1.5. Comparison of EM tomography(right) and P-wave tomography (left) of
the north Pacific region at the depth range
300–800 km at 100 km intervals.



(4) Semi-global mantle electromagnetic tomography
An effective code was developed for forward and inverse problems of 3-D electromagnetic (EM)induction. Using the inversion code, EM response functions estimated from geomagnetic andgeo-electric data derived from OHP geomagnetic stations, the OHP submarine cable network, andexisting geomagnetic observatories, were inverted to a 3-D electrical conductivity distribution (Fig.
2.1.5). This distribution is defined as a perturbation of conductivity, from a semi-globally averaged
1-D reference (Utada et al., 2003), in the upper and mid mantle for the quarter of the globe that iscentered by the North Pacific Basin (Fukao et al., 2004). A joint interpretation was attempted for the
EM and P-wave tomography, assuming that both are ascribed to temperature anomalies. Results of thisinterpretation indicate that major anomalies, such as the conducting and low-velocity anomaly beneath
Hawaii and the resistive and high-velocity anomaly in the lower part of the transition zone beneath thePhilippine Sea, can be simply explained by temperature anomalies of 100–200 K. However, theconducting anomaly in the upper part of the transition zone beneath the Philippine Sea is notaccompanied by a seismic velocity anomaly. This indicates that an additional effect is necessary toconsistently account for these features.
2-1-2. Stagnant Slab Project (SSP)As one of the post-OHP science programs, the Stagnant Slab Project (SSP) was proposed as a
5-year project and funded in 2004 by a Grant-in-Aid for Scientific Research in Priority Areas (MEXT)
to carry out an intensive and multidisciplinary study on mantle dynamics with the key term of“stagnant slab.” The project is organized by eight subgroups (three in seismology, one in EM, two in
high-pressure science, and two in computer simulation). The total budget of approximately 1,400m

Fig. 2.1.6. Locations of BBOBS (red dot) and OBEM (pink cross) sites
deployed as part of the Stagnant Slab Project.

yen will be supported for five years. More than 70 scientists are participating this project from morethan 10 research institutions throughout Japan. Again, the OHRC plays an important role in bothresearch activity and project management of the SSP. Various collaborations in this project are
expected to lead to a better understanding of the mechanism of slab stagnation and subsequent descentof the slab into the lower mantle, as well as further reveal the effects that these processes have had on
the history of plate motions and the entire Earth history.
Scientists in the OHRC are involved in two subgroups that carry out long-term seafloor seismic
and EM observations to provide datasets for high-resolution seismic and EM tomographies,
particularly for the region beneath the Philippine Sea where a vast amount of slab material is stagnant
within the transition zone. In October 2005, the first SSP cruise was carried out by the R/V Kairei of
JAMSTEC (Japan Agency for Marine Science and Technology), in which 16 broadband ocean bottom
seismometers (BBOBSs) and 11 ocean-bottom electro-magnetometers (OBEMs) were installed (Fig.
2.1.6). These instruments will be recovered in 1-years time, when a second set of instruments will be
deployed at mostly the same locations. By repeating such installations, a three-year observation using
BBOBSs and OBEMs is planned to provide more reliable datasets to improve the resolution of seismic
and EM images of the stagnant slab and surrounding area. These two research groups will thus
contribute to the goal of the SSP by determining the distribution of physical parameters that control
mantle dynamics.



2-2. Ocean and Land Observations
2-2-1. Seafloor Seismic Observation
Since initial experimental attempts at the Japan Sea in 1989 and the Atlantic Ocean in 1992, there
have been few broadband seismic observations on the seafloor until the OHP began practicalobservations in 1999 in the northwest Pacific Ocean and the Philippine Sea. From the beginning of the
OHRC in 1997, we have been developing broadband and long-term seismic observation systems forthe seafloor that are able to interpolate the global seismic network, which has only sparse coverageover large oceanic areas; this is a principal goal of the OHP. Two types of systems have beendeveloped: the Seafloor Borehole Seismic Observatory (SBSO) as stable stations within the global
seismic network, and BBOBS for long-term but mobile array observations. These systems are basedon our long experience of ocean bottom seismometer development, and were developed in cooperation
with members of the Earthquake Observation Center (ERI) and JAMSTEC. Figure 2.2.1 shows thelocations of these stations during the OHP period; this represents the first step for ocean bottom
broadband seismology. After several successful experimental observations by the BBOBS, we began
practical array observations in 2003, as described later in the text.

Fig. 2.2.1. Location map ofseafloor seismic stations
employed during the OHP.

The four SBSOs were constructed as part of the ODP program to drill the boreholes and install theheavy main unit on the seafloor, with the final setup and maintenance performed using the ROV"KAIKO" (JAMSTEC). The JT-1 and JT-2 stations were built in 1999, mainly to monitor seismic andgeodetic signals on the continental shelf in areas with different background seismic activities. As partof the global seismic network, WP-1 and WP-2 have been working since 2001 and 2000, respectively.
These stations have two sets of broadband sensor units cemented in place near the bottom of the13

borehole, and data is stored at the re-entry cone (Fig. 2.2.3). At the WP-2 station, we have successfully
obtained data over a period in excess of 400 days, which has yet to be bettered by any seafloor
borehole seismic station. The data quality is also good compared to previous seafloor borehole seismic
stations, as shown in Fig. 2.2.4; noise level is low and stable over time, comparable to a quiet inland
station (Araki et al., 2004; Shinohara et al., 2006).

Fig. 2.2.2. Photographsof the WP-2 during final
setup.

Fig. 2.2.3. System diagram of
the SBSO.

Fig. 2.2.4. Noise levels of theseafloor borehole stations.

14A trade-off of the SBSO's high performance is the difficulty in increasing the number of stations.
To address this problem, the mobile BBOBS and long-term OBS (LTOBS) have been developed tocompensate the global network and perform array observations to achieve better spatial resolution;these are operated by free-falling deployment and self-popup recovery. The BBOBS (Fig. 2.2.5) has atitanium sphere (D=65 cm) that contains a broadband sensor with a leveling unit, a data recorder witha high-precision clock, an acoustic transponder unit, and lithium cells for power supply. The LTOBS
is similar to the BBOBS, although the sensor does not provide long period coverage and it has asmaller sphere (D=50 cm). After the first experiment with the LTOBS at the East Pacific in 1997, thefirst BBOBS (NWPAC1) was deployed at the same position as the WP-2 in 1999, with successful
recovery in 2000. From the 1-year data by the BBOBS, the noise model is indicated with that of the
KIP land station (Fig. 2.2.6), which shows good performance in the vertical component. The reason
for the relatively high noise level in the horizontal components is probably tilt variation of the BBOBS
sphere due to tidal bottom current. To reduce this noise level, we are now developing a new generation
BBOBS system with a separate sensor package that is stuck in the sediment but is still mobile.

Fig. 2.2.5. Photographs of the BBOBS on the ship deck (left) and upon the seafloor

Fig. 2.2.6. BBOBS noise modelof NWPAC1 and 2

15

As examples of research into the Earth interior using seafloor seismic observations, we describe
two results from the Mariana LTOBS array observation (MR2001) and a combined study of several
observations in the Philippine Sea using a receiver function analysis. The former experiment involved
using 10 LTOBS to conduct a 1-year passive observation around the active deep seismic zone toinvestigate the subduction system. The array detected more than 3000 local events, while the PDE list
recorded only about 60. By using a simultaneous inversion method with the event data set, we foundthat the hypocenter distribution shows a clear deep double seismic zone down to 200 km depth, anddetermined the velocity structure of the upper mantle (Fig. 2.2.7). The latter study used data fromvarious observations by LTOBS (PHS1999), BBOBS (NOT1), and SBSO (WP-1) in the Philippine
Sea (Suetsugu et al., 2005). Fig. 2.2.8 shows the main result: shallow (410 km) and deep (660 km)
discontinuities were determined at the site WP-1. Our results also indicate that this variation in depth
within the Philippine Sea may be related to the morphology of the stagnant Pacific Slab.

Fig. 2.2.7. Hypocenters and velocity
structure from the data of MR2001
observation (Vp and Vs, relative to
IASP91).

Fig. 2.2.8. Depth of upper mantle
discontinuities derived from a
global tomography result.

16

From 2003 to 2005, we made cooperative land and ocean-floor observations in French Polynesia
with IFREE (Institute For Research on Earth Evolution) of JAMSTEC and French scientists (thePLUME Project; Suetsugu et al., 2005). Finally, data from nine of the ten deployed BBOBS wereobtained for the entire deployment period with a remarkably low micro-seismic noise level.
Preliminary analysis using a receiver function method shows a normal upper mantle discontinuity
depth, except for the area around the S2 station that is probably related to the Society Hotspot.
Another large-scale observation in the Philippine Sea and northwest Pacific, the Stagnant Slab Project,began in 2005 and is to continue until 2008, using 12 or 16 BBOBS.

Fig. 2.2.9. Location map of
the PLUME Project in French
Polynesia.

2-2-2. Land Seismic Observation Network(1) Permanent network
Japanese researchers began to deploy an international seismic network in the Western Pacificregion as a part of the POSEIDON (Pacific Oriented SEISmic Digital Observation Network) Project in1987. The POSEIDON Project is not a large funded project but a non-funded collaboration between
individual researchers who are interested in global seismic observations. The observation systems,
data formats, and policy for data distribution were not unified, and the POSEIDON network did notessentially function as a single network.
After the establishment of the OHRC, we established a unified and modern seismic network (OHP
Network) by improving the POSEIDON stations in collaboration with researchers from anotherinstitutes. We unified the observation systems to STS-1 and a data logger developed by the Nagoya
University. We have also been deploying real-time data transmission systems since 2002. We
established the Ocean Hemisphere Project Data Management Center (OHPDMC) to distribute theobservational data to the public (see Section 2-6).

17

(2) Portable networks
We are also deploying portable networks to supplement coverage by permanent stations. We
developed a data logger that is suitable for portable observations in foreign countries where asufficient quality of electrical power and communication lines cannot be guaranteed. Our system uses
a compact HDD for data storage, which enables long-term and stable observation. Using this system,
we deployed portable networks in China (four stations deployed between 1998 and 2003) and Vietnam(six stations deployed since 2001; Fig. 2.2.10). This system was also adopted in other projects(JISNET and SPANET).

Fig. 2.2.10. Location map of
portable seismic stations in
Vietnam (blue triangles).
Permanent IRIS and CDSN
stations are indicated by blue
circles.

(3) Education
We provided training for our counterparts in maintaining our stations and analyzing observational
data. We invited two trainees from Indonesia in 1999 and one from Vietnam in 2002. We also invited
14 trainees from the China Seismological Bureau to train in skills for data analysis.
2-2-3. Seafloor Electromagnetic Observations
Electromagnetic observations on the seafloor surrounding Japan have been carried out by the OHP,
the Stagnant Slab Project, and related collaborative studies with other institutions (Fig. 2.2.11). Theseexperiments contribute to the study of semi-global scale mantle dynamics in the region by conjunctanalysis as well as the study of various tectonic settings.
(1) Philippine Sea experimentSix ocean bottom electromagnetometers (OBEMs) were deployed on the seafloor of the West
Philippine Basin, the Parece-Vela Basin, and the Mariana Trough in November 1999 and subsequently
recovered in July 2000. From these data we estimated the 1-D electrical conductivity structure of the18

Fig. 2.2.11. Seafloor electromagnetic
observation sites deployed as part of
the OHP network, Stagnant Slab
Project, and related projects (crosses),superimposed on a bathymetry map of
the Japan region. Triangles represent
onland geomagnetic stations, whilelines indicate submarine cables used
for electric field measurements.

upper mantle at each site (Seama et al., submitted to Phys. Earth Planet. Inter.). The experimentdemonstrated the excellent potential of this method to investigate the geothermal structure and waterand melt content of the upper mantle.
(2) Sea-Floor Electro-Magnetic Station (SFEMS)The SFEMS was developed to achieve continuous seafloor observation of absolute geomagnetic
total force as accurately as land-based observatories, including the vector geomagnetic field andhorizontal electric field (Toh et al., 1998). The SFEMS has been in operation at 41°07’03”N,
159°55’43”E in the northwest Pacific Ocean since August 2001. The data collected to datedemonstrate that the seafloor observatory can contribute to improving the spatial resolution of theexisting geomagnetic observatory network in the middle of the northwest Pacific, where long-term
geomagnetic data are lacking (Toh et al., 2004).
(3) Mariana experiment
Seafloor magnetotelluric (MT) experiments in the central Mariana region have been used toinvestigate mantle dynamics associated with plate subduction, arc volcanism, and back-arc spreading.
A pilot survey was conducted using 10 OBEMs during 2001–2002. The obtained data were inverted,and the resultant electrical conductivity model suggests that the melt generation process within a backarc spreading axis is similar to that of normal oceanic spreading. Further experiments at 40 sites using
47 instruments began in December 2005 under international collaboration.
(4) Japan Sea experiment
The aim of this experiment was to image the back-arc mantle beneath the eastern Japan Sea. Six
OBEMs were utilized during 2002–2003. Useful data were acquired from four OBEMs and analyzed
together with onland data. The obtained conductivity model demonstrates a high conductivity zonethat indicates a relationship with the root of the “hot fingers”, which is as cluster-like distribution of
volcanoes and low velocity anomalies in northeast Japan.

19

(5) Earth’s electric Field Observation System (EFOS)EFOS was developed with the aim of measuring the electric field of the Earth on the seafloor using
a long (10–100 km) cable. It is expected that measurements using EFOS will enable us to detect theelectric field that originated from toroidal magnetic mode penetrated from the outer core. A prototype
with a 10 km cable (EFOS-10) was installed using JAMSTEC’s deep-tow and ROV technology upon
the Daito Ridge, West Philippine Basin. Comparison of the data collected over a 1-year period withdata from the submarine cable TPC2 and data collected from an OBEM demonstrate the successful
operation of the EFOS.
(6) Northwest Pacific experiment
A 1-year seafloor MT survey using OBEMs was conducted during 2003 with several cruises in the
Northwest Pacific Ocean to investigate the electrical conductivity of the upper mantle and transitionzone. In this region, a remarkable low-velocity anomaly was imaged from global seismic tomography,
and very young (within 1 Ma) intra-plate volcanism was detected (Hirano et al., 2001; Obayashi et al.,2006). One of the goals of this experiment is to elucidate the relationship between these phenomena
through the seafloor MT survey. Data have been collected at seven sites to date, and analysis isongoing.
(7) Stagnant slab experiment
As part of Stagnant Slab Project, a long-term semi-global scale seafloor MT experiment using
OBEMs has been planned to image the mantle transition zone beneath the Philippine Sea, where thesubducting Pacific plate appears to have stagnated above the lower mantle. Eleven OBEMs were
deployed in October 2005 at sites covering the northern West Philippine Basin, the Shikoku Basin,
and the northern Parece-Vela Basin, at ~500 km spacings. This survey is just the first phase of theexperiment. The 1-year measurement will be iterated three times to acquire sufficient data to image thetransition zone.
2-2-4. Onland Electromagnetic Observations
We have been conducting geo-electromagnetic observations in northeast China to reveal theelectrical conductivity structure of the upper mantle, especially around the depth of the mantle
transition zone. A highly seismic velocity anomaly documented in the area beneath northeast China–
East China Sea is interpreted as a stagnant slab. Understanding the electrical conductivity structure of
the back-arc region is essential to consider the physical state of the stagnant slab and the origin of
back-arc volcanism in northeast China.
Observation of the magnetic field began in 1998 at Changchun, China, and regional-scale electricfield observations using retired telephone cables of lengths 20–40 km have been performed since thattime in collaboration with the Institute of Geology, China Earthquake Administration. Long-period
geomagnetic depth sounding (GDS) and magnetotelluric (MT) responses have been obtained. The
responses were then inverted to 1-D electrical conductivity structures that are representative of thestructure beneath the magnetic station and cables. The resultant electric conductivity profiles in the
Jilin area, China, are shown in Fig. 2.2.12, together with those for Tucson, USA (red dashed line),
Carty Lake, Canada (blue dotted line), and Honolulu, USA (green chain-shaped line). It is evident thatthe mantle transition zone beneath northeast China is significantly more conductive than those of theother areas. Similar patterns of electrical conductivity were observed in the western and central parts20

of the Liaoning area.

Fig. 2.2.12. Comparison of conductivity
profiles. (from Ichiki et al. 2001).

We calculated profiles of temperature and water content in the upper mantle above the stagnant
slab of the Pacific back-arc from the obtained electrical conductivity and seismic P-wave velocity (Vp)structures. Geothermal profiles were determinedfrom the conductivity and seismic structures
assuming a dry hartzburgite or dry pyrolite
composition. For the deeper part of the upper
mantle (depth > 250 km), it is not possible toobtain a geotherm that satisfies both conductivity
and seismic structures if dry hartzburgite orpyrolite is assumed. This discrepancy can be
explained by allowing for a small amount ofwater (500–1000 ppm H/Si) in the mantle. In
shallower parts of the upper mantle (depth < 250km), the electrical and seismic geotherms are
consistent with each other at 1500–1700 °C for
dry hartzburgite, whereas they are inconsistentby more than 100 °C for dry pyrolite.
Alternatively, a wet pyrolite composition applied
to the deeper part of the upper mantle alsosatisfies the electrical conductivity and seismic
Vp structures in the shallower part. The proposedmodels that are consistent with both electricalconductivity and Vp are shown in Fig. 2.2.13.
Fig. 2.2.13. Models that are consistent
with both the seismic and conductivity
anomalies beneath northeast China(from Ichiki et al., in press in Phys.
Earth Planet. Inter.).

21

2-2-5. Heat Flow and Temperature Observations(1) Thermal structure of the Nankai subduction zone, Southwest Japan
We have been conducting heat flow measurements in the Nankai Trough area to constrain theboundary condition of a thermal model of the Southwest Japan subduction zone. Concentrated
measurements taken off eastern Shikoku (near Muroto) from 1999 to 2001 reveal that heat flow on thefloor of the Nankai Trough is extraordinarily high, almost twice as high as the value expectedconsidering the age of the subducting Philippine Sea plate (Fig. 2.2.14; Yamano et al., 2003). In
contrast, data recently acquired off the eastern Ki-i Peninsula (near Kumano) show that heat flow on
the trough floor in this area is normal. These results indicate that the thermal structure of the
subducting plate varies significantly along the margin. We are now collecting data from the troughfloor between the Muroto and Kumano areas to delineate the extent of the high heat flow anomaly.
Upon the accretionary prism, landward of the trough floor, heat flow profiles across the Muroto
and Kumano margins are similar except for in the vicinity of the deformation front (Fig. 2.2.14). Heatflow decreases landward and reaches about 50 mW/m2 at around 40 km from the deformation front.
These heat flow profiles provide constraints on the thermal structure of the subduction zone and theamount of frictional heating along the plate interface. Comparison of observational data with the heatflow profiles calculated using subduction thermal models indicates that frictional heating (i.e. effective
friction coefficient) is very low in the Nankai subduction zone, consistent with results reported for
Cascadia and other subduction zones.
250

Heat flow (mW/m2)

Off Muroto
200
150
100
50
0
250

Heat flow (mW/m2)

Off Kumano
200
+

150

Deep sea
Shallow sea
BSR depths
ODP holes

100
50
0
80

60

40

20

0

-20

Fig. 2.2.14. Heat flow data from the
Muroto and Kumano margins of the
Nankai subduction zone plotted against
distance from the deformation front.

Distance (km)(2) Heat flow measurements in shallow seas
It is difficult to obtain reliable heat flow measurements in shallow seas where the bottom watertemperature is unstable. Long-term monitoring of temperatures in surface sediments may be a practicalsolution to this problem, and we have developed pop-up type temperature monitoring instruments for22this purpose (Fig. 2.2.15). The instrument can measure the sediment temperature profile to a depth of 2m over a period of about one year. We monitored temperature profiles in shallow sea areas of the
Nankai subduction zone and obtained temperature records for over 200 days at seven stations.
Analysis of the temperature data show that bottom water temperature variations (BTV) propagatedthrough sediments solely by thermal diffusion. We were then able to remove the influence of BTV
from the temperature records and determine the temperature gradient and heat flow at all stations(Hamamoto et al., 2005). The obtained heat flow values generally agree with those estimated from
depths of methane hydrate BSRs in the vicinities of the stations. These results demonstrate that longterm temperature monitoring is a useful method for heat flow determination in shallow seas.

Fig. 2.2.15. Photograph of a popup type temperature monitoring
instrument.

We also conducted temperature monitoring for about one year at three stations around cold-seepbiological communities in the Nankai Trough area, using instruments deployed and recovered bysubmersibles. We estimated the vertical pore-fluid flow rate and heat flux by analyzing the obtained
temperature records.
(3) Long-term temperature monitoring in boreholes
We have monitored the temperature profile in a borehole drilled into the Nojima fault, an active
fault in southwest Japan, since 1997, using the distributed optical fiber temperature sensing technique.
The temperature profile has been very stable until the start of a water injection experiment, and we did
not detect any thermal effect of the 1995 Hyogo-ken Nanbu Earthquake (Yamano and Goto, 2001).
During water injection experiments conducted in 2000, 2003, and 2005, temperature drops associatedwith the injected water were observed only above about 540 m. This clearly shows that the waterleaked out of the hole at this depth (Yamano and Goto, 2005). Changes in the temperature profile werealso observed when groundwater was flowing out of the borehole. The shape of the measured
temperature profile indicates that the discharging water entered into the hole at the same depth (540 m).
(4) Compilation of heat flow data
We have been compiling heat flow data from the northwest Pacific area, including the JapaneseIslands. The most recent version of the compilation covers an area from 0 to 60°N and 120 to 160°E,
and contains heat flow data from 3195 stations. This data set was published with a list of references in2004 as part of a CD-ROM (Yamano, 2004).

23

2-3. Structure and Dynamics of the Earth’s Interior
2-3-1. Electromagnetic Structure and Core Dynamics
We studied the electrical conductivity structure of the upper mantle and uppermost part of thelower mantle beneath the Pacific region using magnetic field observatory data and newly obtained
electromagnetic field data from the OHP (see Section 2-5). Our findings indicate the importance of theeffect of land-sea conductivity contrast in obtaining proper structures (Fujii and Utada, 2000). Thisimplies that the electromagnetic induction problem is inherently a 3-D one related to the effect of
seawater, even if a 1-D mantle is assumed. We developed computer codes that model global andsemi-global electromagnetic induction in a 3-D medium based on an integral equation method; thesemodels are utilized to obtain PREM-like 1-D reference conductivity structure beneath the Pacific(Utada et al., 2003) and for the first successful 3-D inversion of semi-global electrical conductivity
structure in the mantle (Koyama, 2002; Fig. 2.3.1).

Fig. 2.3.1. Cross-section of the 3-D structure of electrical conductivity in the
mantle beneath the Hawaii–Philippines region (right, from Fukao et al., 2004).

It is well known that the toroidal magnetic field must exist in the Earth’s core in order for it to bea dynamo, although the component is confined in the core. Although the field cannot be detected bymagnetic field observations at the surface of Earth, its signature is present in the geo-electric field andcan possibly be detected by electric field observations using cables at the scale of thousands ofkilometers. We began geo-electric field observations in the northwest Pacific with the aim of detecting
the signature of the toroidal field (see Section 2-5). Long-term (decade-scale) variations in electric
potential obtained to date indicate that decade-scale variation in the toroidal magnetic field at the coremantle boundary is 1–10 times that of the poloidal field variation, with reasonable electricalconductivity of the mantle (Shimizu et al., 1998). We confirmed that the amplitude is consistent withthe electrodynamics of the geodynamo (Shimizu and Utada, 2004).
We are studying the small-scale magnetohydrodynamics within a rotating system as a step towards
obtaining better parameterization of subgrid scale phenomena, especially the mean-field electromotive
force, and establishing more realistic geodynamo models as a joint project with David E. Loper(Florida State University, USA) and Arnaud Chulliat (Institut de Physique du Globe de Paris, France).
Structures of the flow and magnetic field driven by a buoyant blob are classified based on the strengthof the rotation of the system and the magnetic field (Shimizu and Loper, 1997); detailed structures are
obtained for the case of rapid rotation that is appropriate for the Earth’s core (Loper et al., 2003;

24

Chulliat et al., 2004; Fig. 2.3.2). Although the integral of the electromotive force to the flow and fieldover entire space is zero, it has a significant non-zero value when integrated over the cross-section of
the Taylor column, and it is significantly anisotropic (Shimizu and Loper, 2000; Chulliat et al., 2003,
2004).

Fig. 2.3.2. An example of the structure offlow driven by a buoyant blob in a rotating
magnetohydrodynamic system (from Loperet al., 2003).

The problem of the state of the inner core boundary (ICB), whether a mushy layer or a slurry layer,
is revisited as a new joint research project with J.-L. LeMouël and J.-P. Poirier (Institut de Physiquedu Globe de Paris, France). Stability analysis of the solid-liquid interface indicates that the condition
that leads to a slurry layer does exist, but it cannot be attained within the Earth’s core; it is mostprobable that a mushy layer exists at the ICB.
Determining core surface flow by inverting geomagnetic secular variation with a frozen flux
assumption has unavoidable physical non-uniqueness. We studied the use of length of the day (l.o.d.)data to overcome the non-uniqueness by assuming topographic (Asari et al., 2006) or electromagnetic
core–mantle coupling. Cylindrical torque, which should be balanced in decade-scale core dynamics, isadditionally considered both as a constraint to reduce the non-uniqueness and to obtain dynamicallyappropriate core surface flow (Asari, 2006). We found that only topographic core–mantle coupling iscompatible with decadal cylindrical core dynamics.
2-3-2. Seismic Tomography
The highlight of tomographic studies within the OHP is the introduction of a unified story for thefate of subducting slabs (Fukao et al., 2001; Fig. 2.1.2–3). After Fukao et al. (1992) suggested theexistence of stagnant slabs, various tomographic studies attempted to reveal further detailed slab
structures. Compiling those results, we found that every major subduction zone in the world can be
explained as a snapshot of the following subduction process: subducting slabs tend to be deflected
within the Bullen transition zone (400–1000 km depth), but can sink into the lower mantle via an
instability. To confirm the plausibility of this idea, we are now conducting a nation-wide project toreveal the nature and role of stagnant slabs.

25

Tomographic studies conducted within the OHP have also been characterized by efforts to develop
unique techniques. We developed an efficient method to compute accurate synthetic seismograms for
3-D heterogeneous Earth models (Takeuchi et al., 2000), an appropriate weighting method to correct
heterogeneous data sampling (Takeuchi and Kobayashi, 2004), and accurate methods for travel-timemeasurements to correct systematic bias in conventional methods (Fukao et al., 2002; Oki et al., 2004).
These unique techniques have revealed important features of the Earth structure, including (i)boundary layers at 670 km depth indicated by the predominance of lateral heterogeneities with longer
horizontal scale lengths in this region, and (ii) low R (=dlnVs/dlnVp) values in subducting slabs
obtained by highly accurate S–P time measurements (Fig. 2.3.3).

Fig. 2.3.3. (left; presented at the 2005 AGU fall meeting) S-wave velocity models obtained
via the full waveform inversion of Takeuchi & Kobayashi (2004). Note the longer
horizontal scale lengths of heterogeneities in the vicinity of 670 km depth, especially in
upwelling regions. (right; presented by Oki, 2006, Ph.D. thesis) R (= dlnVs/dlnVp) modelobtained from the accurate S–P time measurements of Oki et al. (2004). Note the smaller
R-values in the subducting regions, which indicate that R-values are lower in high-Q media.

2-3-3. Array Seismological Studies of the Earth's Deep Interior
Although seismic tomography is a powerful tool to unravel hidden structures within the Earth's
deep interior, its resolution remains limited. To supplement existing information on the Earth's deep
interior, we used the array seismological technique, which is sensitive to small-scale (~10 km)
heterogeneities. We have particularly focused on utilizing dense and high-quality Japanese network
data (J-array, F-net, Hi-net) in an attempt to detect structures that were not detected in previous studies.
Our earlier studies indicated the existence of discontinuities/reflectors in the mid-mantle (Kawakatsu
and Niu, 1994; Kawakatsu and Niu, 1997; Vinnik et al., 1998, 2001; Fig. 2.1.4) and multiple
discontinuities at the bottom of the mantle transition zone (Niu and Kawakatsu, 1996); otherresearchers have investigated both of these features in subsequent studies. We also succeeded inestimating the local density contrast at discontinuities in the mantle transition zone, which is a difficult
parameter to estimate seismologically (Kato and Kawakatsu, 2001).

26

(1) Reflection seismology of the upper mantle
Recent studies utilize the large amount of high-quality Hi-net data. For example, to delineate theeffect of the presence of a cold "deep mantle slab" on the mantle transition zone discontinuities, we
conducted a receiver function analysis to finely map the seismic discontinuities beneath the JapaneseIslands (Kawakatsu and Watada, 2005; Fig. 2.3.4). The obtained image shows remarkably clear
continuous features corresponding to the 410 km and 660 km transition zone discontinuities; inaddition, the top surface of the subducting Pacific plate is traceable to at least 300 km depth. Whilethese general features are expected for a penetrating subduction, the observed 500-km-wide depressionof the 660 km discontinuity beneath southwest Japan appears to be consistent with neither a simple
penetrating slab nor a flat stagnating slab near the boundary. A model that involves a more
complicated morphology of the stagnating slab structure may be required.

Fig. 2.3.4. Cross-section of P–S
conversion energy beneathsouthwest Japan.

(2) Reflection seismology of the inner core
We have also made an unusual entire array observation of the near-vertical PKiKP phase, which is
known to be very difficult to observe (Kawakatsu 2006, in press). Array analyses of this rare data set
show a sharp (<~1 km) inner core boundary. Utilizing the PKiKP as a reference phase, a reflection
seismological study of the inner core was possible for the first time. This study identified a possible
discontinuity beneath the ICB.

Fig. 2.3.5. Hi-net observationof PKiKP.

27

2-4. Solid-fluid Complex Systems in Earth Science (“Blue Earth Seismology”)
2-4-1. Continuous Free Oscillations of the Earth
The discovery of incessant excitation of the free oscillations of the Earth started from the analysisof 3-year superconducting gravimeter data recorded at Syowa, Antarctica, as part of the OHP (Nawa etal., 1998). Numerous subsequent papers (e.g., Suda et al., 1998; Kobayashi and Nishida, 1998)
confirmed that incessant oscillation is a global-scale phenomenon by analyzing continuous recordingsof global seismic networks. The power spectrum of the continuous free oscillation shows clear
seasonal changes, with the power being larger in the Northern Hemisphere winter and smaller insummer; this trend is common to all quiet seismic stations. Another prominent feature of the power
spectrum of the oscillation is the excess amplitude of specific spheroidal normal modes, which are
known to couple to long-wavelength acoustic modes trapped in the atmosphere. This resonant
oscillation between the solid Earth and the atmosphere is termed Earth’s hum (Nishida et al., 2000).

Fig. 2.4.1. Ground motion spectrum forseismically quiet days. Days are
represented in the ordinate andfrequencies in the abscissa. The verticalyellow lines indicate the incessant freeoscillations of the Earth.

These observations led the idea that the main source of the continuous oscillations is atmosphericturbulence within thermal convective motion in the troposphere, as powered by solar influx via
heating of the ground beneath the turbulent atmosphere. Modeling of the frequency-dependence of theobserved power spectrum was conducted by assuming the ground surface is randomly andhomogeneously buffeted in space and time by the atmospheric turbulence (Fukao et al., 2002).
Parametric fitting in the modeling was successful only in the limited frequency band below 10 mHz
and by assuming a characteristic scale length of the atmospheric turbulent motion. A 2-year local
barograph array observation was conducted to measure the characteristic scale length of theatmospheric turbulence (Nishida et al., 2005). The barographic array observation demonstrated thatthe largest source of the atmospheric turbulence is wintertime wind, while measurement of thecharacteristic scale length was inconclusive in determining if atmospheric turbulence is the primarycause of the continuous free oscillations of the Earth.
Another powerful source of the continuous free oscillations of the Earth is within the ocean.
Watada and Masters (2001) analyzed continuous ocean bottom pressure records from the deep sea and28

identified three characteristic features that support the oceanic excitation hypothesis: (i) the pressure
power spectrum at the ocean bottom in the frequency range of 1-10 mHz is larger than the atmospheric
power by one or two orders of magnitude, (ii) the frequency dependence of the power spectrum at theocean bottom resembles that of the observed power spectrum of the continuous free oscillations,
including the sharp drop in power beyond 10 mHz, and (iii) the spectrum power reaches a maximum
during winter. The source regions of the continuous free oscillations can be directly determined fromthe analysis of the global broadband seismic array. The results show that seasonal and geographic
changes of the excitation sources from the Pacific during November to February to high latitudes of
the Southern Hemisphere from June to September, consistent to the oceanic excitation hypothesis.
2-4-2. Seismology in the Atmosphere
There are thee kinds of waves in the atmosphere in thefrequency band above sub-milihertz: acoustic waves, Lambwaves, and gravity waves. These atmospheric waves are
excited by various sources and mechanisms such as
volcanic eruptions and earthquakes, and are observed asatmosphericpressurechangesandionosphericperturbations at distance sites. In research into active
volcanoes, a simple pulse-like acoustic signal of a volcanic
explosion directly indicates that the explosion process was
simple, while a complex seismogram during a volcanic
explosion reflects the propagation effect of seismic waves
through the strong heterogeneous body. The harmonic
excitation of seismic surface waves following the eruptionof Mt. Pinatubo in 1991 at two specific frequencies is an
example of resonant oscillations of atmospheric acoustic
waves and the spheroidal normal mode of the Earth.
Watadaetal.
(2006)
demonstrated
frommicrobarometers deployed along the Japanese Islands that
atmospheric pressure fluctuated during the passage of theRayleigh waves following the 2003 M8.3 Tokachi-Oki
Earthquake. By examining co-located broadband
seismograms and barograms, the authors revealed that thepressure change is caused by ground motion beneath themicrobarometers, and thereby constructed a transfer
function between the vertical ground motion and pressurechange up to a period of about 50 seconds. The observed
transfer function is in good agreement with the theoreticaltransfer function.
Observation and analysis of these atmospheric waves,together with ground motion and ionospheric density
perturbation records, will help to quantify the excitation
sources and reveal previously poorly known behaviors of
the Earth.

29

Fig. 2.4.2. Record sections of original
velocity seismograms of the verticalcomponent (upper) and filteredmicrobarograms (lower). Atmosphericpressure change travels at the same
speed as seismic Rayleigh waves:approximately 3.2 km/sec.

2-4-3. Broadband Seismometry at Active Volcanoes
We have been deploying broadband seismometers, together with other geophysical instruments, at
active volcanoes in Japan as part of cooperative research projects with Kyoto University, Tokyo
Institute of Technology, Tohoku University, Kyushu University, and Hokkaido University. Of theresearch sites, the deployment at Aso Volcano is notable as it is recognized as one of the best
examples in the field of volcano seismology in demonstrating the significance of broadband
seismometry at active volcanoes.
Our observations using broadband sensors reveal the existence of unusually long-period (15 s)
tremors (Fig. 2.4.3) that are continually emitted from the volcano regardless of surface activity andhave several common spectral peaks that align with almost equal spacing (15, 7.5, 5, 4 s) (e.g.,
Kawakatsu et al., 2000). From the amplitude variation of these tremors, we detected the presence of acrack-like conduit whose strike and width are almost the same as those of the chain of craters
illustrated in Fig. 2.4.4; this indicates that the chain of craters is the surface expression of a buried
crack-like conduit (Yamamoto et al., 1999). Broadband records for phreatic eruptions (Fig. 2.4.5)
show that this crack-like conduit also acts as a pressure buffer leading up to eruptions, and thus
monitoring of the conduit may be the key to forecasting future activity at Aso Volcano.

Fig. 2.4.3. Broadband records oflong-period tremor(s) at Aso Volcano.

Fig. 2.4.4. Sketch of thecrack-like conduit model.

30

Fig. 2.4.5. Broadband waveformsof a phreatic eruption observed at
Aso Volcano.

The existence of the crack-like conduit is also supported by other studies such as the reflection
study, and the crack-like conduit is considered as a subsurface path that connects surface craters with apostulated magma chamber at a depth of around 5 km. We also studied the nature of other short-period
seismic signals with periods of about 0.5 s and 0.4–0.1 s, using modern digital data obtained fromdense array observations. Fig. 2.4.4 schematically summarizes the system beneath Aso Volcano as
revealed by seismological analyses (Yamamoto, 2005). Such a line of volcanic conduit systems
connecting the magma chamber and the surface has not been detected at any other active volcano in
the world. Considering that the various volcanic signals are manifestations of dynamic interactions
between volcanic fluids and the volcanic edifice in the conduit system, Aso Volcano appears to be oneof the best sites to study the dynamic behavior of the volcanic fluid system beneath active volcanoes.
We are currently constructing a real-time monitoring system of the activity of the crack-like conduit
and fluid migration beneath Aso Volcano to understand how this system operates when magma rises
to the surface in the leadup to an eruption.
2-4-4. Field and Laboratory Experiments to Study Oscillation of Multi-phase Systems Beneath
Volcanoes
Volcanoes are good test sites at which to observe the geophysical phenomena of multi-phasesystems. In this project we are studying the dynamics of magma transport and oscillation, taking
account of coupling with surrounding rocks and the coexistence of solid, liquid, and gas phases within
the magma.
(1) Propagation and attenuation of acoustic waves within bubble-bearing magma
Seismic signals passing from or through a volcano commonly indicate existence of a body or areas
with slow sound speed and/or large attenuation, which are interpreted as bubble-bearing magma.
Although it is well known in engineering systems that bubbles significantly decrease the speed andincrease the attenuation of acoustic waves within the liquid, volcanic systems are more complicatedand variable. We have proposed a mathematical model to evaluate the speed and attenuation of
acoustic waves in bubbly magma that takes into account the large variation in viscosity and pressure
within magma, as well as the diffusion of heat and volatiles, and time-dependent mechanicalproperties (Ichihara and Kameda, 2004). The mathematical model has been confirmed by laboratory
experiments in terms of the effect of viscosity and viscoelasticity (Ichihara et al., 2004).

31

(2) Magma fragmentation and the generation of air-waves
Magma fragmentation occurs during explosive eruptions. Such fragmentation transmits pressure
waves and magma fragments (pumices and ash) to the air and can cause natural disasters. We areconducting experimental studies of this process that focus on the mechanical properties of magma that
control the conditions and behavior of magma, and the generation of pressure waves in the air bymagma fragmentation at the ground surface. We measured and analyzed pressure waves in the airtransmitted by underwater explosions and detected the influence of the disintegration of the water
surface (Ichihara et al., 2005, proceedings).
(3) Bubble oscillation in magma and geothermal water
The oscillation of a single bubble is a fundamental process within more complicated systems that
contain many bubbles. We are investigating the roles of a bubble or bubbles in the onset of volcanic
eruptions and earthquakes (Ichihara and Brodsky, 2006).

32

2-5. Long-term Observation Network in the Ocean Hemisphere(1) Seafloor borehole seismic observatory
During the life of the OHP, four seafloor borehole seismic observation sites were installed (e.g.,
Araki et al., 2004): JT-1 and JT-2 offshore from the Sanriku district, northeast Japan, installed inAugust 1999; WP-2 installed in the Northwest Pacific Basin in August 2000; and WP-1 installed in theWest Philippine Basin in April 2001. The systems at JT-1 and JT-2 include strain and tilt sensors aswell as seismometers. Observations by high-quality broadband seismogram at WP-2 continued formore than 400 days, which is a record duration for such observations. This long-term record enabled
us to estimate how noise levels change throughout a year and prove the high efficiency of borehole
seismometry at this site, which can detect seismic signals from earthquakes of greater than M3.5 that
occur at any location in the world.
(2) Onland broadband seismic observatories
There were many difficulties in maintaining broadband seismic stations installed by Japanesegroup prior to the OHP, as they were equipped with many different types of instruments, including
varying recording system hardware, power supply, and data format. It was therefore one of the mostimportant tasks of the OHP to unify the total observation system so that each station is of world
standard performance. Finally, the OHP seismic network, with 11 broadband stations, was constructedin the western Pacific region (Fig. 2.1.1). Upon completion of the OHP, this seismic network wastransferred to the IFREE of JAMSTEC for more efficient long-term operation and cooperative
maintenance.
(3) Onland geomagnetic observatories
A network of geomagnetic observatories was constructed during the 6 years of the OHP (Fig.
2.1.1), consisting of nine sites equipped with a geomagnetic system (RFP-523) developed within the
OHP and one site in Antarctica (Syowa Station) whose data are provided by the National Institute for
Polar Research. RFP-523 provides stable observations for the study of geomagnetic secular variation
with a typical baseline drift that is smaller than 5 nT/yr for three components, with only one absolutecalibration every year (Shimizu and Utada, 1999). Data from the OHP geomagnetic network havebeen and will be used to image the 3-D distribution of mantle conductivity. The OHP geomagnetic
network is also maintained in collaboration with IFREE/JAMSTEC.
(4) Submarine cable network for measurement of the geo-electric field
Seven submarine cables (one in the Japan Sea and six in the Pacific Ocean) are used for long-termmonitoring of the geo-electric field after retirement from telecommunications use. Data exchange
protocol exists between the OHP and US research group who run a similar experiment in the eastern
Pacific region. One of the goals of this experiment is to constrain the strength and spatial distributionof the toroidal magnetic field at the core surface. Theoretical studies show that a detectable electric
signal can be generated by a rapid (decadal scale) oscillation of the outer core fluid that is comparable
to observed changes in voltage differences (Shimizu and Utada, 2004). Geo-electric data are also usedin 3-D EM tomography and will be used to further investigate mantle conductivity together withgeomagnetic data.

33

(5) Seafloor magnetic observatory
A seafloor instrument was developed within the OHP that enables us nearly permanent observationof four components of the geomagnetic field (three components and total intensity) and two
components of the geo-electric field using measurements of the instrument’s attitude (orientation andtilt). After 5 years of development, this instrument was deployed at site WP-2 (near the borehole
seismic site in the Northwest Pacific Basin) in 2001 and recovered one year later, when an identical
instrument was installed for a further year's operation. Our investigation has shown that the repetitionof such deployment will provide a continuous magnetogram that is nearly equivalent to that of astandard onland geomagnetic observatory (Toh et al., 2004).
(6) GPS network in the western PacificMore than 10 continuous GPS observation sites were deployed in the western Pacific region by the
OHP, and continuous positioning data from over 40 sites were analyzed as well as campaign results inseveral areas. As a result, we found that the motion of oceanic plates such as the Pacific and Philippine
Sea plates is generally consistent with geological models of plate motion, while the continental platesof Asia is subject to large-scale deformation, mostly due to collision with the Indian subcontinent. We
precisely determined the spreading rate of a back-arc basin, the Mariana Trough, which is also
consistent with the geologically estimated rate.
(7) Seafloor geodesy
A precise seafloor positioning system was developed in collaboration with the Scripps Institutionof Oceanography and installed on both sides of the Japan Trench, off the Sanriku coast, as a test
experiment. This system realizes a resolution of horizontal location as high as a few mm at a slant
range of 14 km using a GPS-acoustic link; this enables us positioning control for the deep ocean floor
at sites such as close to the trench axis. This technology is expected to measure the convergence rateof the Pacific Plate in the vicinity of the trench axis, about 300 km offshore.
(8) Super-conducting gravimetry
The OHP constructed a network of super-conducting gravimeters at Syowa in the Antarctica,
Canberra in Australia, Bandong in Indonesia, Esashi in Japan, and Ny-Alesund in Norway. This
network is the first in the world that extends from the polar regions, where the Earth’s rotation effect isminimum, to the equatorial region where the effect is at a maximum. Analysis of data from thisnetwork reveal the coupling between the solid and liquid Earth over a frequency range that is muchwider than that considered previously. Long-term measurement in Norway is also expected to detect
post-glacial rebound.


34

2-6. Ocean Hemisphere Network Data Center
The Ocean Hemisphere Project Data Management Center (OHPDMC) was established in 1997 toprovide research communities with an easy-to-use data distribution service. The OHPDMC provides
seismic, geomagnetic, and geodetic data from the OHP network and other related networks. In
addition to these required tasks, we developed a new data distribution system by applying JAVA RMI,a recent IT technology for networked systems (Takeuchi et al., 2002). This system realizes a network
data center and provides a unified interface such that a user can download data from various data
centers on the Internet.
Following the establishment of IFREE within JAMSTEC, OHPDMC began to closely cooperatewith IFREE in running the data center. The OHPDMC plays an important role, especially in real-timedata transmission. We have been replacing the observation system since 2002, and seven of ten
broadband seismic stations, four of nine geomagnetic stations, and all four submarine cables(measuring voltage differences) are now online. These data are beginning to be applied to apreliminary real-time data analysis (Fig. 2.6.1).

Fig. 2.6.1 Preliminary results of real-time hypocenter determinations
obtained using real-time data from the OHP seismic network(Mizutani, 2006, personal communications).

35 Future Plan of Ocean Hemisphere Research Center (OHRC)As ERI plans to have an institutional reorganization of its research centers and divisions at the endof the 2009 fiscal year (i.e., March, 2010), which coincides with the end of the university’s 6-year
term, it is our plan to keep OHRC in the present form till then even after the end of its originallyplanned 10-year period. In the following, we will discuss what our main research focus will be during
the extended term of our center (5 years or so). It is our hope that the review committee will evaluate
this plan, and gives us advices and/or recommendations, so that we may utilize the resources that we
possess in maximum for better understanding of the earth’s internal processes.
FiscalYear

Land component

2006

NECESSArray
Project

2007

pilot
observation

2008



2009

 

2010



2011

Marine component
Stagnant Slab
Project (2005-2008)
OHRC 10th anniversary

deployment
Future big project
ERI reorganization

demobilization
Future project

Seismic and electromagnetic joint ocean-bottom mobile array research:Through the OHP project, OHRC has established permanent geophysical networks in the western
Pacific region, which are now largely maintained by IFREE/JAMSTEC in cooperation with OHRC. In
the post-OHP era, our focus has shifted toward more mobile array type observational science, such as
one currently performed in the Stagnant Slab project (see Section 2-1-2). In the project, the long-term
ocean bottom broadband seismic and electromagnetic observations are conducted together as a teameffort for the first time to map out the integrated image of the mantle transition zone beneath thePhilippine Sea. We believe that it is OHRC’s great strength to be able to conduct such science using
our own instruments, and this should be pursued further for the coming decade to make major
contribution in the mantle dynamics research. New systems for marine seismic, EM and geothermal
observations are under development by individual research program in OHRC. Deployment of these
new instruments is expected to lead us to another breakthrough in terms of the quality of observation
data.
Although at present we have no definite plan for the next big project after SSP, one of the possible
target areas may be the northwestern Pacific, seaward of the Japanese subduction zone, whereland-based seismic tomography studies with limited resolution have provided images of a low velocity
anomaly around a depth of 400km. Recent geophysical and geological observations have also found36

fresh volcanoes in the old lithosphere and seismic activities in the middle of the northwestern Pacific.
Marine geophysical observations in this region will contribute to improve the resolution of the mantle
image and to elucidate the relation among these phenomena. Seismic observations using BBOBSs,
electromagnetic observations using OBEMs, and heat flow measurements should be carried out insuch regions. The seismic and EM tomography can provide seismic velocity/attenuation and electricalconductivity structure models, respectively, while the heat flow measurements will provide boundary
conditions of thermal modeling. Integrated analysis of these data allows us to separate effects ofthermal and compositional heterogeneities of the mantle, and helps understanding of the origins ofpeculiar activities observed.
Following the Stagnant Slab project which ends in FY2008, to pursue the line of research, we hopeto get funding by applying for large funding sources such as Monbu Kagakusho’s Grants-in-Aid for
Scientific Research for “Specially Promoted Research” (“tokubetsu suishin kenkyu”).

Land-based mobile broadband seismic array research:
While the ocean-based approach provides the OHRC's unique contribution to the mantle dynamics
research, we also plan to strengthen our land-based approach, which provides high quality and large
quantity data set. One of such projects is now about to be launched.
NorthEast China Extended SeiSmic Array (NECESSArray) is a passive broadband seismic
experiment aiming to reveal the seismic structure beneath the northeastern China, where the
Sino-Korea craton and unusual volcanism in the continent are tectonically quite interesting. The
structure of the upper mantle is the main target of NECESSArray to clarify the tectonic roots of thisinteresting and seismologically unexploited area and its relation to the stagnant slab beneath thisregion. This project may be considered, to some extent, as a land-based component of the Stagnant

Fig. 3.1. Station of the planned temporary
NECESSArray (black squares) are shown
together with the permanent stations (someare planned) in China.

37

Slab project; or at least it will greatly complement the achievements of SSP. NECESSArray runs
under the collaboration with the China Earthquake Administration (led by Dr. Y. Chen) and USA
scientists (Drs. S. Grand, F.-L. Niu, J. Ni), and OHRC will take the leading role of the Japanese side.
140 mobile broadband stations are planned to be deployed for two years, and with 140 permanent
stations of Chinese regional seismic networks, NECESSArray will consists of 280 stations (Fig. 3.1),
the scale and density comparable to one foot of the USArray; thus it is a frontier and challenging
observational seismology conducted in the best accessible area to study deep subduction processes.
Moreover, the experiment will play an important role to image deep mantle structures beneath thewestern Pacific where the largest cold and hot superplumes are mutually interacting. The investigationin northeast China will be eventually integrated to more institutional project including not only the
NECESSArray but also EM, GPS and gravity observations, geochemical and petrological studies, andcomputer simulations, which are currently conducted rather independently.
As to a future project in this context, we consider a mobile broadband array experiment in Vietnam(see Section 2-2-2; Fig. 2.2.10), where we currently operate 6 stations, as a potential target. Vietnam islocated near the antipode of the south America where deep seismicity is high, and thus this array will
greatly improve our knowledge of the deepest part of the inner core which is least known in thisplanet.
It is our plan as a center to spend resources available to OHRC mainly to these lines of research,while keep conducting other research activities discussed in Section 2 of this report. For the purpose,
we plan to apply for funding supports from various external agencies, and also hope to receive
financial and personnel support from the institute.

38

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