GRAVITY ANOMALIES IN THE WESTERN PACIFIC OF THEIR …

J. Phys. Earth, 29, 387-419, 1981

GRAVITY ANOMALIES IN THE WESTERN PACIFIC

AND GEOPHYSICAL INTERPRETATION

OF THEIR ORIGIN

Yoshibumi TOMODA and Hiromi FUJIMOTO

Ocean Research Institute, the University of Tokyo, Tokyo, Japan

(Received April 22, 1981; Revised August 19, 1981)

Maps of free air and Bouguer gravity anomalies in the western Pacific arecompiled on the basis of the sea gravity data obtained during a period 1963-1980 by use of the Tokyo Surface Ship Gravity Meter.

Outer gravity high seaward of trench and negative gravity moat around

seamounts can be interpreted as caused by anomalous thickness of the litho-sphere.

Mutual interference between trench and seamount or rise is discussed fromthe view point of thickness of the lithosphere. Seamounts or rises can be

classified into three types;1) seamount which can easily subduct as represented by Kashima No. 1

Seamount,

2) sizable seamounts which take a long time to subduct,3) rises which will never subduct.

It is suggested that a new subduction zone seaward of the old trench isformed if the rises of the type 3 encounter a trench. In such a case a relic of

trench and a fore-arc ridge may be left in topography. After seamount of type2 eventually subduct, they give rise to an acute bending of trench accompanied

by a zone of negative gravity anomaly in the landward side of the trench.

1. Gravity Anomalies in the Western Pacific

1.1 Gravity measurements at sea and mapping of gravity anomaliesGravity data at sea have been obtained mostly in the western Pacific since

1963 by use of a vibrating string gravity meter stabilized by a vertical gyroscope-T.S.S.G.

We have now 18 years' collection of sea gravity data measured by the T.S.S.G.

The data obtained before 1971 have already been published (TOMODA, 1974),and maps of free air and Bouguer gravity anomalies in and around Japan have

also been published (TOMODA, 1973a). Since 1972, stabilization of the meter's

plarform and data aquisition system have been greatly improved. Since thenship's position has been determined by use of N.N.S.S., and, therefore, the qualityof gravity data since 1972 has been greatly improved (FUJIMOTO, 1976). Ship's

tracks along which the sea gravity data were obtained by T.S.S.G. chiefly onboard the R/V Hakuho-maru (3,200t) are shown in Fig. 1.

387

388 Y. TOMODA and H. FUJIMOTO

Fig. 1. Ship's tracks along which sea gravity data were obtained by T .S.S.G. chieflyon board the R/V Hakuho-maru.

The first step of compilation of these gravity data was "debugging" of the"raw data

," most of "bugs" being caused by unexpected errors in N.N.S.S.Ship's position determined by N.N.S.S. and dead-reckoning navigation with theaid of gyrocompass and an electromagnetic log is not always sufficiently accurate

for the Eotvos correction in the order of a few mgal, although the accuracy hasbeen dramatically improved compared with that prior to 1971. For this reasonit was necessary to adjust the data by hand with careful reference to bottom to-

pography and ship's log.The second step of data compilation was the adjustment of inconsistency

between data obtained at different times at the same position. An example ofsuch inconsistency at an intersection is shown in Fig. 2. T.S.S.G. data in its earlystage (before 1971) have sometimes systematic errors more than 10 mgal . Theerrors seem to have occurred when an unexpected vibration of the vertical gyro-scope was existent or when the ship set sail without enough warming -up time fora temperature regulating housing of the string gravimeter. Gravity data meas-

Gravity Anomalies in the Western Pacific 389

Fig. 2. An example of inconsistency of gravity or bathymetric data at intersections of

different cruises.

Fig. 3. Free air gravity anomaly in the northwestern Pacific.


Fig. 4. Bouguer gravity anomaly in the northwestern Pacific.

ured by T.S.S.G. on board the Hakuho-maru generally well agree with eachother.

These inconsistencies were carefully adjusted and maps of gravity anomalyin the northern part of the western Pacific were compiled (TOMODA and FUJIMOTO ,1981). Free air and Bouguer gravity anomalies are simplified and shown forthree regions; northwestern Pacific east of the Japan trench (Figs. 3 and 4), centralpart of the western Pacific in and around the Izu-Ogasawara trench (Figs. 5 and6), and northern part of the Philippine Sea (Figs. 7 and 8).

1.2 Characteristics of gravity anomaly in the western Pacifica) "Outer gravity high" seaward of the Japan trench (WATTS and TALWANI ,

1974) was observed by the sea gravity measurement on board the submarine


Fig. 5. Free air gravity anomaly in the central part of the western Pacific in and around

the Izu-Ogasawara trench.

Fig. 6. Bouguer gravity anomaly in the central part of the western Pacific in and around

the Izu-Ogasawara trench.


Fig. 7. Free air gravity anomaly in the northern part of the Philippine sea.

Ro-57 (MATUYAMA, 1934). It was mentioned that it cannot be interpreted as

caused by crustal structure (KUMAGAI, 1953). In the map of free air gravity

anomaly in and around Japan (TOMODA, 1973a), the distribution of the outer

gravity high is clearly shown, though partially. It is also shown that the outer

gravity high seaward of the trench is compensated by gravimetric low around a

seamount if it is situated close to the trench.

Figure 3 shows the general features of the outer gravity high seaward of the

Japan trench, where the gravity high larger than 20 mgal extends seawards more

than 200km. The maximum height of the positive gravity anomaly amounts to

50 mgal in the region east of the Japan trench along 39° N .


Fig. 8. Bouguer gravity anomaly in the northern part of the Philippine sea.

b) The western Pacific is characterized by the existence of many seamounts

and seamount chains as represented by the Emperor seamounts, Marcus-Necker

seamounts, and Magellan seamounts. It is well known that significant positive

gravity anomalies as large as 200-300 mgal due to the body of seamounts areobserved. However, more important gravity characteristics for the interpreta-tion of its root seem to be the negative free air gravity anomaly observed around

seamounts (TOMODA, 1973b).


As illustrated in the gravity anomalies seaward of the Japan trench, positiveanomalies seaward of the trench are compensated by the negative free air gravity

anomalies around seamounts close to the trench axis. Although this situationis recognized in the southern part of the Japan trench, the interference betweenseamounts and trench is observed typically in the region of the Ogasawara rise

where the Ogasawara trench and Marcus-Necker rise are supposed to collide witheach other (Figs. 5 and 6).

c) Northern part of the Shikoku basin in the Philippine sea is bordered by

the Nankai trough. The outer gravity high seems to exist seaward of the trough,but the magnitude is not so large as that seaward of the Japan trench and the

Philippine trench. The maximum value of free air gravity anomalies seawardof the trough is about 25 mgal, and its mean value is about 10 mgal. As de-scribed by WATTS and TALWANI (1974) the outer gravity high seaward of the trough

is different in its character from that of the Japan trench.The negative gravity anomaly lower than -20 mgal in the northern part of

the Ryukyu trench appears to extend to the gravity low off Miyazaki in Kyushudistrict and further north to the Japan Sea through the Bungo and Kanmon

Strait between Honshu and Kyushu islands.The northern part of the Philippine basin is bordered by the Palau-Kyushu

ridge in its west. The Daito ridge runs nearly perpendicular to the Palau-Kyushu

ridge. Gravity lows are observed at both sides of the Daito ridge, similarlywith those observed in the western end of the Marcus-Necker rise.

2. Gravity Anomaly Expected from the Crustal Structure Disclosed by Explosion

Seismology

A great number of seismic works have been carried out and the structureabove the Moho is determined at more than 470 points in the Pacific region

(SHOR, 1960, 1962, 1964; FISHER and RAITT, 1962; RAITT, 1963; SHOR and POLLARD,1964; FURUMOTO et al., 1965, 1968, 1970, 1971, 1973; LUDWIG et al., 1966, 19711973; MURAUCHI et al., 1967, 1968, 1973; SHOR et al., 1968, 1970, 1971; RYALL

and BENNETT, 1968; DEN et al., 1969, 1971a, b; HILL, 1969; SHOR and RAITT,1969; EWING et al., 1970; HOTTA, 1970; LUDWIG, 1970; YOSHII et al., 1973;

HOUTZ et al., 1980; MURAUCHI and LUDWIG, 1980).Recently, a long shot experiment as long as 1,800km was carried out at the

Mid Pacific as one of the items of the Geodynamics Project (ASADA et al., 1981;NAGUMO et al., 1981). Long velocity profiles are also determined from the Japantrench to Shatsky rise (HOUTZ et al., 1980). Now, the general feature of the

crustal structure above the Moho in the western Pacific basin seems to be clarified.By use of the subterranean velocity structure obtained by explosion seis-

mology, the density structure can be brought out by use of the velocity-densityrelationship and so gravity anomaly caused by the structure below the Moho canbe estimated. The difference between gravity anomaly actually measured and


that estimated from the crustal density structure was called "the residual gravity

anomaly (R.G.A.)" by YOSHII (1972). The R.G.A. puts forth the information

on the density anomaly below the Moho, which is not easily determined by ex-

plosion seismology.Figure 9 shows a map of the free air gravity anomaly in the Pacific basin calcu-

lated from the crustal structure determined by the above-mentioned explosion

seismology works. The gravity values at 470 points in the Pacific were calculatedby use of the velocity-density relationship compiled by LUDWIG et al. (1970), and

the standard gravity value calculated by use of the standard crust given by SHORand RAITT (1969) for the Pacific and Indian basins was subtracted. Figure 10

shows the profiles of the crustal structure and the residual gravity anomaly fromthe Japan Sea to Gorda ridge off the West Coast of U.S.A. along the line A to B

in Fig. 9. East part of the crustal structure section is the same as that shown by

HOTTA (1972). As shown by these profiles, the R.G.A. in the western Pacificbasin is systematically about 300 mgal larger than that in the eastern part. Thisdiscrepancy is more than 10 times as large as the observed gravity anomaly.

Based on these facts, YOSHII (1973) postulated a new model of thickening

lithosphere. According to his postulation the lithosphere produced at the midoceanic ridge becomes thicker toward the trench as material of the asthenosphere

is condensed at its bottom. Analysis by use of the R.G.A. is a useful means forestimation of the regional thickness of lithosphere. We think that the R.G.A.

Fig. 9. Map of the free air gravity anomaly in the Pacific basin calculated from the

crustal structure determined by explosion seismology.


Fig. 10. Profiles of crustal structure and the residual gravity anomaly in the northern

Pacific.

is also useful for inferring the deep structure beneath a minor topographic struc-

ture such as a seamount.It is naturally expected that a seismological analysis puts forward possible

crustal structures that satisfy the time travel curve, resulting in various kinds ofestimation of the R.G.A. According to Yoshii (personal communication), how-

ever, the R.G.A. is always the same as far as the structure strictly satisfies theobserved time travel curve.

The R.G.A. will be applied to the interpretation of gravity lows around

seamounts or outer gravity high seaward of the trench.

3. Outer Gravity High Seaward of the Trench

3.1 Outer gravity high and R.G.A.The results of explosion seismology by LUDWIG et al. (1966), HOUTZ et al.

(1980), and MURAUCHI and LUDWIG (1980) seem to provide sufficient data to in-vestigate the origin of the outer gravity high and the thickness of lithosphere

seaward of the Japan trench, if they are correlated to the gravity data. Figure 11

shows the profiles of velocity structure, free air gravity anomaly, and R.G.A.


Fig. 11. Profiles of velocity structure, free air gravity anomaly and R.G.A. deviation

from the standard value seaward of the Japan trench along the 33° N parallel.

along the 33° N latitude. The R.G.A. was calculated according to the following

formula.

R.G.A.=F.G.A.+2πkk2Hi(ρm-ρi),

where F.G.A. and kk2 indicate free air gravity anomaly and the gravitational con-

stant, respectively. The density of the upper mantle ρm is assumed to be 3.3g/

cm3, which value corresponds to the Pn velocity 8.1km/sec. Hi and ρi indicate

the thickness and the density of the i-th layer, respectively. The R.G.A. thus

obtained minus 587 mgal is shown in Fig. 11. This is the R.G.A. deviation from

the standard value estimated with the standard crust given by SHOR and RAITT

(1969). The profiles of the R.G.A. deviation are also calculated by the same

method along the parallel of 39° N and shown in Fig. 12. The R.G.A. seaward

of the trench is not so complicated as that landward of the trench, and it is pos-

sible to compile a map of relative values of R.G.A. Figure 13 (a) shows the iso-

anomaly lines, drawn by use of 2.5° means of R.G.A. Figure 13 (b) shows the

distribution of depth to the Moho.

The map of R.G.A. as well as its profiles shows that the thickness of litho-

sphere becomes larger toward the trench. According to the model presented by

YOSHII (1973), changes in thickness of lithosphere and accordingly in the theo-

retical R.G.A. value are nearly constant, since the ages of this region are older

than 100 million year. Therefore, variations in the estimated R.G.A. should be

caused by other origins. The observed increase in lithosphere thickness may


Fig. 12. Profiles of velocity structure, free air gravity anomaly and R.G.A. deviation

seaward of the Japan trench along 39°N parallel.

possibly be attributed to a rapid cooling of the oceanic lithosphere close to the

trench.

This fact seems to suggest that extra heat loss must exist in the lithosphere

near the trench axis in addition to heat transfer by conduction, if thickening of

lithosphere is due to cooling process.

Although the R.G.A. landward of the trench seems very important for in-

terpreting the origin of mass excess or deficiency at the trench region, the seismic

data are not as yet sufficient for such analysis.

3.2 Origin of the outer gravity high and driving force of subduction

In order to explain the outer gravity high seaward of the trench, two models

have so far been proposed.

One of the models can be schematically shown in Fig. 14(a). In this model

the trench is formed by the subduction of a mass heavier than its surrounding

material, and the positive free air gravity anomaly seaward of the trench or the

positive Bouguer anomaly at the trench represents this mass excess (MORGAN,

1965). The mass excess can also be interpreted as caused by the density contrast

between the sinking lithosphere and the asthenosphere, or it may also be inter-

preted as caused by the lithosphere which suddenly thickens near the trench


Fig. 13. (a) Map of the R.G.A. deviation seaward of the Japan trench. (b) Map ofthe depth to the Moho seaward of the Japan trench.


Fig. 14. (a) A model showing that the driving force of subduction is produced by thepossible negative buoyancy due to the density contrast near the trench, and that theouter gravity high shows this mass excess (MORGAN, 1965). (b) A model showingthat the outer gravity high is caused by the swollen mantle material near the trench(WATTS and TALWANI, 1974).

(FUJIMOTO, 1976; FUJIMOTO and TOMODA, 1977). The model represented byFig. 14(a) shows essentially that the driving force of subduction is produced bynegative buoyancy due to the density contrast near the trench, and that the outer

gravity high shows this mass excess.On the other hand, Fig. 14(b) shows the model presented by WATTS and

TALWANI (1974). In their model, the driving force of subduction does not directly

connected to outer gravity high, because they explain the gravity high as causedby the swollen mantle material near the trench, which is caused by the elastic

bending of the lithosphere. The structure of the Moho seismically determined,however, does not disclose such an elastic bending of the lithosphere (LUDWIGet al., 1966; HOUTZ et al., 1980; MURAUCHI and LUDWIG, 1980). The outer

gravity high is recognized independently of the outer topographic high which iscaused by the elastic bending.

The essential difference between the two models depends on the assumption

whether the lithosphere is an elastic plate or a plastic plate where isostatic equi-librium is easily achieved.

As shown in Figs. 11 and 12, the R.G.A. shows that the outer gravity highseaward of the trench is caused by density anomaly below the Moho. The

lithosphere thickens towards the trench, so that the horizontal gradient of thick-ness of the lithosphere near the trench is twice or three times as large as the litho-sphere near to the ridge crest (Fig. 10). This extraordinary thickening of the

lithosphere near the trench may possibly cause a driving force of subduction oflithosphere. However, the cause of such an enhanced cooling process is notcertain yet.

4. Gravity Lows around Seamounts

4.1 Gravity anomaly of a seamountPositive free air gravity anomalies relevant to seamounts in the western


Pacific are generally correlated with the topography, but we should also pay

attention to the negative anomaly zones extending over a wider area around the

seamount. The horizontal extent of the negative anomaly zone is generally

more than twice as large as the topographic extent of seamount. These features

are shown by a long north-south profile crossing the crests of seamounts along

about 150°E longitude (Fig. 15). The horizontal extent of the gravimetric moat

is very large and its amplitude is 20-50 mgal.

In order to investigate the origin of the negative free air anomaly surrounding

a seamount, detailed gravity surveys were carried out at the Suiko seamount located

in the Emperor ridge (TOMODA et al., 1968b; SEGAWA, 1970) and at the Shunsetsu

seamount (23°54′N, 148°50′E) in the Marcus-Necker seamount chain (FUJIMOTO,

1976).

One of the gravity profiles across the Shunsetsu seamount is shown in Fig. 16.

The positive free air gravity anomaly has a very good correlation with the seamount

topography. Using this correlation, the density of the seamount has been de-

termined on the basis of the principle that the correlation between Bouguer

anomaly and topography becomes minimum when an optimum density is selected.

As is shown in Fig. 16, the density is 2.9g/cm3 for the Shunsetsu seamount. The

same value is also obtained for the Suiko seamount (SEGAWA, 1970; FUJIMOTO,

1976). Using this density, the Bouguer gravity anomalies are calculated for the

two seamounts. The horizontal extents of Bouguer anomalies probably rep-

resenting roots of Suiko and Shunsetsu seamounts are about 230 and 150km,

respectively. Three models of sub-bottom structure seem to satisfy the gravi-

metric results (Fig. 17), if the structure above the Moho is taken into account.

A) First model (Airy-Heiskanen's model): This is one of the idealized

models of isostasy, where mass of a seamount body is supported by buoyancy

caused by the root in the crust. Gravimetric effect can easily be calculated if

depth of compensation and density contrast between the crust and the mantle

are assumed. The results of calculation for the Shunsetsu seamount (Fig. 18(A))

show that this model does not satisfy the gravimetric results, as far as the com-

pensation depth is in the order of 10km, as expected from the thickness of the

Fig. 15. Profile of the free air gravity anomaly along about the 150°E meridian cross-

ing the crests of seamounts (TOMODA, 1973b).


Fig. 16. Gravity and bathymetric profiles of Shunsetsu seamount.

crust. Airy-Heiskanen's model seems unreasonable, when the isostatic equilibri-um above the Moho is assumed. However, the model fits in the observed results

when a compensation depth of 100km is assumed, as will be mentioned in thefollowing section.

B) Second model ("elastic bending of the crust"): The crust is bent bythe weight of a seamount forming a topographic moat around the seamount asschematically shown in Fig. 17(B), the moat may be filled with sediments of lowdensity amounting to about 2.0g/cm3. In such a model, the bending of the crust

can easily be verified by a acoustic reflection profiling (air gun) around the sea-mount. Figure 19 shows an example of acoustic reflection profiling suggesting

that the thickness of sedimentary layer becomes thicker with the distance fromthe seamount. This is inconsistent with the model.

C) In the third model, the thickness of crust is made large over a wide area

(twice or three times of horizontal extent of the seamount) around a seamountto satisfy the Bouguer gravity anomaly, as shown in Fig. 17(C). The concept ofisostasy is not taken into account explicitly in this model.


Fig. 17. Three types of sea-

mount models viewed

from the gravimetric and

seismic stand points.

Fig. 18. Bouguer gravity anomaly expected from models A, B,

and C shown in Fig. 17.

Fig. 19. Example of acoustic reflection profiling near the foot of a seamount.

4.2 Gravity low around seamounts and anomalous structure of the lithosphereExplosion seismic surveys of the crust are usually carried out in the region

which are away from seamounts. However, if the horizontal extent of the root

of seamount is twice to four times as large as that of the seamount body, the rootof the seamounts can be observed.

As far as the seismic results so far obtained is concerned, it is difficult to

prove that the crust near the seamount becomes thicker. In the case of theEmperor ridge, the crust becomes thinner near the seamount chain as shown in

Fig. 20(a) (DEN et al., 1969). In the case of Bikini Atoll, it seems difficult to

point out any change of crustal structure near the seamount, as shown in Fig.20(b) (RAITT, 1954).

404 Y. TOMODA H. and FUJIMOTO

Fig. 20. Examples of structure crossing near seamounts. (a) Emperor ridge (DENet al., 1969), (b) Bikini Atoll (RAITT, 1954).

Fig. 21. Proposed structure of a seamount. The weight of seamount is supported bythe buoyancy of the root in the asthenosphere underneath the seamount.

Taking such seismic results into consideration it is necessary to attribute the

origin of gravity low to the structure beneath the Moho. Judging from the

horizontal scale of the gravity low, it is reasonably concluded that the lithosphere

beneath a seamount is 20-30km thinner than the surrounding lithosphere as shown

in Fig. 21. In this model weight of a seamount is supported by buoyancy of the

root in the asthenosphere underneath the seamount.

According to the theory of plate tectonics, a seamount chain is produced by


a volcanism when the lithosphere moves above the hot spot (WILSON , 1963;

MORGAN, 1972). Therefore, it will be quite reasonable to assume that the thick-

ness of lithosphere becomes thinner when heated by the hot spot (KONO and YOSHII ,

1975; OGAWARA and KONO, 1981).

A long distance seismic explosion work was carried out at Mariana region in

1976 (ASADA et al., 1981). In this experiment, the line of survey was 1,800km

long, crossing the Marcus-Necker seamount chain, and the maximum explosives

fired were 8.5t. Extraordinarily large attenuation of seismic wave was observed

for the area across the seamounts, compared with that observed in the north-

western Pacific, where there is no seamount chains.

In the northern Pacific region initial phases can easily be detected in seis-

mogram obtained from an explosion of 0.5t by a ocean bottom seismogram

deployed 1,000km apart from the shot point, even if the bottom noise was the

same as that of the Mariana's region. However, when the explosives are fired

across the Marcus-Necker seamount chain, we could not obtain such a seismo-

gram in spite of the large amount of explosive of 8.5t. One of the interpretations

of the fact is that the asthenosphere beneath the Marcus-Necker seamount chain

is responsible for this large attenuation of seismic waves.

Whether or not this idea is also plausible for the seamounts in the Philippine

Sea is an important problem. It is difficult to recognize gravity lows around

seamounts as seen in Fig. 7 which represents the case of the Kinan No. 2 seamount

(30°15′N, 136°40′E). The result may be consistent with magnetic and micro-

paleontological data showing the ages of ocean floor and seamount are nearly

equal there (FURUTA et al., 1980).

4.3 Subducting seamount-Kashima No. 1 seamount

The Kashima No. 1 seamount is located exactly at the axis of the Japan

trench. The seamount has been surveyed in detail by the Hydrographic Depart-

ment, Maritime Safety Agency, Japan, and it was found that the landward half

of the seamount has broken down and subsided by about 1,500m (MOGI and

NISHIZAWA, 1980). Detailed gravity surveys were carried out along tracks with

a spacing of 5 nautical miles in this area. The bottom topography and ship's

tracks along which the gravity profiles are made are shown in Fig. 22. Six sec-

tions crossing above or near the seamount are prepared with respect to the free

air anomaly, the Bouguer anomaly, and the bottom topography, and shown in

(a) through (f) of Fig. 23 (TOMODA et al., 1980). As mentioned above, most

seamounts in the western Pacific have large roots of compensation, which are

represented by relatively smaller Bouguer anomalies or negative free air anomalies

around them. However, the Bouguer gravity anomaly of Kashima No. 1 sea-

mount does not show such characteristics. This is clearly seen in the profile of

mean Bouguer anomaly of the 6 sections of Fig. 23, in which we see only a linear

trend (Fig. 24) of gravity change.

If a depression of Bouguer anomaly around a seamount should reveal the


Fig. 22. Bottom topography and ship's track along which gravity profiles are con-structed at the Kashima No. 1 seamount.

presence of a root of the seamount-a thinner lithosphere beneath the seamount,it can be said that the Kashima No. 1 seamount is lacking in such a root.

A seamount must be supported by buoyancy of asthenosphere. The results

of the Kashima No. 1 seamount suggest that there exists no buoyancy whichsupports this seamount, and this may be the reason why the Kashima No. 1seamount has broken down. The results of acoustic profiling and bottom topo-

graphy show that the breakdown is confined to the seamount body, and that nodepression of the basin around the seamount is recognized. Although this fact

sounds somewhat strange, it would not be impossible that the seamount calmlysinks down when the root which existed just beneath the seamount has disappeared.

The reason why the root of a seamount has disappeared may be attributedto a process by which the lithosphere becomes thicker by cooling due to the hydro-thermal process in the rock (ANDERSON et al., 1979). Gigantic faults observed

seaward of active trenches seem to provide one of the processes releasing theheat preserved there (FUJIMOTO, 1976).

5. Mutual Interference between Seamount and Trench

5.1 Positive gravity anomaly seaward of a trench and negative gravity anomalyaround a seamount

In order to satisfy both seismic and gravimetric results, it would be necessary


Fig. 23. Profiles of free air, Bouguer, and bottom topography along 6 sections cross-

ing above or near the seamount.

to assume that (1) the lithosphere is thinner beneath a seamount due to the rise

of a partially molten layer, and that (2) the lithosphere is thicker near the trenchdue to the possible accretion of high density materials at the bottom of the litho-

sphere. These two phenomena must mutually interfere each other at the trenchmargin.

Figure 25 shows gravity profiles for a case when a seamount exists far from

the trench. When the distance between a trench and a seamount is larger than500km, their gravimetric characteristics can be recognized separately. When

a seamount is near a trench, the outer gravity high seems to be disturbed by thenegative anomaly zone around the seamount, as shown in Fig. 26. Figure 27

shows that the positive anomaly is completely cancelled by the negative anomaly.


Fig. 24. Mean Bouguer anomaly of 6 sections of Fig. 23. It should be noted that onlylinear trend can be seen.

One of the typical examples of gravimetric interference between seamounts

and a trench can be seen between the outer gravity high seaward of the Ogasawaratrench and negative anomaly around the Marcus-Necker seamount chain as shown

in the maps of free air anomaly (Fig. 5) and Bouguer anomaly (Fig. 6). It isreadily concluded that this kind of gravimetric interference must represent thedegeneration of lithospheric anomaly.

5.2 Bending of trench axisIf the thickening of lithosphere is the main cause of the driving force of

lithospheric subduction, and if the thickness anomaly is cancelled by the root of

a seamount, the subduction should stop. In that case the seamount becomes anobstacle of subduction and gives rise to a bending of the trench axis (VOGT, 1973).

An example of such a bending is seen at the intersection of the Kuril and the

Aleutian trenches where the Emperor seamount chain meets with the trenches,and at the intersection of the Ryukyu trench and the Nankai trough where thePalau-Kyushu ridge meets with the trench and trough.

As described before, Kashima No. 1 seamount would subduct without beingaccompanied by a bending of trench axis. Many profiles of acoustic reflection

profiling surveys at the foot of this seamount do not show any faulting whichsuggests the possible effect of horizontal stress near the foot of the seamount

(MURAUCHI and ASANUMA, 1977). A seamount was also found at the trenchaxis to the north of Kashima No. 1 seamount, where it is buried in the sediment

(Kagami, personal communication).This fact shows that seamounts of such a small scale causes no bending of

trench axis. This seems to suggest that bending of trench axis due to difference


Fig. 25. Gravity profiles when a seamount is located far from the trench.

Fig. 26. Gravity profiles when a seamount is located near the trench.

Fig. 27. An example of cancellation of the outer gravity high seaward of the trench

by the gravity low around the seamount.

in the thickness of lithosphere depends on the scale of the seamount or on the

characteristics of the crust of the seamount.

Suppose that a sizable seamount approaches a trench in the course of a plate


motion. The seamount would not break down until the thickness of the litho-

sphere beneath the seamount reaches a certain limit by cooling. Assuming thatthe model is two-dimensional, the thickness of lithosphere beneath a seamount

as high as 5km is 10km thinner than the surrounding lithosphere. It wouldtake about 10 million years for the negative root of seamount of 10km thick

to disappear, if the normal cooling process of lithosphere continues. However,the negative root would persist only for about 1 million year, because, heat transfernear the trench seems about 10 times larger than in the normal place as estimated

from the thickening rate of the lithosphere near the trench as previously shownin Fig. 10. Therefore when the subducting force around the seamount is large,the trench axis would be compelled to flip over the seamount, leaving the seamount

behind. About 1 million year later, root of the seamount would disappear andthe normal subduction would take place.

5.3 Relics of seamount landward of trench

The maximum negative free air gravity anomaly off Urakawa in Hokkaidois -200 mgal and horizontal extent of the region where gravity value is lowerthan -20 mgal is larger than 100km. The scale of the negative zone is com-

parable to that of the trench, but the water depth is smaller than 200m.Asimilar negative anomaly zone off Miyazaki in Kyushu district (TSUBOI

et al., 1956; TOMODA et al., 1968a), which is not so large as that off Urakawa,

indicates the lowest negative value amounting to about -150 mgal. It is im-

portant that these negative zones are both located at the landward side of junctionof two trenches.

These negative zones can be interpreted as a result of delayed subduction ofrelics of seamounts, started to subduct later to form an anomalously swollen

subduction zone. Such a configuration of subduction zone may cause a negativebuoyancy resulting in the observed negative free air anomaly at the landward

side of trench junctions.

5.4 Non-subducting seamount

There are a number of oceanic ridges or rises in the western Pacific. Oneof the largest ones is the Shatsky rise the horizontal extent of which is more than400km, and the thickness of the crust determined by explosion seismic surveys

is about 18km (Fig. 28).The crustal structure of the rise brought out by the explosion seismic surveys

and the free air gravity anomaly show that the rise is quite isostatic.The R.G.A. over the Shatsky rise is almost the same as that in the west of

the rise, showing that the isostatic equilibrium is perfectly achieved above the

Moho. If such a ridge or rise arrives at the trench, a collision must occur as wecan see in the case of the collision between the Eurasian and the Indian plates.

In the case of the Ogasawara rise, however, the situation seems to be in-termediate stage between a seamount supported by the asthenosphere and a rise


Fig. 28. Profiles of velocity structure (DEN et al., 1969), free air gravity anomaly, and

R.G.A. deviation from the standard value over the Shatsky rise.

supported by the upper mantle.The density of a seamount is usually 2.8-2.9g/cm3, as described in Sec. 4.

The density estimated from the correlation between free air gravity anomaly andtopography of a seamount is likely to give a lower density according to the degree

of isostatic compensation of crustal structure. In the central continental districtof Japan, for example, a density of 2.67g/cm3 is obtained from the area with a

topography smaller than 20km in scale, but a density of 2.0-1.5g/cm3 with thetopography of 50-200km in scale (YAMAMOTO et al., 1980).

Profiles of free air, Bouguer, and bottom topography along the Ogasawaratrench axis is shown in Fig. 29. If the same analysis as that in Sec. 4 is applied

to the Ogasawara rise, extraordinarily small value of density less than 2.0g/cm3

will be given. Such an apparently low density given for the Ogasawara rise sug-

gests that the depth of compensation is not so deep as that of isolated seamounts,and that the crustal structure in this region is partially isostatic.


Fig. 29. Profiles of free air, Bouguer, and bottom topography along the Ogasawara

trench axis.

The depth of compensation can be determined, as Hayford calculated assum-ing Airy isostasy, i.e., depth is given as an optimum depth which gives the best cor-

relation between topography and equivalent Bouguer anomaly.As seen in Fig. 29, the correlation between Bouguer gravity anomaly and

topography is good unlike the case of the isolated seamount mentioned above,and so it is concluded that the depth of compensation is shallow.

The Ogasawara rise is partially isostatic equilibrium by buoyancy of the crust,

and the rise is not so easily subducted as the Kashima No. 1 seamount.When such a fragment arrives at the trench, subduction will stop at least

temporarily. Seaward jump of trench axis will take place, if subduction process

proceeds in the surrounding trench.


Fig. 30. Relics of trench in the west of Hokkaido district.

5.5 Relics of trench

When jump of the trench axis takes place, the old trench continues to existas the fragment of lithosphere heavier than asthenosphere still subducts. Arelic of trench will be recognized until the fragment subducts deeper into the

asthenosphere.An example of such relics of trench is seen in the west of Hokkaido district,

where a negative zone of free air gravity anomaly runs from the Urakawa lownorth to the west of Sakhalin as schematically shown in Fig. 30.

In the case of the Ogasawara trench, we can see double arc: One is representedby Iwozima islands and the other by Ogasawara islands. The Ogasawara troughbetween these two topographic highs is interpreted as a relic of trench. TheOgasawara trench must have stopped its subduction near the Ogasawara islands

after the islands collided with the trench. As shown by the outer gravity highrunning from the north to the south in this collisional region (Fig. 5), the drivingforce of lithospheric subduction there must be larger than that for a normallithosphere, should the outer gravity high seaward of trench be a measure of the

driving force of subduction. These situation must have resulted in jump of


Fig. 31. Role of "A," "sizable A," and "B" types seamount or rises at a trench.

trench axis toward east, and the Ogasawara trough was formed as relics of the

old trench.

6. Summary-Role of Seamount and Rise at a Plate Boundary

Seamounts and rises are classified into three classes-A type, sizable A type,

and B type as shown in Fig. 31. An "A" type seamount is supported by the

asthenosphere. A "sizable A" type seamount is supported by the asthenosphere,

although it is hard to be broken down by a cooling process as in the case of type

"A." A type "B" seamount is a rise supported by the crust.

When these seamounts are in line and move towards the trench (Fig. 31),

the type "A" would easily be subducted with the slab, the type "sizable A" would

not be subducted so easily as the type "A" and jumping of trench axis would take

place and relics of trench or seamount would be seen. The type "B" would never

be subducted and jumping of trench axis would take place and consequently the

type "B" will be accreted.

In a three-dimensional case (Fig. 32), suppose that plate α and plate β are

bounded by a trench and moving in different directions.

The type A will easily sink at the trench and the type B will be accreted to

plate a at the trench and jump of the trench axis will take place.

In such case, type "B" changes its plate from β to α, and accordingly the


Fig. 32. Role of "A" type seamounts and "B" type rises at a trench. Type "B"

changes its plate from α to β, md accordingly the "B" type rise changes its moving

direction.

"B" type rise changes its moving direction.The Philippine sea, especially its northern part, seems to be very complicated,

and the authors suggest that at least some complexities of the Philippine basin

are due to situation mentioned above.For example, Ogasawara islands including Chichijima and Hahajima islands

and Daito ridge are similar to each other from the view points of geochronology

(KANEOKA et al., 1970; OZIMA et al., 1980) and paleontology.If these islands and ridge are assumed to be separated by the opening of the

north Philippine basin, it seems difficult to explain why the inactive Ogasawara

islands and the Ogasawara trough now exist east of the active Iwojima islandson the Philippine plate. However, this fact may be explained as that the Ogasa-

wara islands which had previously belonged to the Marcus-Necker seamounts,changed their plate from the Pacific to the Philippine by eastward jumping of the

trench axis by collision of Ogasawara islands. Low heat flow at the Ogasawara

trough and westward penetration of Bouguer anomaly over the Ogasawara riseseem to support these hypothesis.


We wish to thank Professor Kazuo Kobayashi for many helpful discussions and for readingthe manuscript critically. We thank Professors Sadanori Murauchi, Toshi Asada and Toshi-

katsu Yoshii for helpful discussions on interpretation and evaluation of results of explosionseismology.

We thank Dr. Jiro Segawa, Dr. Kazuhiro Kitazawa, Messrs. Kin-ichiro Koizumi, TakaoIgarashi, Akinori Uchiyama, and Takeshi Matsumoto for their able assistance in gravity meas-

urement at sea.The R/V Hakuho-Maru is opened for all kinds of basic research for the ocean and therefore

only 13% of the whole cruise is available for solid geophysical use. To cover the West Pacific,measurement was carried out in almost all cruises, and we wish to thank chief scientists of the

cruises for giving us chances of gravity measurement even if the cruises were planned for fisherysciences, biology, chemistry, physical oceanography or meteorology.

REFERENCES

ANDERSON, R.N., M.A. HOBART, and M.G. LANGSETH, Geothermal convection through oceaniccrust and sediments in the Indian Ocean, Science, 204, 828-832, 1979.

ASADA, T., H. SHIMAMURA, S. ASANO, K. KOBAYASHI, and Y. TOMODA, Explosion seismologicalexperiments on long-range profiles in the North Western Pacific and the Marianas Sea, in

The Geodynamics Program W.G.-1 Final Report, ed. T.W.C. Hilde, 1981 (in press).

DEN, N., H. HOTTA, S. ASANO, T. YOSHII, N. SAKAJIRI, and Y. ICHINOSE, Seismic refraction

and reflection measurements around Hokkaido. Part 1. Crustal structure of the continentalslope off Tokachi, J. Phys. Earth, 19, 329-345, 1971 a.

DEN, N, W.J. LUDWIG, S. MURAUCHI, M. EWING, H. HOTTA, T, ASANUMA, T. YOSHII, A.

KUBOTERA, and K. HAGIWARA, Sediments and structure of the Eauripik-New Guinea Rise,

J. Geophys. Res., 76, 4711-4723, 1971 b.DEN, N., W.J. LUDWIG, S. MURAUCHI, J.I. EWING, H. HOTTA, N.T. EDGAR, T. YOSHII, T.

ASANUMA, K. HAGIWARA, T. SATO, and S. ANDO, Seismic refraction measurements in the

Northwest Pacific Basin, J. Geophys. Res., 74, 1421-1434, 1969.EWING, M., L.V. HAWKINS, and W.J. LUDWIG, Crustal structure of the Coral Sea, J. Geophys.

Res., 75, 1953-1962, 1970.

FISHER, R.L. and R.W. RAITT, Topography and structure of the Peru-Chile trench, Deep-SeaRes., 9, 423-443, 1962.

FUJIMOTO, H., Processing of gravity data at sea and their geophysical interpretation in the regionof the Western Pacific, Bull. Ocean Res. Inst., Univ. Tokyo, 8, 81 pp., 1976.

FUJIMOTO, H. and Y. TOMODA, Lithosphere anomalies beneath seamounts and trenches, derived

from difference between observed gravity and expected gravity from subterranean structure,

and mutual interaction between the trenches and seamounts, J. Seismol. Soc. Japan, 30,359-368, 1977 (in Japanese).

FURUMOTO, A.S., N.J. THOMPSON, and G.P. WOOLLARD, The structure of Koolau Volcano

from seismic refraction studies, Pac. Sci., 19, 306-314, 1965.FURUMOTO, A.S., J.F. CAMPBELL, and D.M. HUSSONG, Seismic refraction surveys along the

Hawaiian Ridge, Kauai to Midway Island, Bull. Seismol. Soc. Am., 61, 147-166, 1971.FURUMOTO, A.S., G.P. WOOLLARD, J.F. CAMPBELL, and D.M. HUSSONG, Variation in the thick-

ness of the crust in the Hawaiian Archipelago, in The Crust and Upper Mantle of the PacificArea, ed. L. Knopoff, C. Drake, and P. Hart, AGU Monogr., 12, 94-111, 1968.

FURUMOTO, A.S., W.A. WIEBENGA, J. P. WEBB, and G.H. SUTTON, Crustal structure of the

Hawaiian Archipelago, Northern Melanesia, and the Central Pacific Basin by seismic re-

fraction methods, Tectonophysics, 20, 153-164, 1973.FURUMOTO, A.S., D.M. HUSSONG, J.F. CAMPBELL, G.H. SUTTON, A. MALAHOFF, J.C. ROSE ,


and G.P. WOOLLARD, Crustal and upper mantle structure of the Solomon Islands as revealed

by seismic refraction survey of November-December 1966, Pac. Sci., 24, 315-332, 1970.

FURUTA, T., S. TONOUCHI, and M. NAKADA, Magnetic Properties of Pillow Basalt from the

Kinan Seamount Chain, the Shikoku Basin, J. Geomag. Geoelectr., 32, 567, 573, 1980.HILL, D.P., Crustal structure of the Island of Hawaii from seismic refraction measurements,

Bull. Seismol. Soc. Am., 59, 101-130, 1969.

HOTTA, H., A crustal section across the Izu-Ogasawara Arc and Trench, J. Phys. Earth, 18,125-141, 1970.

HOTTA, H., Crustal structure determined by seismic refraction method, in Submarine Geophysics,ed. Y. Tomoda, pp. 31-66, University of Tokyo Press, Tokyo, 1972 (in Japanese).

HOUTZ, R., C. WINDISCH, and S. MURAUCHI, Changes in the crust and upper mantle near the

Japan-Bonin Trench, J. Geophys. Res., 85, 267-274, 1980.

KANEOKA, I., N. ISSHIKI, and S. ZASHU, K-Ar ages of the Izu-Bonin Islands, Geochem. J., 4,53-60, 1970.

KONO, Y. and T. YOSHII, Numerical experiments on the thickening plate model, J. Phys. Earth,23, 63-75, 1975.

KUMAGAI, N., Results of measurements of gravity in Japan and her vicinity, Report of Japanese

Geodetic Commission, New Series No. 4, 1953.

LUDWIG, W.J., The Manila Trench and west Luzon Trough. III. Seismic-refraction measure-ments, Deep-Sea Res., 17, 553-571, 1970.

LUDWIG, W.J., J.E. NAFE, and C.L. DRAKE, Seismic refraction, in The Sea, Vol. 4, Part I,ed. A.E. Maxwell, pp. 53-84, Interscience, New York, 1970.

LUDWIG, W.J., S. MURAUCHI, N. DEN, P. BUHL, H. HOTTA, M. EWING, T. ASANUMA, T. YOSHII,

and N. SAKAJIRI, Structure of East China Sea, West Philippine Sea margin off Southern

Kyushu, Japan, J. Geophys. Res., 78, 2526-2536, 1973.

LUDWIG, W.J., J.I. EWING, M. EWING, S. MURAUCHI, N. DEN, S. ASANO, H. HOTTA, M.HAYAKAWA, T. ASANUMA, K. ICHIKAWA, and I. NOGUCHI, Sediments and structure of theJapan Trench, J. Geophys. Res., 71, 2121-2137, 1966.

LUDWIG, W.J., S. MURAUCHI, N. DEN, M. EWING, H. HOTTA, R.E. HOUTZ, T. YOSHII, T.

ASANUMA, K. HAGIWARA, T. SATO, and S. ANDO, Structure of Bowers Ridge, Bering Sea,J. Geophys. Res., 76, 6350-6366, 1971.

MATUYAMA, M., Measurements of gravity over the Nippon Trench on board the J.M. SubmarineRo-57, preliminary report, Proc. Imp. Acad. Japan, 10, 626-628, 1934.

MOGI, A. and K. NISHIZAWA, Breakdown of a seamount on the slope of the Japan Trench,

Proc. Japan Acad., Ser. B, 56, 257-259, 1980.

MORGAN, W.J., Gravity anomalies and convection currents, J. Geophys. Res., 70, 6175-6204,1965.

MORGAN, W.J., Deep mantle convection plumes and plate motions, Am. Assoc. Petrol. Geol.

Bull., 56, 203-213, 1972.MURAUCHI, S. and T. ASANUMA, Seismic Reflection Profiles in the Western Pacific, 1965-74,

232 pp., University of Tokyo Press, Tokyo, 1977.MURAUCHI, S. and W.J. LUDWIG, Crustal structure of the Japan trench: The effect of sub-

duction of ocean crust, in Initial Reports of the Deep Sea Drilling Project, ed. E. Honza

et al., Vol. 56, 57, pp. 463-469, U.S. Government Printing Office, Washington, 1980.MURAUCHI, S., N. DEN, S. ASANO, H. HOTTA, T. ASANUMA, T. YOSHII, K. HAGIWARA, and M.

YASUI, Crustal structure of the Japan Sea derived from the deep-sea seismic observations,1967 (in preparation).

MURAUCHI, S., W.J. LUDWIG, N. DEN, H. HOTTA, T. ASANUMA, T. YOSHII, A. KUBOTERA, andK. HAGIWARA, Seismic refraction measurements on the Ontong Java plateau northeast of

New Ireland, J. Geophys. Res., 78, 8653-8663, 1973.MURAUCHI, S., N. DEN. S. ASANO, H. HOTTA, T. YOSHII, T. ASANUMA, K. HAGIWARA, K. ICHI-


KAWA, T. SATO, W.J. LUDWIG, J.I. EWING, N.T. EDGAR, and R.E. HOUTZ, Crustal struc-ture of the Philippine Sea, J. Geophys. Res., 73, 3143-3171, 1968.

NAGUMO, S., T. OUCHI, J. KASAHARA, S. KORESAWA, Y. TOMODA, K. KOBAYASHI, A.S. FURU-MOTO, M.E. ODEGARD, and G.H. SUTTON, Earth Planet Sci. Lett., 53, 93-102, 1981.

OGAWARA, H. and Y. KONO, Thinning of the oceanic lithosphere-evolution of the Hawaiian-Emperor Chain, J. Seismol. Soc. Japan, 34, 385-400, 1981 (in Japanese)

OZIMA, M., Y. TAKIGAMI, and I. KANEOKA, 40Ar-39Ar geochronological studies on rocks of

Deep Sea Drilling Project Sites 443, 445, and 446, in Initial Reports of the Deep Sea DrillingProject, ed. G. de Vries Klein, K. Kobayashi et al., Vol. 58, pp. 917-920, U.S. GovernmentPrinting Office, Washington, 1980.

RAITT, R.W., Seismic refraction studies of Bikini and Kwajalein Atolls and Sylvania Guyot,U.S. Geol. Surv. Prof. Pap., 260-K, 507-527, 1954.

RAITT, R.W., The crustal rocks, in The Sea, Vol. 3, ed. M.N. Hill, pp. 85-102, Interscience,

New York, 1963.RYALL, A. and D.L. BENNETT, Crustal structure of Southern Hawaii related to volcanic pro-

cesses in the upper mantle, J. Geophys. Res., 73, 4561-4582, 1968.

SEGAWA, J., Gravity measurements at sea by use of the T.S.S.G. Part 2, J. Phys. Earth, 18,203-284, 1970.

SHOR, G.G., JR., Crustal structure of the Hawaiian Ridge near Gardner Pinnacles, Bull. Seismol.

Soc. Am., 50, 563-573, 1960.SHOR, G.G., JR., Seismic refraction studies off the coast of Alaska: 1956-1957, Bull. Seismol.

Soc. Am., 52, 37-57, 1962.

SHOR, G.G., JR., Structure of the Bering Sea and the Aleutian Ridge, Mar. Geol., 1, 213-219,1964.

SHOR, G.G., JR. and D.D. POLLARD, Mohole site selection studies north of Maui, J. Geophys.Res., 69, 1627-1637, 1964.

SHOR, G.G., JR. and R.W. RAITT, Explosion seismic refraction studies of the crust and upper

mantle in the Pacific and Indian oceans, in The Earth's Crust and Upper Mantle, ed. A.R.Ritsema, AGU Monogr., Vol. 13, pp. 225-230, 1969.

SHOR, G.G., JR., H.W. MENARD, and R.W. RAITT, Structure of the Pacific Basin, in The Sea,ed. A.E. Maxwell, Vol. 4, Part II, pp. 3-27, Interscience, New York, 1970.

SHOR, G.G., JR., H.K. KIRK, and H.W. MENARD, Crustal structure of the Melanesian area,

J. Geophys. Res., 76, 2562-2586, 1971.SHOR, G.G., JR., P. DEHLINGER, H.K. KIRK, and W.S. FRENCH, Sesimic refraction studies off

Oregon and northern California, J. Geophys. Res., 73, 2175-2194, 1968.

TOMODA, Y., Maps of Free Air and Bouguer Gravity Anomalies in and around Japan, Univer-sity of Tokyo Press, Tokyo, 1973 a.

TOMODA, Y., Gravity anomalies in the Pacific Ocean, in The Western Pacific: Island Arcs, Mar-

ginal Seas, Geochemistry, ed. P.J. Coleman, pp. 5-20, University of Western AustraliaPress, 1973 b.

TOMODA, Y., Reference Book for Gravity, Magnetic and Bathymetric Data of the Pacific Oceanand Adjacent Seas, 1963-71, 158 pp., University of Tokyo Press, Tokyo, 1974.

TOMODA, Y. and H. FUJIMOTO, Maps of Free Air and Bouguer Gravity Anomalies and Bathy-metry in the West Pacific, Bull. Ocean Res. Inst., Univ. Tokyo, 14, 1981 (in press).

TOMODA, Y., H. FUJIMOTO, and Y. GANEKO, Interaction between trench and seamounts in case ofKashima Seamount No. 1, J. Seismol. Soc. Japan, 33, 493-499, 1980 (in Japanese).

TOMODA, Y., K. OZAWA, and J. SEGAWA, Measurement of gravity and magnetic field on boarda cruising vessel, Bull. Ocean Res. Inst., Univ. Tokyo, 3, 1-170, 1968 a.

TOMODA, Y., J. SEGAWA, S. OSHIMA, Y. GANEKO, Y. SUZUKI, and O. ISEZAKI, The results of

grid survey on the Suiko seamount, in Preliminary Report of the Hakuho Maru CruiseKH-68-3, ed. Y. Tomoda, pp. 13-27, Ocean Res.. Inst., Univ. Tokyo, 1968 b.


TSUBOI, C., A. JITSUKAWA, and H. TAJIMA, Gravity survey along the lines of precise levels through-

out Japan by means of a Worden gravimeter, Part 9. Kyushu District, Bull. Earthq. Res.Inst., Univ. Tokyo, 4 Suppl., 475-552, 1956.

VOGT, P.R., Subduction and aseismic ridges, Nature, 241, 189-191, 1973.WATTS, A.B. and M. TALWANI, Gravity anomalies of deep-sea trenches and their tectonic im-

plications, Geophys. J.R. Astron. Soc., 36, 57-90, 1974.WILSON, J.T., A possible origin of the Hawaiian Islands, Can. J. Phys., 41, 863-870, 1963.

YAMAMOTO, A., Y. FUKAO, and K. NOZAKI, A method for estimation of crustal density necessary

for Bouguer gravity reduction, Programme and Abstracts, 1980, No. 2, Seismol. Soc. Japan,

p. 52, 1980 (in Japanese).YOSHII, T., Terrestrial heat flow and features of the upper mantle beneath the Pacific and the

Sea of Japan, J. Phys. Earth, 20, 271-285, 1972.

YOSHII, T., Upper mantle structure beneath the North Pacific and the marginal seas, J. Phys.Earth, 21, 313-328, 1973.

YOSHII, T., W.J. LUDWIG, N. DEN, S. MURAUCHI, M. EWING, H. HOTTA, P. BUHL, T. ASANUMA,and N. SAKAJIRI, Structure of southwest Japan margin off Shikoku, J. Geophys. Res., 78,2517-2525, 1973.

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