IＡＭＳＴＥＣ掫尚研寛 第1(盻

「し ん か い6500 」 と海底 地 震計 に よる

南 部 マ リ アナ トラ フ 海 山 に お け る

強 い熱 的 湧昇 流 の 証 拠

笠原 順三＊l 佐藤 利典＊l 藤岡換太郎１2

北枇18 度に位 置する爾 留マリフ ナトラフ軸東齠 の海山 の頂上に 光磁気 デスク型 海底地 震叶

（ＭｏｏＢｓ）を 設置 し た。 海暖地 震ltは207℃ の灌ぽを示す 熱水 ベンドの １ｍ脇に落 下した。海

山 直下 の鰉露 活助は 大変低 かうたが。 多数 の圧力･りレスが海底地 露itの/ヽイドロフ ォｙに よ９

て 観測さ れた。 圧力 パル スは圧 力の急 激な低下を示 し，ま たその圃有周期 は3a秒 から １分 で

あ りた。観測の途中 で「しんかい6500 」によ ってベン ドの南 部30m に海 底 地震叶を移動 した。

移 ●售 兩カパ ル スの 数と振 帽は急 激な減少を示 した。 この観 測陪果 ぽ圧力 パルスが為 的ぺｙ Ｆ

からの爨昇洫 によ うてできたことを示 している のだろ う。熱 的ベンドには フルピニ コンカ の群

落が見つか･ｸ た。精 宙海廖洫形には ２本の リ５ アメ ントが認められる。１本は南郎 マリアナト

ラ フの軸上に ，他は トラフ岫の東側にあ る。後 者はヱシ・ 口ｙ状配列を示 し，熟的 ベンド群が

見 つかっ だりｓ アｊ ントのある海山を 縦断しを いる。 熱的活 動の存 在は過 去のト ラフ軸と考え

ら れる地形上 の海嶺 の18km 東 に， リ フティ ング過程 かあ る同席 進行して いることを意味 する

の だろう。

キ ーワー ド： 熟水べ ｙ卜，圧カ パ４,ス．海底地震叶， マ９ アナトラ フ， 推水躙

Intensive Thermal Upwelling at a Seamount

in the South Mariana Trough Observed by

Ocean Bottom Seismic Instruments Using

"Shink&i 6500" Submersible

Junzo KASAHARA*3 Toshinori SATO*3 Kanttro FUJIOKA'4

An MOOBS (Magneto Optical Ocean Bottom Seismometer) wag deployed at just 1 m

beside of * hoc thermal vent whose temperature was 207*0 on the summit region of the

seamount located in 18km east ol the south Marians Trough axis. Although ihe seismi

city just eatfc the sesmcunt wg$ very w≫sk. oumcrous pressure events were observed

by hydrophone channel showing sudden pressure decrease. The characteristic period of

the pressure pulses is from 30 seconds to Imin. Submersible "Sinhai

6500* relocated

MOOBS at 30m south of the vent and the sei&mie records show a

drastic change of

j醇

＊ １ 東 京 大 学地 震 研究 所

＊ ２ 海 路 科 学技 術 セ ン タ ー深 海 研 究 部

＊ Ｓ Ｅｌｒtｈｑｕａｋｅ Ｒ恚ＳＳａrｃｈ ｌｎstitｕ１４ University ０１Ｔｏｋy ｏ

* 4 Deep Sajl Re ｓｅ●rＣｈ Ｄｅｐａrtｘｎとｎt.Ｊａｐａｎ Ｍｌｒine Science aod Tech' てＫｇＯｇｙ Ｃｅｎｌａr

ＪＡ ＭＳＴＥＣ Ｊ.Ｄ●●ｐ Ｓ●Ｉ Ｒ軸｡10 (1994)

number and amplitude of pressure events. This suggests that pressure events might be

generated by the hot water up writing from the thermal vent, where an almiccncha

colony was found. Tftere are two set of topographic lineaments, the one for axis region

of the south Mariana Trough, arid the other located eastward of the axis. The eastward

lineament, which forms echelon arrangements, cuts the summit of seaxnouni where

thermal vent chain were found along the Hnesmept The existence of thermal activity

implies some extent of rifting process occurring in the 18km east of the topographic ridge

which Is supposed to be the previous Trough axis.

Hey words: Hot thermal vent. Pressure pulse, Ocean bottom seismometer, the Mariana

Trough, Submersible

1, Introduction

Oceanic spreading and rifting are thought to be

major sources of global climate changes because heat

flux generated by submarine volcanic activities

seems one hundred times of land volcanoes. Ocean*

ic ridges and troughs are major constituents for such

submarine volcanoes. Although there are extensive

researches on volcanic activity of oceanic ridges and

troughs done by portable instruments, none of

surveys have been done continuously and in real

time,

Mariana Trough is a good example of spreading

center or rifting center of backarc regions (Pig, 1),

In the north of Mariana Trough, several researches

were made using deep-tow camera and rock dregs

(Hawkins, et al, 1990), submersible dives (Craig

et aL 1987) and multi-narrow-beam teg , Kasahara

etal, 1993). Thermal vents were found on the

summit of ridge located at axis region of the Mariana

Trough at 18"N by submersible Alvin dive (Craig

et al, 1987). Temperature was measured as 28?*C at

one of these vents. Similar hydrothertnal chimneys

and biological colonies were confirmed by submersi*

ble "Shinkai 6500" dives at the same location, where

the temperature was 2801D (Gamo et al, 1993). The

topographic survey of Mariana Trough shows close

resemblance to the Mid-Atlantic Ridge, which is a

typical slow spreading center (Hawkins et aL, 1990;

Kong etaU 1993), Magnetic survey also supports

that the Mariana Trough is a type of very slow

spreading (Seama and Fujiwara, 1993). Intense

seismicity was found at the Mariana Trough around

164

18* N (Hussong and Sinton, 1982; Kasahara et aJ,

199$). Crustal structure near the Pagan fracture

sone at the 1763<W shows a typical oceanic crust,

suggesting young oceanic crust formation at this

location (Blbee et si., 1980)- Although there are ex*

tensive researches on volcanic activity of oceanic

ridges and troughs done by portable instruments,

none of surveys have been done continuously and in

real-tine. To monitor submarine volcanic activity

in the Marina Trough region, the authors are propos-

ing to install an integrated geophysical station at one

of the most active thermal vent as a part of G≪0-TOC

(Geophysical and Oceanographical Trans Ocean

Cable) project (Kasahara etaL, 1994).

In the southern part of the Mariana Trough, active

thermal-vents, active chimneys and vent biological

colonies were also found on a summit of seamount

Cl43°65' E. 18*24' N, 1.470m) located at the eastern

side of the trough axis by a "Shinkai 6500" #161 dive

(Garro et al, 1993). The temper was tou:vi as

202 °C at one of the most active thermal vent Al-

though a number of researches revealed strong evi-

dences of backarc spreading or rifting in the north-

ern part of Mariana Trough, southern half of the

trough remains as unsolved problem in stage of

backarc tectonics. Finding of hydrothermal vents

in the 18km east of the southern Mariana Trough

axis might be an evidence of active spreading and

detailed survey of these thermal vents will give solu-

tion for this question. It has been proposed that

spreading of Mariana Trough occurred at the south

and has migrated toward north (Karig, 1971). Ac-

JAMSTEC J Deec SeaRes , 10 {1994)

Fig. 1 (a) Mariana backarc region and hydrothecmal vent area. Number* show previous "Shinkai 6500" dives at

Mariana Trough (after Game et al, 1993). (b) Detail map of south Mariana Trough and location ol dive (9S.

Dive 185 ia almost the same location as this scale. Dotted line shows Mariana Trough axis.

cording Co this hypothesis, the southern part of Mari-

ana Trough already stopped the spreading. It is

interesting to examine whether this is correct or not

In August 8, 1993, a large earthquake, whose mag-

nitude is reported as M 8.2, occurred at forearc side of

Mariana Islands and made some damages on build*

ings, During aftershocks sequence, some events

occurred in the south Mariana Trough area (Fig. 2),

Focal plane solutions of main shock and two large

aftershocks were estimated as thrust having vertical

and horizontal fault plane (PDE) aJthough princi*

pal axes show different directions in the horizontal

plane among them. Sugi (1993) concluded that the

horizontal plane was rupture plane because after*

shocks formed nearly horizontal distribution.

Ad this experiment was carried out in two months

after the main shock (October 17 and 2$), it was

expected to detect some aftershock activities of the

M 8.2 main shock occurring in the south Mariana

Trough ares. In this experiment, one digital OBS

with a hydrophone was deployed on the summit of

seamount, where active thermal vents were previ*

JAM ST EC J. Deep Sea Res.. 10 (1994)

ously found by "Shinkai" dive, located eastern side of

Che southern Mariana Trough (Photo 1). It was

planned to observe OBS and to replace it near an

active thermal vent by submersible "Shinkai 6505".

The purpose of this experiment is to monitor nature

of thermal vent activity just at neighbor of vent by

seismic tools and measure the potential of backarc

rifting process of the south Mariana Trough.

Identifying earthquakes beneath the seamount could

be a good measure for tectonic activity if it exists.

By use of submersible, it is realized to set OBS at a

previously known thermal vent. Hydrophone is ex≫

pected to detect any volcanic T-phase signals if vol-

canic activity can generate. Although a hydro-

phone detects earthquakes, it can also sense pressure

variation because a hydrophone responds to water

pressure change. It is also expected to detect vol-

canic tremors if the seamount is close to the stage of

volcanic eruption.

2. Field Experiments

A digital ocean bottom seismometer called MOOBS

165

Pig, t Main (Solid diamond) and its aftershocks (solid circles) in August 6, 1993 obtained by PDE (USGS). al

Mariana for≪arc and Mariana baclurc. Bathymetry obtained by "Yokwuka* multi-narrow- teams during th≪

present study. Contours are each 200m.

(Magneto Optical Ocean Bottom Seismometer)

(Kasahara etal., 1992) was used. MOOBS has

three-component 2H2 seismometer (L-22E) and

one hydrophone as seismic sensors, and an MO disk

drive for data storage device. After digitising: seis-

mic signals by 16-tut A/D converter, seismic data are

stored in 256Kbyte temporary memory. MOOBS

has two recording modes, event picking and continu-

ous modes. In this experiment the continuous re-

cording mode was selected (or data-save except of

noisy period due to shaking by MO disk drive. The

sensitivities are 1.67 V/(cm/sec) and-8?dB/(V/

/rbar) (or-18? dB/(V/*iPa)) for seismometer and

hydrophone, respectively. Amplification factor for

all channels is 40dB and sampling rate is selected as

50H&. Response of amplifier is flat from DC to 30Hs.

Instead of using glass sphere for pressure housing of

OBS, an aluminum one was used in the viewpoint of

safety in order to avoid a sudden implosive collapse

of sphere, when "Shinkai 6600" carries an OBS,

Approximate deployment site was chosen by

referencing the previous dive ("Shinkai 6500" Dive

#161), where is the former active chimney location

on the seamount at 143°55'E. 13*23'N and depth of

1,500m. Before the deployment, the exact observa*

IS$

tion site was carefully selected using multi-narrow-

beam bathymetric map. Summit area (named

"SNAIL Haven") was chosen as the target site rather

than slope side because seamount slope is very steep

for OBS deployment. Altho ugh t he summit appears

as flat on multi-narrow-beam map within measure*

ment resolution, which is approximately 20m in

depth, the horizontal size is extremely small, nearly

80m square at depth of l,S00m. Later, a number of

"Sinkai 6500" dives reveaJed that the summit area is

very rough in the viewpoint of OBS deployment,

although we expect the fiat top with a sediment

cover. The deployment of OBS was performed by

R/V "Yokosuka", which is a support vessel of

"Shinkai 6500".

After the deployment of OBS, Kasahara, one of

authors, dove on the "SNAIL Haven" in order to

move the OBS ("Shinkai 6500" Dive, #1*3), but the

OBS was not found during his dive. It happened

that the OBS was found by a later dive (#185, by R.

Vrijenhoeh), just lm beside of an active thermal

vent. Temperature of the thermal vent was meas-

ured as 20?°C. An Alviniatncfia colony was found at

this vent (seen as white in Photo 1). Alvinkoncha

is a kind of snail family and first found in the north

JAMSTEC J. D≪ep Sea Fte., 10 ()9M)

Mariana thermal vent by Alvin. Because sitting

pose of 08$ seemed too unstable to fulfill function as

seismometer, it was decided better to move OBS to

more stable place. Although a location, where

should be wide and stable enough to reset the 08$,

was sought around the vent by "Shinkai". there were

so few. Finally. OBS was fixed at one tee&tiori at the

30m south of the previous site by dive # 185. Seel n g

the video tapes taken by #189 dive, the sitting shape

st the first site is better than the second one, OBS

records, however, showed that two horizontal seis-

mometers ace almost dead at the first site, and one

horizontal seismometer is extremely unstable at the

second sitfc showing low-frequency (ca. 0.3Hz) os≪

cilia lion with large amplitude.

The 7 daj's record was obtained from J?; & (local

=GMT+ 8 hours) of October 17 to 11:05 (local) of

October 25, 1993. The OBS was located at

143*55.241' E. IS0 23.615' K and depth ol 1.4?0m.

During the diving study, the multi-narrow- beam

bathymetric survey was carried out by R/V

"Yokosuaka".

3. Results

Data, however, have some losses at savins due to

malfunction of MO disk drive. Amount of data ob-

tained is nea≫y one third c4 the total periods during

one week. By visual examination of video tapes

obtained by "Shinkai 6500", basement of the summit

region is extremely rough in the viewpoint of OBS

landing point The bottom surface of seamount is

covered by mass of pillow lava, specially at the

summit region and along seamount slope. There

were only a few locations covered with thin sedi≫

mente near the summit. There no tfiermat vente

at this flat region. OBS was found at the location

fust 1m beside of an active thermal vent (called

'Geo-phonic vent" site, Photo 1) by "Shinkai 6500"

Dive (#185, by R. Vrijenhoeh) in the narrow

summit region with 30m square. Pillows are ap-

proximately 80cm to 1 m In diameter and form 1- 2

m up and down topographic change. A biological

colony oi Ah/iniconcha was seen at the "Geo-phonic"

thermal vent. Smearing of water was also observed

JAMSTEC J, De@D Sea Res., 10 {1934)

at the vent during dive #185. By several dives, it

was observed that the vents are forming a lineament

and distributed along nearly N-5 strike near the

western wall side of summit.

One hundred and thirty seismic events were ob-

served during 8 days period. Fig. $ shows S-P time

distribution of events. Most of S-P times are dis-

tributed between 10 and 50 seconds. None of events,

which occurred inside of this seamount were found.

The S-P time for the nearest event is ca. 3-4 seconds.

The P and S readings of events were compared to the

fist prepared by USGS Guam station CP.Hactori. per*

sonal communication), where is located at approxi*

mateJy 90km east of the CBS, and the results indicate

very few events having earlier arrivals than in

Guam. Examples of earthquake records are shown

in Fig. 4.

During the current observation, numerous pulse

shape events are recorded only on hydrophone trace,

hereafter, these are called "pressure event(s)", be*

cause these are picked by pressure censor (hydro*

phone), and none of pressure pulses were identified

on seismometer traces even for the largest pulses.

Pig. 6 shows three examples of pressure events.

The characteristic period of pressure pulses is 90

seconds to 1 minute. The MEM spectrum also

shows the same characteristics as visual inspection

(Pig. 6). It is also confirmed that hydrophone

worked correctly seeing natural earthquake records.

167

Sequential nunBw of ≪vert≫

S-P tirne distribution of earthquakes observed.

$-P seconds in vertical and sequential number of

events in horizontal.

Fig 4 Example oi natural earthquakes. Channel I <top trace): vertical; channel 2: homontal #1, channel &

horizontal #2 aod channel 4 (bottom trace) : hydrophone. Vertical relative scales are shown far below after

channel number. Tick on vertical scale corresponds to 5.3roV voltage scale. Two honaontels are near)y dead

due lo Inclined OBS settlement at ale I. (a) Nearest earthquake with 6S5 seconds for (oil length and Cb)

with 40 seconds for full length at site I, (c) further distant earthquake with 655 seconds at site 2 and (d)

low frequency noise observed at site 2 with 40 seconds (or full length. Vertical scalcs axe 4, 1024, 4 and 16

from top to bottom. Extremely large noise on horizontal #1 is due to unstable OBS settlement.

At the second site, mimDer of pressure pulses and

specially amplitude of them quickly decreased.

Pressure pulses tend to stiow negative first breaks.

The above results strongly suggest that pressure

pulses were generated by episodic and rapid up well-

ing of hot water generated by the "Geo-phonic vent",

168

not by smooth and continuos smearing. Negative

first break might indicate local pressure decrease by

Ihis upwelling. Pig. 7 shows the time sequence of

pressure pulses. Although the record is not com-

pletely continuos, only we can say it docs not show

an clear periodic nature except there is a possibility

JAMSTec J. De≫p See Res.. 10 41994}

Fig. 5 Examples c/ pressure events. Note on vertical scale difference between channels 1 and 4:6 and 128.

respectively. Polarity for hydrophone is negative for up.

of 1 day or 12 hours period icily.

4. Discussions

It, however, is necessary to examine whether pres-

sure pulses can be observed by seismometer or not.

Typical amplitude ratio between vertical seismome-

ter and hydrophone at 2-8 Hz is approximately 2.5 :

1 as seen in Fig. 4. During low frequency oscillation

period seen in Fig. 4 (d>, the ratio of vertical seis-

mometer and hydrophone decreased to I :4 at 3.5

seconds. This amplitude decrease can be explained

by low frequency response of 2 Hi seismometer be-

cause sensitivity of 2 Hi seismometer at 4 seconds is

1/8 of that at 2 Hz. The observed ratio is nearly this

level Considering this effect, the sensitivity at 32

seconds is approximately 1/64 of 2Hs. The observed

amplitude of pressure pulse is 1.2V seen in Pig. S.

This corresponds to 0.02V at vertical seismometer

channel. Because this is five times greater than

noise level (4mV) on vertical channel, pressure

U9

Fig. 6 An example of MEM spectrum oi pressure events.

Peak fit 30 second*! mln corresponds to pressure

pulses.

pulse should be seen if these are seismic nature.

Pressure pulses with much larger amplitude are also

net identified on vertical channel. Horizontal #2

channel has much larger amplitude than of vertical

channel when it is active as seen in Fig. 4. This also

supports no evidence of pressure events on horizon-

tal #2 channel. If pressure pulses are generated by

pressure change of surrounding water due to local

up welling of hot (or warm) water from the vent,

the above observation can be explained. It is also

supported by the evidence that change of OBS's loca-

tion shows drastic decrease of number and amplitude

of events The nature that most of pressure pulses

have negative first breaks seems difficult to happen

for acoustic signals. If pressure pulses are shaking

noise caused by unstable sitting pose, these should

be observed simultaneously by seismometers- All

of observations suggest that pressure pulses are not

acoustic signals, but are generated by upwelling of

hot water from the thermal vent intermittently and

the circulation of water generates negative pressure

change at the location of OBS, It is not likely that

seismic oscillation such as in Fig. 4 (d) is volcanic

170

Fl?, 7 Time sequence o( pressure evenu. In the middle

of observation. DBS w≪ moved ty 'SKinhai 6S00*.

Negative procure means decrease of pressure.

Note mo&t 0/ pulses with negative fkrsi break.

tremors generated by volcanism. The most possible

cause for such oscillation is bottom water currents.

Using multi-narrow-beam bathymetry data (un-

published data) taken by the current "Yokosuka"

cruise combined with the results of cruise in 1993

(Camo ctal., 1993), the detailed topographic maps

(Figs. 8 and 9) were generated for south of Mariana

Trough and Mariana forearc region, respectively.

In Fig. 8 (a), there are two different kinds of topo-

graphic lineaments, the on* with NNE-SSW strike

located in the westward and the other with echelon

like lineaments crossing: seamounts with N-S strike

located in the eastward. Westward lineamen t corre-

sponds to the axis of Mariana Trough. The other

group of lineaments exactly corresponds to the ther-

mal vent chain found by a series of the current dives

(e,g. #18S) just on the summit of 'SNAIL Haven*

seamount. The eastside lineament is a little wider

width between 13*25' and 13*40'. N-S trend is also

noticed just on the seamount at 13 ° 10' (

"Usagiyama* seamount). At this seamount one of

"Shinkai 6500* dive (#191. Ms. Masuda) found

hydrothermal yellowish deposits along one of very

steep walls, which cut the seamount by N-S strike.

The yellowish deposits are clear evidence of

hydrothermal activity (Masuda, personal communi-

cation). The Usagiyama seamount is cut by an an*

JAMSTEC J. Deep Sea Res.. 10 (19M)

Pjg. 6 Bathymetry map of Marina Trough, 'a) 50m contours And (b) color map with 200>n contours, N-S faults cutsubmarine volcanos in ihc middle of mapping soon In (a). Mariana Trough a*is runs with NNE-SSW siriko

in ihc tcft side of tho map. B-W linentions across a scamounl (U$a≪iyame) in 13s 10'

JAMSTEC J. DHp Sm fits｡1011994｝ /77

Fig. 9 Batymetric map or Mariana lore arc region with 100m contoure. The area corresponds to main and

aftershocks of August 8.1993 event.

other lineament with the strike of E-W around

13*10'. This seems a conjugate fault set of N-$

lineaments In the south of 13° 10', the Mariana

trough aits changes the strike westward. The N-S

strike of eastward lineament is slightly different

from the one of westward lineament which has been

thought as spreading axis. The N-$ strike linea-

ment is similar to the main strike of the middle and

north Mariana Trough.

OBS at so called "SNAIL Haven" seamount

recorded numerous pressure pulses, which are es-

timated as evidence of pressure decrease due to

hydcathermal upweUiag. The seamount is located

13km eastward of the south Mariana Trough arid it is

cut by N-S trend fault found in the bathymetric map.

The observation of pressure pulses matches with the

observation of high temperature <207"C> at "Geo-

phonic venl". at 1 m distance from OBS. Any earth-

quakes beneath the seamount, however, were not

observed during 7 days period, suggesting rather

low intensity volcanism occurring on this seamount

caused by the faulting activtt?* This might suggest

that the eastward lineament is a kind of presently

active rifting zone. The rifting stage of the east

lineament seems very Juvenile or extinction stage.

172

Acknowledgments

This project was carried out by a pari of

JAMSTEC "Shinkai 6500' dive operations. The pre-

sent authors give their great thanks for nice supports

by people of JAMSTEC, crews and operational stuffs

gt submersible "Shiokai 6500". awl captala and his

crews of R/V "Yokosuka". Multi-narrow-beam

maps were obtained by R/V ■Yofcosuka" of

JAMSTEC during the leg. Bathymeteric maps

were generated by GMT mapping software de-

veloped by University of Hawaii.

References

Bibee, LD., C.G. Shor, Jr. and R.S. Lu (1980) : inter-

arc spreading in the Marina Trough. Maine Geol-

ogy, 35,133-197.

Craig, Y,, K, Horibe, A. Farfey, J.A. Welhan. K-R. Kim

and R N, Hey (1987) : Hydrothermal vents in the

Marina Trough; results of the first ALVIN dives.

Eos Trans., AGU, 68, p< 1531.

OawiO.T. and Shipboard Scientific party ot the Y9204

cruise (1993) : InterRidge News, 2 <1). 11-14

Hawkins, J.W., P.P. Lonsdale, J.D. Macdougall and A

M. Volpe (1990) : Petrology of the axial ridge of

the Mariana Trough ftackarc spreading center.

Earth Planet. Sci Letter. 100,226-250.

Hussoftg, DM. and J.B, Sinton (1982) : 'Seismicity

JAMSTEC J. Ow $4i R*$., 10 (1994)

associated with back arc cruatal spreading in the

central Mariana Trough." p 217-295. In: Tectonic

and geological evolution of southeast Asia seas

and islands, Part 2. Edited by D.E, Hays.

Karig, D.E. <1971) i Structural history of the Man*

ana island arc system. Geol. Sot Am. Bull., 83,

323-344.

Kasahara, J., K. Mochifuki, T. Sato, S. Koresawa and

T, Matsubara (1992): A new digital OBS

(MOOBS) using MO disk and the depth depend*

ence of noise spectrum on deep sea floor ob-

tained by this. AGU Fall Meeting.

Kasahara. J, L. Kong, S. Koresawa. C. Igarashi T.

Yamaguchi, M Suzuki and R. F≪ng-Lu (1998):

Seismic observation at the Mariana Trough 18R N

and its relation to the detailed bathymetry. Pre*

liminary Report of the Hakuho-Maru Cruise KK

92-1, Ocean Research Institute, University of

Tokyo, 77-98.

JAMSTEC J. D* ≫p Ｓ●● Ｒ●Ｓ･, 10（1994 ）

Kasahara, 0 . H. Utada aud H. Kmoshila (1994):

Reuse of submarine cable for seismic and

geoelectrical measurements Oceans 94 Proceed*

in* I-T08-m

Kong, L, N. Seama, R Fujimoto, J. Kasahara and KH

92-1 shipboard scientific party (1992): Segmen-

tation of the Mariana Trough back-arc spread-

ing center at 18N. interRidge News, 1,2-5.

Searoa, N. and T. Fujiwara (1993) ? Geomagnetic

anomalies in the Mariana Trough. 18N. Pre)imi>

nary Report of the Hafcuho-Maru Cruise KH92-1.

Ocean Research Institute, University of Tokyo.

70-73.

Sugi, Y. (199$) : Abstract of Japanese seismological

Society Fall Meeting.

(Manuscript received 30 June 1994)

(Nottce) Photos are given on the following page.

173

174

(a)

(b)

Pholol OBS b ≪･sidc or lk･rmal VC'lll r Ｇt刃|)h りnic sitc“}surroundcd by

pillow 1av;i. Alviniconcha co1 り111い 肬･t'11 as whiu*. QI)ドnr vil･w ilnd

山I near view.

JAMSTEC J. Dnp Sea Res.. 10 1199ね