The Ocean Hemisphere Network Project (OHP)
Yoshio Fukao∗1 , Yuichi Morita∗1 , Masanao Shinohara∗1 , Toshihiko Kanazawa∗1 ,Hisashi Utada∗1 , Hiroaki Toh∗2 , Teruyuki Kato∗1 , Tadahiro Sato∗3 , Hajime
Shiobara∗1 , Nobukazu Seama∗4 , Hiromi Fujimoto∗5 , and Nozomu Takeuchi∗1
Earthquake Research Institute, University of Tokyo
Department of Earth Sciences, Toyama University∗3
National Astronomical Observatory, Mizusawa∗4
Research Center for Inland Seas, Kobe University∗5
Graduate School of Science, Tohoku University∗1∗2

Abstract. The Ocean Hemisphere Network Project(OHP) is a five-year long multidisciplinary project
funded by the Ministry of Education, Science,Sports and Culture Japan. The project started in
April 1996 in a schedule to end in March 2001, but
its one-year extension has now been approved. The
OHP aims to study and to build a new paradigm
on the structure and dynamics of the Earth’s deep
interior by constructing a global network of multidisciplinary geophysical observations on the hemisphere including the whole Pacific where 90by ocean.
The OHP network consists of three components, i.e.,
Seismic, Electromagnetic and Geodetic networks.
Each network is further composed of observationsof different technologies and/or methods. This article highlights topics of networking efforts of eachcomponent as well as status of data distribution andpreliminary report of observational results.

KMS

60˚

TAY
TAA

30˚

WP-2

JT-1
JT-2

BTO
TJN
TSK

INU

NAJ
OGS

MCSJ

ISG
HAI

WP-1
TGY

PALU

PATS

PSI

0˚

JAY
PMG

1

Seismic Network

Japanese seismologists began to construct a seismic
network at the Northwestern Pacific region in late1980’s, and named it as POSEIDON (Pacific Orient
SEIsmic Digital Observation Network). The POSEIDON was not an organized network but cooperative observations by seismologists who managed
oversea stations. The main purpose of the project
was to exchange the seismic data and the know-how
on the seismic observations in foreign countries. The
instruments used there were not uniform, because
maintenance of each station was left to the individual participant. It caused difficulties in systematic
operation of the network and systematic management of the data in spite of great effort by the POSEIDON participants.
In 1996, the OHP took over the POSEIDON

90˚

120˚
OHP Seismic Stations
OHP Cooperative Stations
OHP Temporal Stations

150˚

180˚

OHP Temporal OBS
OHP Long-term OBS
IRIS Stations

Fig. 1. Location map of stations of the OHP seismic network
and IRIS one in the Northwestern Pacific region.

seismic network, and began to unify the observation system and improve the network. We designed
the standard recording system with its total response equivalent to that of the IRIS network, andintroduced it to every station. The seismometers
were also changed to three components of STS-1(360sec type) at stations where STS-2 was used.
The seismometer vaults were also reconstructed, if
necessary. Eleven stations in Japan, Russia, Korea,
Philippine, Micronesia, Palau, Vietnam and Indone-sia (solid triangles in Fig. 1) are now in operation, in
addition to five cooperative stations (reverse triangles) with the IRIS, PRI (Polar Research Institute,Japan) and MRI (Meteorological Research Institute,Japan).
The seismic waveform data are sent to the OHP
data center by mail once a month to compile there.
The recording system has facilities for instrumental control and data transmission through telephoneline and/or computer network, but we rarely retrieve
the data by telephone line, because most of our stations are located at under-developed area, where thequality of telephone line is not good enough for datatransmission. The situation about computer networking is even worse. The waveform data are only
available at many stations, therefore, two or threemonths after they were recorded.
Besides the fixed OHP seismic network, we developed a portable observation system like PASCAL, which is composed of a set of broadband
seismometer (CMG-3T: 100sec type) and portable
recording system. The portable recording systemdeveloped here has remarkable advantages in highportability, easy handling, and low power consumption. Using this observation system, the temporalseismic observation has been carried out in China
since Nov. 1999 under the collaboration with the
China Seismological Bureau (Diamonds in Fig. 1).
We also deployed fifteen semi-broadband ocean bottom seismometers (OBS) in the Philippine Sea fora period of 9 months in 2000–2001 (stars in Fig. 1)
along a great circle path from the Tonga seismic zone
to the Inner Mongolia region in China. The main
purpose of this temporal observation was to sharpen
the seismic images of deeply subducting slabs andthe overlying mantle beneath the Philippine Sea.
In conclusion, during these five years of the OHP
project, we have improved and expanded the broadband seismic network in the Northwestern Pacificregion, which is augmented by the temporal arrayobservation system of broadband seismometers.

2

Borehole Geophysical Observatories

Tomographic studies using seismic waves have revolutionized our understanding of mantle structureand dynamics. A great limitation on existing tomographic images of the Earth’s interior is the uneven
distribution of seismic stations, especially the lackof stations in large expanses of ocean such as the
Pacific. The Ocean Hemisphere Network Project(OHP) has selected the western Pacific area for in-stallation of ocean-bottom sensors because it is ideal
for addressing problems related to plate subduction.
The installation of four seafloor borehole geophysicalstations in three areas was planned by OHP (Fig. 2).
At present, three borehole geophysical stations were
established by Ocean Drilling Program (ODP) andone borehole seismic station is scheduled to be installed in April 2001 during ODP Leg 195.
YSS

WP-2
NMR

40

N

PHN

JT-1
JT-2
HCH

ISG
OGS

20

MCSJ

WP-1

N
TGY

PATS

0

PMG

100

E

120

E

140

E

160

E

180

Fig. 2. Map of western Pacific area showing borehole geophysical sites. Solid circles indicate land seismic stations,whereas open circles are current and proposed seafloor borehole observatories. A few borehole stations effectively complement and expand the existing network.

Two stations (JT-1 and JT-2) are located immediately above the inter-plate earthquake generation zone on the landward side of the Japan Trench,
and were installed in July 1999 to monitor ongoing tectonic processes in the deep sea terrace on
the landward slope. JT-1 was located above theactive portion of the seismogenic zone where large
interplate thrust earthquakes recur and JT-2 is positioned above an aseismic portion of the seismogenic zone. JT-1 and JT-2 should improve greatly
determinations of source locations and rupture processes of earthquakes near the Japan Trench. Twoother sites (WP-1 and WP-2) are located to complete a 1000-km span network in the western Pacificarea. The WP-2 station is situated on the normal
oceanic Mesozoic crust in the northwestern PacificBasin and was installed in August, 2000. WP-2 isof special scientific importance, since no other landsite can replace this site. Accumulation of broadband seismic data from within the basin part of the
Pacific Plate will help us to resolve structures of thelithosphere and asthenosphere of the Pacific Plate.
The WP-1 site is in the west Philippine Basin, and ismore than about 1000 km away from land stations.
WP-1 will be a crucial network component in deter-Emining whether the Pacific plate is penetrating into
the lower mantle in the Marianas Trench but not in
the Izu-Ogasawara (Bonin) Trench.

seafloor unit to the borehole unit for activation of
the system.
The ROV, Dolphin 3K, of the Japan Marine Science and Technology Center visited JT-1 and JT-2
and activated the systems in September 1999. Preliminary data covering an 8-hour interval were recovered from JT-1. Many seismic events were foundin the record (Fig. 3). In October 2000, the ROV,
Kaiko, visited WP-2 to activate the system. In a
one-day visit of Kaiko, we recovered one hour of
preliminary data from the borehole seismometers.
It is confirmed from these preliminary records from
JT-1 and WP-2 that seismic noise in seafloor borehole is low enough for highly sensitive, broadband
observations.

3Fig. 3. Example of the data recorded by broadband seismometer at the JT-1 station. The data from nearby land
stations are also shown. The component of each seismic trace
is approximately radial to the hypocenter. The locations of
the teleseismic event and the seismic station are also shown.

The observatories are designed to last for many
years as a stand-alone system. For JT-1 and JT2, the sensors consist of a strainmeter, a tiltmeter,
and two kinds of broadband seismometers. The
same broadband seismometers are used for the WP1 and the WP-2. The sensors are housed in separate pressure vessels near the bottom of drill holes
and are permanently cemented in as required forthe strainmeter and tiltmeter operation and to assure good coupling for the seismometers. Separate
cables connect the sensors uphole and the signals
from the borehole sensors are sent to the seafloor.
A data recorder and batteries to supply power tothe whole system are installed on the seafloor. The
data recorder is replaced using a Remotely Operated
Vehicle (ROV) before the data storage becomes full.
After installation, an ROV is needed to connect the

Seismic Observation System
on The Sea Floor

Under the OHP project, three types of long-term
ocean-bottom seismometers (OBS) have been developed for seafloor observations over one year;
1) the very broadband OBS (LT-VBB OBS) covering periods of 360s∼0.05s,2) the broadband OBS (LT-BB OBS) covering periods of 30s∼0.05s, and3) the OBS (LT OBS) covering periods of 1s∼0.05s.
Like our standard OBS using a short-period sensor of 4.5 Hz, the developed OBSs have been designed to be of free-fall and self pop-up type andto be deployed in the sea down to 6000 m in depth.
“Long-term”, “broadband”, “compact”, “portable”,“reliable” and “robust” have been the key words inour developments of the OBS systems for the OHP
project. The outlines of the three types of the OBS
systems are as follows.
1) Long-Term Very Broadband Ocean Bottom Seismometer (360s∼0.05s)
In order to cover the western Pacific with adenser seismic network, the OHP planned to have
two quasi-permanent seafloor observatories in addition to the four borehole broadband seismic observatories by placing ocean bottom seismometers
repetitively (Fig. 4). The LT-VBB OBSs were developed for these observatories (Fig. 5), which successfully continued to record seismic data over 400
days on the sea floor. The modified version of theGuralp CMG-1T is used as the three-component
broadband velocity sensor of the LT-VBB OBS.
The power consumption of the CMG-1T is lowered to achieve long-term observation. Throughthe RS232C serial interface to the CMG-1T, theOBS digital recorder (HDDR) controls intelligently

Fig. 4. The station map of the OHP observatories in thewestern Pacific. JT-1, JT-2, WP-1 and WP-2 are the borehole observatories. BBOBS stations #1 and #2 are the quasipermanent observatories with using the long-term very broadband ocean-bottom seismometers (LT-VBB OBS) and havebeen planned to be installed in 2001. The experimental observations for developing the LT-VBB OBS were held fourtimes at the sites of BBOBS1 to BBOBS4 between 1999 and2000.

the on/off of the power of the sensor, the locking/unlocking of the pendulum, and the centeringof the mass of the pendulum. The CMG-1T ismounted on the active-controlled leveling systemthat we developed specifically for the LT-VBB OBS(Fig. 6). Each velocity component of the CMG-1T
is digitized by the 20-bits analog-to-digital converterof the HDDR at a sampling rate of 128 Hz. The
sampled data are recorded continuously with a time
tag on the four 2.5-inch hard disks using memory
buffering of 5M bytes. The HDDR recorder, theCMG-1T sensor mounted on the leveling system, theelectronic circuit boards of the acoustic transponder,and Lithium batteries are contained in a 65 cm diameter sphere made of titanium alloy. The acoustic
transponder has a function of data transmission thatenables us to check the health of the LT-VBB OBS
on the seafloor remotely from a surface ship and tochange the observation parameters of the LT-VBB
OBS. Such communications between the seafloor instruments and the surface ship is essential for the
long-term observations on the seafloor in order tomake the observations being flexible and reliable.
Forced electric corrosion is utilized for releasing theanchors being attached outside the titanium sphereof the LT-VBB OBS. Once a command is sent to
the LT-VBB OBS acoustically from a surface ship,

Fig. 5. The exterior view of the long-term very broadband
ocean-bottom seismometer (LT-VBB OBS).

Fig. 6. The view of the active-controlled leveling system for
the CMG-1T sensor installed on the titanium hemisphere.

it activates the forced electric corrosion to dissolve
the titanium plates of 0.5 mm thickness to release
the anchors. The LT-VBB OBS then begins to ascend to the sea surface to be retrieved by the surface ship. All the LT-VBB OBSs were successfully
retrieved after more than one-year observations on
the seafloor, demonstrating a high reliability of oursystem.
2) Long-term Broadband Ocean Bottom Seismometer (30s∼0.05s)
We developed a long-term broadband OBS (LTBB OBS) using the PMD sensor covering periods of
30s∼0.05s. The HDDR recorder, the PMD sensor,

the acoustic transponder and Lithium batteries arecontained in a 50-cm diameter sphere of titanium
alloy. The period of continuous recording is about
540 days. The LT-BB OBS is compact and costs
lower than the LT-VBB OBS. The 15 OBSs were
deployed along a 2,500 km long profile in the Philippine Sea and succeeded in the one-year observationsof teleseismic events during 1999–2000.
3) Long-term Ocean Bottom Seismometer(1s∼0.05s)
The Lennartz 1 Hz sensor is used for the longterm OBS (LT OBS). The sensor is mounted on theactive-controlled leveling system, the mechanism ofwhich is similar to that of the LT-VBB OBS. The
observation period is about 500 days in a systemwith the HDDR recorder and the titanium sphereof 50 cm diameter as a pressure housing.

4

Geoelectromagnetic
work

Net-

The OHP geoelectromagnetic network (Fig. 7)
aims to study electrical conductivity structure of
the Earth’s mantle, Earth’s main field variations
and core-mantle dynamics. It consists of longterm (nearly permanent) geomagnetic and geoelectric observatories, and supplementary mobile
surveys. Eight geomagnetic sites have been installed in the Pacific region, where pre-existingpermanent observatories are few. Locations are
Pohnpei (FSM), Kiritimati (Kiribati), Huancayo(Peru), Changchun (China), Tongatapu (Tonga),
Marcus (Japan), Muntinlupa (Philippines), and Majuro (Marshall Islands). These observatories areequipped with an identical magnetometer system,
the OHP magnetometer (Shimizu and Utada, 1999).
The OHP magnetometer is designed for a longterm observation with needs of little maintenance(monthly visit to change disk, and absolute measurement once a year). It consists of a proton
magnetometer to measure the total intensity anda three-component fluxgate magnetometer (Shimizu
and Utada, 2000). The three component sensor
is highly sensitive (resolution 0.01 nT, noise level
0.05 nT) so that data can be used for induction studies by analyzing rapid geomagnetic variations or forionospheric studies. It is also possible to control
three-component absolute values with an accuracyof a few nT, only by conducting an absolute measurement every year. Therefore the OHP magnetic
data can be used for study of secular variations.
Submarine coaxial cables retired from telecommunications services are utilized to get information

Fig. 7. Location map of stations of the OHP geoelectromagnetic network which consists of geomagnetic observations onland and on ocean bottom and electric field measurements bysubmarine cables.

on the Earth’s deep interior. By analyzing rapid
variations (10 minutes to 10 days of period), we try
to determine the electrical conductivity distribution
beneath the Pacific. For example, one-dimensional
inversion of magnetotelluric (MT) response from the
Hawaii-Midway cable indicated a sharp increase of
conductivity at around 400 km depth. Another purpose of the cable voltage measurements is to detect an electric field variation related to the toroidal
magnetic field at the surface of the outer core, whichis possible to exist with a finite lower mantle conductivity. Such field variation is probably detectable
but needs many decades of observation as alreadypointed out by Shimizu et al. (1997).
Distribution of these permanent observatories issometimes not dense enough for detailed studies, especially to study the mantle electrical conductivity
structure in the subduction zone of the northwest
Pacific. In order to increase the spatial resolution,
we have carried out two kinds of temporary arrayobservations: Ocean Bottom ElectroMagnetometer(OBEM) array in the Philippine Sea and NetworkMT experiment in Northeast China.
In northeast district of China, there exists active Quaternary volcanism that is neither of hot
spot origin nor of island arc type. Corresponding to
the location of volcanoes in this region, global seismic tomography delineates a presence of high velocity anomaly at around the mantle transition zone,
which is interpreted as a stagnant slab (e.g., Fukao
et al., 1992). This situation is geologically interest-ing enough to conduct further studies on the mantle structure in details, for example, Network-MT(Uyeshima et al., 2001) experiment. Electric field
variations are measured by using telephone lines toconnect electrodes. This enables us long (>10 km)
electrode spacing that provides high S/N especiallyat lower frequencies. Since the summer of 1998, we
have conducted electric field measurements on seventelephone lines in this region and obtained good datafrom four lines. These data were processed with thegeomagnetic data at Changchun (OHP magnetometer) to obtain magnetotelluric responses from 100
seconds to several days. One-dimensional inversionof the responses indicated the presence of anomalous
conductor in the upper mantle and deeper (600–800 km) conductivity jump than those found beneath the Pacific.

5

Seafloor
Electromagnetic
Station: The Third Generation

The third generation of SeaFloor ElectroMagnetic
Station (SFEMS) has been delivered to us (Fig. 8).
SFEMS was developed for:1. Probing the electrical conductivity structureof the deep Earth via long-term seafloor magnetotellurics.
2. Estimation of the lower mantle conductivity
by detection of geomagnetic secular variations.
3. Understanding dynamics in the Earth’s coreby improving the distribution of the existing geomagnetic observatories. In addition,
a combination of the geomagnetic total force
measured by SFEMS’s absolute scalar magnetometer with the vertical magnetic component of a magnetotelluric (MT) variograph of
SFEMS may provide us with the correct geomagnetic potential in regions where continuous EM observations have never been carriedout (e.g., the northwest Pacific).
The prototype SFEMS (the first generation) was
equipped with an Overhauser absolute magnetic
sensor and an acoustic telemetry modem (ATM)in addition to its basic platform made of titanium.
It was used to confirm reliability of the absolute
magnetometer and of the realtime data transmission by ATM in two experiments (1996-1997, KH963 by R/V Hakuho, ORI, U. Tokyo, cruises by #7
Kaiko and R/V Ten’yo of Maritime Safety Agency,

Fig. 8. Outerview of SFEMS3.

Japan). An MT variograph was further implemented to the prototype to form the second generation of SFEMS, which was tested in KH98-1 aroundthe East Mariana Basin to yield a 40-day long 8component dataset successfully. In the third generation, a fibre optical gyro (FOG) was also attached
to know instrument’s geographical orientation. All
the client sensors (the Overhauser magnetometer,
the MT variograph and the FOG) can now be controlled from the sea surface by means of ATM.
The third generation of SFEMS was deployed in
the northwest Pacific in the summer of 1999 (KH993). It was originally planned to be recovered by
MR00-K03 in May-June, 2000 (R/V Mirai, JAMSTEC) but was unfortunately not retrieved since
the acoustic release system failed to function well.
Hence, it was towed to the surface by ROV Kaiko
under the support of its tendership, R/V Kairei(JAMSTEC), at the end of October, 2000. The failure turned out to be due to current leakage from theacoustic releaser. The renewed SFEMS was tested
in the Gulf of Aden, Indian Ocean during KH00-5,
which successfully provided us with a 21-day long
8-component time series (Fig. 9) with an instrument’s orientation of 189.50 ◦ from the geographical north. Although the SFEMS is still a pop-up
type instrument using primary lithium cells for main
power supply, it can be made to a semi-permanent
seafloor observatory by periodical servicing using remotely operated vehicles (ROVs) which will replace
the SFEMS’s primary cells and the data recorder.

Fig. 9. Four-component magnetic data collected in the Gulfof Aden.

6

GPS Network and Result ofData Analysis

The Japanese research group of tectonics using the
Global Positioning System (GPS) has constructed
an array of permanent tracking stations in the western Pacific and the eastern Asia since around 1995.
The project was promoted under the Ocean Hemisphere Project (OHP) as well as other researchfunds. Western Pacific to eastern Asia is the areaof convergence among several mega-plates including Eurasia, Pacific, Indian-Australian and North
American plates. A variety of geodynamic phenomena take place in this area such as continental
collision in the Himalayan area and large scale intraplate deformation in Chinese continent, subduction of oceanic plates and back-arc spreading behind the subducting plates as well as instantaneous
events such as earthquakes and volcanic eruptions.
Although geological plate motion models have been
proposed for these large plates such as NUVEL-1(DeMets et al., 1990) to investigate tectonics in thisregion, those large scale model lacks in including
proposed smaller scale plates such as Amurian plate,
Okhotsk plate, Caroline plates etc. It is thus primarily important to clarify instantaneous displacement
rate field in this region to clarify tectonics and investigate geodynamics in the region.
Among various space geodetic techniques, theGlobal Positioning System (GPS) is the most appropriate technique considering accuracy and logistical expenses. We have established more than 10
permanent GPS stations since 1995. The network
was named as Western Pacific Integrated Networkof GPS (WING). In addition, more than 10 sta-tions of International GPS Service for Geodynamics (IGS) have been established by other institutes
in this region in the same time period. Also, theGeographical Survey Institute of Japan has established a dense national array called GEONET sincearound 1996 with about 1000 permanent stations in
the Japanese islands; from which we used only sevensites that are open to IGS community.
In the present study, we used data at 38 stations in total from above GPS networks for baseline analysis for the period July 1995 ∼ June 1998,
together with 6 IGS global fiducial sites. We employed the Bernese software ver.4.0 and used the
Melborne-Webbena linear combination for fixing initial phase ambiguities for long baselines. A fiducialfree approach was adopted to obtain the most accurate baseline estimates. To fix the estimatedcoordinates to the terrestrial reference frame, theTsukuba IGS site was assumed to be moving about
2 cm/yr westward relative to the stable Eurasiancontinent (Heki, 1996). Fig. 1 shows the summaryof the displacement field thus obtained. The figure includes not only results from the permanent
array but also those from campaign observations
conducted in the same region (Kato et al., 1998).
The figure portrays the displacement rate field in thewestern Pacific and eastern Asian region. We findthat velocities of sites that are within the oceanic
plate and far from boundaries are in good agreement with rigid plate motion models. On the other
hand, Ishigaki and Guam seem to be moving trenchward relative to the Philippine Sea plate, suggesting on-going back-arc openings. Stations in the
Asian continent such as Lhasa, Xian, Wuhan, andShanghai are moving NE-E relative to the stable
Eurasian continent, due possibly to the collision between India and Asia and the stress propagation toward east. Then, the Euler vector of the Philippine Sea plate relative to the Eurasian stable craton was estimated using thus obtained GPS data.
For this purpose, we used repeated observation data
at Okino Torishima (Parece Vera) and velocity vectors at some sites on the Philippine Sea plate in
the nationwide continuous monitoring network by
GSI. The estimated location of the Euler pole is(41.55 ◦N±0.42◦, 152.46 ◦E±0.43◦) and the angularvelocity is -1.50 ◦±0.04◦/ma taking counterclockwise
rotation as positive (Kotake et al., 1998). n addition to establishing a permanent GPS array in thewestern Pacific region, we have conducted various
campaign observations along the plate boundaries
such as in the Mariana, Yap and Palau area (seeFig. 10) under international cooperative projects.
We are also trying to synthesize these velocity fields
under the unified kinematic reference frame based

Fig. 10. Velocities at the GPS sites in the western Pacific area. Black arrows: estimated velocity by GPS, Yellow: estimatedvelocity by plate motion models.

on published data. The obtained velocity fields will
provide us with fundamental information to help
understand complicated tectonic process in the region. Given this permanent array of GPS, we haveto think further developments of study in the area.
First, the array may have to be augmented to obtain more detailed picture of tectonic deformationsin the area. Since the area is under fragmented tectonic blocks and widely spread deforming area in
China, many more sites are clearly necessary to fully
understand the tectonic motions in the area. Secondly, we have to consider that GPS is now used
not only as a tool for solid earth physics but also asthat for atmospheric sciences. The signal transmitted from the GPS satellites keeps information on
atmosphere, namely, content of water vapor along

the path. Estimated water vapor content over thesite provides important data for meteorology andhydrology. Thus a number of projects of using GPS
as an observation tool for atmospheric sciences inrecent years. A part of our array in the western
Pacific and Asia will be handed over to the Hydrological Cycle Observational Research Program under the Frontier Observational Research System for
Global Change, which is aiming at the mechanismof Asian Monsoon. Considering these, GPS regional
array established in this study should be combinedwith other regional arrays on the globe to be a global
infrastructure of earth science.

7

OHP
Superconducting
Gravimeter Array

Under the support of the OHP, the Japanese superconducting gravimeter group is operating an international observation network with superconducting
gravimeters (SG) called GGP-Japan Network. This
network consists of the following seven observationsites; from the north, Ny-Alesund (Svalbard, Norway), Esashi, Matsushiro and Kyoto (Japan), Bandung (Indonesia), Canberra (Australia), and Syowa
Station (Antarctica). An important characteristic of
the GGP-Japan Network is its spatial distributionof the sites on the globe, that covers a wide range oflatitude from 79◦ north (Ny-Alesund) to 69◦ south(Syowa) including a station near the equator (Bandung). This unique characteristic is important forthe studies of latitude dependency on the Earth’s
free oscillations and the Earth tides.
Through changes in currents and ocean-bottompressure, oceans have significant impacts on many
global geophysical processes of the Earth such as
the polar motion excitation, measurable change inthe length of the day, changes in the Earth’s gravitational field and the motion of center-of-mass of thesolid Earth. Sato et al. (2001) discussed the effectof sea surface height variations on superconducting
gravimeter measurements made at the three different sites of GGP-Japan Network, namely Esashi,
Canberra and Syowa. Although there remain somesystematic discrepancies in both amplitude andphase, good agreements with gravity measurements
at all of these sites were obtained using the results from an ocean model (POCM by Stammer andChervin, 1992) and from the TOPEX/POSEIDON
measurement which had been corrected for the stericchanges in sea surface height that have no gravitational signature (Figs. 11 and 12). Their results
suggest that the GGP-Japan network may give a
useful data set that can be used for the comparison
and/or the combination with the data obtained fromthe satellite gravity missions as CHAMP, GRACE
and GOCE.

8

Long-Term OBS Array Observation across The Philippine Sea

As a part of the Ocean Hemisphere network Project,a long term ocean bottom seismic observation was
performed along the trans-Philippine Sea profile.
The main aim of this experiment is to reveal animage of inhomogeneous mantle structure beneathFig. 11. Phasor plots of the observed annual components
and predicted ones. The phase angle of each vector is referred
to an analysis epoch of 00h UTC, January 1, 2000, and theangle is measured from the cosine axis, counter clockwise fora lag. The three vectors in black, red and green show theobservation, the polar motion effect and the effect of solidtide, respectively. The effects of the Sa ocean wave are verysmall to display with the scale of this plot. The three vectorsof yellow, blue and pink show the effect of SSH variations
evaluated from the POCM data using the steric coefficientsof 0.0 × 10−2m/deg, 0.60 × 10−2m/deg and 1.0 × 10−2m/deg,
respectively.

Fig. 12. Similar plots as Fig. 11, but the SSH vectors evaluated from the TOPEX/POSEIDON data are displayed.

the west Pacific region from analyses of body wave
records of the events that occurred on the elongation of this profile, mainly around Tonga and Fiji
Islands.
From Nov. 1999 until July 2000, we conducted
the observation by using 15 long term ocean bottom
seismometers (LTOBS) equipped with semi broadband sensors (WB2023LP, PMD) along the profile (Fig. 13). The profile is about 2800 km longand NW-SE trending across the Saipan Island. Its
NW end (28◦ N, 130◦E) is located near the Amami
Ohshima Island and the SE end (10◦ N, 151◦E) islocated oceanward away from the Mariana Trough.
Six ocean bottom electro-magnetometers were simultaneously deployed on the same profile to improve the image from an individual information of
the deep structure. We also installed four landbroadband seismic stations along the extension ofthis profile in China.
During the deployment, we performed a precise
positioning of each LTOBS. We also conducted onedimensional refraction with an airgun (1000 cu.in.)and single channel reflection surveys at many stations as possible to understand the crustal structure

40˚N

BTO

TAY
TJN PHN

TAA

TSK
INU

NAJ

30˚N

ISG

20˚N
BAG

OBS15
OBS14
OBEM6
OBS13
OGS
OBS12
OBEM5
OBS11
OBS10
OBEM4
OBS9
OBS8
OBEM3
OBS7
OBS6
OBEM2
OBS5
OBEM1
OBS4

TGY

MCSJ

OBS3
OBS2
OBS1

10˚N
PALU

110˚E

120˚E

130˚E

-8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000

140˚E
0

1000

150˚E
2000

3000

4000

Topography (m)Fig. 13. Location map of OBS array and land stations. 15 LTOBSs and 6 OB EMs are indicated as circles and crosses,
respectively.

(centered on OBS11)

300–700 km
100–300 km
0–100 kmFig. 14. Quality map of the LTOBS array data. Signal to noise (S/N) ratios are examined for records of events with Mb>5.5(PDE), filtered in 10∼100 mHz band (Note: this may not be a suitable band to identify onset of direct P ). Filled symbols
are for “good” events, for which S/N ratios are high at more than three stations. Scale-over occurs for local events (<15◦ ),especially for deep events.

curves are superimposed on traces of vertical component, band pass filtered in 10∼80 mHz. Traces
Vertical
2000/01/08 16:47:19.7 Lat.:-17.02˚ Lon.:-174.22˚ Depth:183km Mw:6.5 BPF(Hz):0.010-0.080

45
JAY
PHS3

50
PHS4
PHS5

55
PALU

PHS8

60

OGS
NWPAC1
PHS11

65

PHS12

Distance(˚)

beneath the OBS position. We finally recovered 13
OBSs, but 4 of them did not work due to malfunction of a sensor control unit. We retrieved the datafrom the other OBSs which were fortunately locatedalmost evenly on three different basins, the West
Philippine, Shikoku and Mariana Basins.
We will report the result of the detailed noise
analyses elsewhere. The records show commonly ahigh noise level in 0.1∼1 Hz, that can be an obstacle for body wave analyses, especially in horizontal components. For this reason, the analyses forthe moment are limited in a frequency band below
0.1 Hz. The data quality is preliminarily examinedwith the records band-passed in 30∼100 mHz band,using PDE catalog (Fig. 14). Events with Mb 5.5 orlarger within epicentral distances of 70 degrees arewell recorded with adequately high S/N. Some large
events are well recorded in lower frequency bands,
though the noise level is slightly high in horizontal
components. Most of them are shallow events, butseveral deep events in Japan, Izu-Mariana and Fijiare recorded as well. Majority of the recorded regional events are located in the equatorial southern
Pacific, from the Philippine to Fiji islands. Some
events in Alaska are well recorded with high S/N.
Dispersion of Rayleigh waves from these shallow
events are characterized by the oceanic signature,and our new data set will provide a unique opportunity to investigate wave propagation and the upper
mantle structure in oceanic regions.
An example record section of this array, land stations and a broad band OBS (BBOBS) in the northwestern Pacific is shown in Fig. 15, for the eventthat occurred in Tonga. The theoretical travel time

PHS13
INU

70

ISG

75
TJN

P

pP

80

NAJ

TAA

85
TAY

90

BTO

PcP

PP

PKiKP

95
0

120

240

360

480

600

T-D/16(s)Fig. 15. Example record section for an event in Tonga.
Traces are of vertical component. Theoretical travel time
curves are superimposed. Blue, red and green traces aredata from this LTOBS array (PHS3-13), land stations andthe broadband OBS in the northwestern Pacific (NWPAC1),
respectively. The travel time is reduced with a velocity of
16 km/s.

of the LTOBSs with the PMD sensors are rich incoda-like waves and look different from those of theothers including the land stations and the BBOBS
with the CMG-1T sensor. These coda-like waves

may be an artifact due to a complicated transfer
function of the sensor, but it is still under the investigation. The first arrival to NWPAC1 is apparently
faster than those to other stations, with respect tothe theoretical travel time curve, and this observation might indicate a direction-dependent velocity
variation (anisotropy) of the upper mantle in thewestern Pacific region.
Our first experiment for the long-term seismic
observation on the trans-Philippine Sea profile has
thus been made almost successfully. The usefulnessof this type of experiment for resolving the deep
Earth’s interior has become apparent even from thepreliminary result. For further understanding of the
mantle structure beneath the whole western Pacific,
several long-term ocean bottom observations should
be conducted systematically.

9Long-Term OBEM Array
Observation
acrossThe
Philippine Sea

The Philippine Sea is one of the major marginal seasin the western Pacific and consists of three majorbasins; the West Philippine Basin, Shikoku-Parece
Vela Basin, and Mariana Trough. Previous studies
indicate that these basins have been formed by successive episodes of west to east opening (e.g., Karig,1971). As a part of the Ocean Hemisphere Projectin Japan, we developed six Ocean Bottom ElectroMagnetometers (OBEMs) across the Philippine
Sea during Dai-5 Kaikou-maru cruise in November,
1999; two OBEMs were in the northern West Philippine basin, two OBEMs in the northern Parece
Vela Basin, and two OBEMs in the central Marina
Trough (Fig. 16). The OBEMs measured two horizontal components of electric field and three components of geomagnetic field every one-minute forabout eight months.
All the OBEMs were recovered during ShintatuMaru cruise in July 2000 and the fairly high-quality
data were obtained with some exceptions: 1) The
OBEM3 record is not available except for a period of
one month, because the OBEM3 was flooded when
it was recovered. 2) The electric field record from
OBEM1 is unstable in the initial period of about
50 days. 3) The OBEM2 record has a data gap of
12 days. The electric field data have much larger
drifts than the geomagnetic field data, which are
due mainly to electrode problems.
We analyzed the data through the following procedure. First, we removed the drifts and then two
horizontal components are rotated to obtain northand east components using the geomagnetic field
data, because each OBEM had been set by a free
fall. Figs. 17(a) and 17(b) show the record of eachcomponent of the OBEM. The geomagnetic fielddata are of very high quality, while the electric fielddata are contaminated by noise spikes, but still theeffect of the magnetic storm is well visible. Then,we estimated the magnetotelluric (MT) impedance
which is a frequency dependent transfer function
from magnetic field to electric field that contains
the information on electrical structure in the Earth.
Five seafloor MT impedances with jackknife error
bars are obtained from the original time series datausing the robust remote reference method (Chave
and Thomson, 1989). All the data show relatively
small errors bar in a period range between 600 s anda few ten thousands, even though the electric fielddata are contaminated by the noise spikes.
Once the MT impedance is obtained, the electric field can be predicted from the magnetic field.
For all the OBEMs the coherence between observed
and predicted electric fields is fairly high for Ey
but low for Ex. Thus, we use the Zyx element of
the MT impedance, Z, to estimate 1-D conductivity
structure. We use the MT impedance in a period
range from 600 to 20,000 s for the estimation, because of the low coherence at periods shorter than
600 s and the possible contamination due to tidal effect at periods longer than 20,000 s. Finally, a 1-D
conductivity structural model is obtained for eachsite by Occam’s inversion (Constable et al., 1987).
Each structural model well explains the corresponding MT impedance obtained from the observation.
These preliminary structural models (Fig. 18) indicate that 1) the Philippine Sea plate with older
crustal age has lower conductivity structure in general, and 2) the spreading axis Mariana Trough has
extremely low conductive structure. Conductivity
becomes lower in the order of the OBEM4, OBEM2,
OBEM5, OBEM6, and OBEM1. The order is related to the crustal age except for the OBEM1. The
OBEM4 is located on the oceanic crust, while the
OBEM2, OBEM5, and OBEM6 are located on theold island arcs with their ages increasing in thisorder. The OBEM1 is located near the spreading
axis of the Mariana Trough, showing extremely low
conductivity structure. Results from the MELT experiment also show low conductivity structure beneath the southern East Pacific Rise at 17◦S (Evanset al., 1999). A low conductivity structure has also
been reported beneath the Tahiti islands (Nolasco
et al., 1998). Thus, low conductivity structure may
be a general feature beneath spreading axes and hot
spots, where the uppermost mantle may be depleted
in melt and volatile.

Fig. 16. Locations of OBEM sites (stars) in the Philippine Sea with their site names.

Our analysis remains very preliminary, requiring
further studies:
1) More carefully edited time series data should
give an estimate of more precise magnetotelluric
impedances, because the electric field data are contaminated by noise spikes.
2) Effect of seafloor topography should be takenin account to estimate more reliable conductivity
structure using the method of Baba and Seama(2001).
3) Another survey with a more dense instrument array is required.
The conductivity model after these studies is expected to show vestiges of the tectonic evolution of
the Philippine Sea in relation to the age differenceof the lithosphere, and would provide a key to understand the mechanism of lithospheric evolution.

Fig. 17. The magnetic field (a) and the electric field (b) fromeach OBEM site is plotted against the time. Subscripts of x,y, and z are denote north, east, and downward components.
Note that big magnetic storms are observed at 144th and150th days.

10

Development of Geodetic
Measurement System on
The Seafloor

Observation of crustal deformation on the seafloor
is critical to understand the dynamic mechanismof subduction of an oceanic plate. Scripps Institution of Oceanography (SIO) was the first to develop
precision acoustic transponders with sub-cm resolution in the ranging of 4∼5 km and combined themwith kinematic GPS positioning. Their results of
GPS/Acoustic (GPS/A) experiments about 100 km
west of the coast of Oregon roughly agree with theplate motion in the geological time scale (Spiess etal., 1998).
We have been developing instruments for11Fig. 18. Conductivity structural model of each OBEM site
obtained by Occam’s inversion (Constable et al., 1987). The
Occam’s inversion gives smooth models by two ways, which
are shown by solid (first derivative minimization) and dashedlines (second derivative minimization).

seafloor geodesy (Fujimoto et al., 1998), and recently developed two acoustic transponder systems
for GPS/A positioning by using the method of linear
pulse compression under the OHP Program. One is
the most up-to-date system for positioning on the
deep ocean (6 km) developed jointly with SIO. Encouraged with the results of its sea trials, SIO produced 10 units of similar acoustic transponders anddeployed them on the southeastern slope of Hawaii
Island. Preliminary results show that the transponders can measure precise slant ranges over 14 km,
indicating possibility of a GPS/A experiment on theseafloor of 10 km water depth. We plan to deploy
three units of the acoustic transponders at 6000 m
water depth (required slant range of about 9 km)
on the seaward slope of the Japan Trench in 2001
for repeated seafloor positioning during the coming
several years. We have also developed a simpler system for shallower seafloor and plan to deploy it on
the landward slope of the Japan Trench in 2001.

Seismic Waveform Distribution System

Digital seismic waveform data are being accumulated in an accelerating ratio by various networks.
Seismic data analysis using these huge amount of
dataset is increasingly getting popular. For example, in the Western Pacific region, a large numberof broadband digital seismographic stations are recently installed by various institutions (Fig. 19). If
the data are appropriately gathered from these networks, they effectively constitutes a dataset from avery dense broadband network, and such a dataset
should be very useful to determine high resolution
seismic structures and seismic processes. However,in the current waveform distribution system, suchstudies require somewhat tedious procedures for collecting the waveform data of each data center withdifferent interfaces.
Looking at the field of computer science, distributed object technology (The Object Management Group, 2000) is one of the most important technology in the modern system programming
practice as it can handle a huge system in straightforward. Java RMI (Gosling and Steele, 1996; Sun
Microsystems, 1999) is one of the most notably successful technologies to build a distributed object
management system because of its highly expansiveness and security. We apply Java RMI technology to develop networked data distribution systemproviding a unified interface for various networks
data. The overall structure is shown in Fig. 20,
where the gray colored squares show objects. In
this system, seismic waveform data are managed byrespective data center so that they are always updated. Various request interfaces and data servers
are networked via the Internet with each other. If auser send a request to a data request interface (“dispatcher”, the left two gray squares in Fig. 20), thedispatcher dispatches the request to objects in various data center (“RMI server”, the right two graysquares in Fig. 20), integrates the requested waveform data in internationally standard format (SEED
format; Federation of Digital Seismographic Networks et al., 1993), and returns the resultant SEED
volumes to the user. Thus, a user can download
every latest seismic waveform data with a unified
interface.
The current available dispatchers and RMI
servers are shown in Table 1. Two applet dispatchers and six RMI servers are running. We have
distributed many numbers of CD-ROM in which
the application dispatchers software is stored. RMI
servers are for PACIFIC21 (partly), BATS and IRIS

Fig. 19. Broadband seismographic station map (a) in the Western Pacific region, and (b) in Japan and Taiwan. Black stars
indicate OHP network, gray stars indicate stations of IRIS network (IRIS/USGS, IRIS/IDA and IRIS/CDSN) and white stars
indicate those of other networks.

Fig. 20. Block diagram of our networked data distribution
system. Gray squares indicate objects. “Win file” is a data
file in Japanese domestic format.

data. Available PACIFIC21 data are OHP (formerly POSEIDON) and GEOTOC data. The domestic broadband data from JMA and EOC are alsoavailable. In the future, we have a plan to distribute
every PACIFIC21 data. Providing a unified interface for various networks data greatly improves theconvenience of users. Our communication method
through firewall and automatic information synchronization mechanism between dispatchers does not
require special setup by a user, thus our system isquite user friendly.

Java RMI is suitable for system expansion. It
is straightforward to develop a new function, because it is architecture neutral and we do not haveto prepare source codes for each platform (Sun Microsystems, 1999). Our information synchronization
mechanism is also expansive. It is straightforward toincrease the number of dispatchers and RMI servers
because we do not have to change any setting of preexisting dispatchers nor RMI servers. Of course oursystem can be applied not only to seismological data
but to any geophysical time series data including
geomagnetic data, GPS data and superconducting
gravimeter data. Thus our system is highly expansive.

12

Reference

Baba, K., and N. Seama, A new technique for theincorporation of seafloor topography in electromagnetic modeling, Geophys. J. Int., submitted, 2001.
Constable, Parker, and Constable, Occam’s inversion: A practical algorithm for generatingsmooth models from electromagnetic sounding
data, Geophysics, 52, 289–300, 1987.
DeMets, C., R. G. Gordon, D. F. Argus, and
S. Stein, Current plate motions, Geophys.
J. Int., 101, 425–478, 1990.
Evans, R. L., P. Tarits, A. D. Chave, A. White,
G. Heinson, J, H. Filloux, H. Toh, N. Seama,
H. Utada, J. R. Booker, and M. J. Unsworth,Asymmetric Electrical Structure in the Mantle
Beneath the East Pacific Rise at 17◦S, Science,
286, 752–756, 1999.
Federation of Digital Seismographic Networks, Incorporated Research Institutions for Seismology and United States Geological Survey,
Standard for the Exchange of Earthquake
Data — Reference Manual, 203 pp, 1993.
Fujimoto, H., K. Koizumi, Y. Osada, and
T. Kanazawa, Development of instruments forseafloor geodesy, Earth Planets Space, 50,
905–911, 1998.
Gosling, J., B. Joy, and G. Steele, The Java Language Specification, Addison-Wesley, 1996.
Heki K., Horizontal and vertical crustal movementsfrom three-dimensional very long baseline interferometry kinematic reference frame: Implication for the reversal time scale revision,
J. Geophys. Res., 101, 3187–3198, 1996.
Karig, D. E., Origin and development of themarginal basins of the western Pacific, J. Geophys. Res., 76, 2542–2561, 1971.
Kato T., Y. Kotake, S. Nakao, J. Beavan, K. Hirahara, M. Okada, M. Hoshiba, O. Kamigaichi,
R. B. Feir, P. H. Park, M. D. Gerasimenko,
and M. Kasahara, Initial results from WING,
the continuous GPS network in the western
Pacific area, Geophys. Res. Lett., 25, 369–
372, 1998.
Kotake, T., T. Kato, S. Miyazaki, and A. Sengoku,
Relative motion of the Philippine Sea plate derived from GPS observations and tectonics ofthe south-western Japan, Zisin (J. Seismol.
Soc. Jap.), 51, 171–180, 1998 (in Japanesewith English abstract).

Nolasco, R., P. Tarits, J. H. Filloux, andA. D. Chave, Magnetotelluric Imageing of theSociety Islands hotspot, J. Geophys. Res.,
103, 30287–30309, 1998.
Sato, T., Y. Fukuda, Y. Aoyama, H. McQueen,
K. Shibuya, Y. Tamura, K. Asari, and M. Ooe,
On the observed annual gravity variation andthe effect of sea surface height variations,Phys. Earth Planet. Inter., 123, 45–63, 2001.
Spiess, F. N., C. D. Chadwell, J. A. Hildebrand,
L. E. Young, G. H. Purcell Jr., and H. Dragert,
Precise GPS/Acoustic positioning of seafloor
reference points for tectonic studies, Phys.
Earth, Planet. Inter., 108, 101–112, 1998.
Stammer,A. J., and R. M. Chervin, Ocean general circulation from a global eddy-resolving
model, J. Geophys. Res., 97, 5493–5550, 1992.
Sun Microsystems, Java Remote Method Invocation Specification, Revision 1.7, 1999.
The Object Management Group, The Common
Object Request Broker: Architecture andSpecification, Revision 2.4, 2000.