Lateral variations in the upper mantle velocity structure in the northwestern pacific margin

Tectonophysics,

112 (1985) 227-253

Elsevier Science Publishers

LATERAL

227

B.V.. Amsterdam

VARIATIONS

VELOCITY

STRUCTURE

PACIFIC MARGIN

and V.G. KRISHNA

Natronal Geophysical Research Institute, (Received

in The Netherlands

IN THE UPPER MANTLE

IN THE NORTHWESTERN

K.L. KAILA

- Printed

July 10, 1984; accepted

Hyderabad

August

500 007 (India)

6, 1984)

ABSTRACT Kaila.

K.L. and

Krishna,

northwestern

V.G.,

Pacific

margin.

1985. Lateral

Subduction

Zones. Tectonophysics

The upper

mantle

Pacific times

velocity

has been studied data

obtained Okhotsk

Pacific margin:

7.80 km/set

are found

northern Ryukyu nearly

(1969)

determined

regions

in the Ryukyu

to about

P. velocities,

are quite similar found

velocity

Sea region, jumps

comprising

functions,

reveal a sharp

first-order

Japan

and the Japan

365 km for P and 345 km for S waves with associated regions

are found to be considerably

marginal km depth.

(e.g. Sakurazima

seas of Japan

and Okhotsk,

There is no evidence

velocity jumps

of a significant

Pacific margin.

0040-1951/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

constant

to

also in the

range of 290-640

km. in the

In the central

Japan

discontinuity

The observed region

also,

at depths

of

of 8.6% for P and 4.8% for S waves.

to about 640 km depth in the central may be a high temperature/high

than the

of the southwest

resulting

velocity

lower. The entire area, comprising

for the presence

island,

255 km, in the

to remain

chamber

from

Kurile Islands

in the Hokkaido

islands

Is.,

The S

higher, about

at 390 km depth.

first-order

8.20

volcano).

in the depth

discontinuity

Sea, there is a sharp

The S velocities below this discontinuity

regions.

5% higher on the average,

are 9.2% for P and 5.6% for S waves in this region.

the Honshu

to be about

and the Japanese

of about

are found

in

slab in

where it is varying

depths

of a large magma

determined velocity

margin

and Shikoku

P and S wave velocities

in southwest

lithospheric

is found

to a depth

and are about

(4)

Islands,

of the order of 8-10%.

185 km depth in the southern

at comparable

were of the

(3) Hokkaido,

and Taiwan-Luzon

Pacific

in the Kyushu

In the southwest

The P- and S-wave velocity-depth

units

(8) Ryukyu

Kurile Islands,

determined

255 km depth. This might be due to the presence

Okhotsk

both

velocities

in several

Kurile Is., is also found to be relatively

almost

observed

in

of the northwestern

the subducting

Islands,

in the northwestern

of 7.88 km/set region,

method

at 40 km depth,

in the northern

Japan.

activity

in the

of P- and S-wave travel

Sea, (2) Kyushu-Shikoku,

higher than those obtained

Japan

structure

and Processes

35 to 640 km. Wave

analytical

250 km depth within

determined

and Kamchatka.

P velocity

volcanic

sea regions

Kurile Islands, (7) Kamchatka,

7.90 km/set

to all other regions

and Taiwan-Luzon

extensive

velocity

Structures

evidence for large lateral variations,

The P velocity

P and S velocities

from

Bonin-Japan

at 40 km depth, in the southern

Islands

constant

varying

Kaila’s

and 8.05 km/set

to be 3-5%

Kurile

arc-marginal

depths

to about

Kurile Islands,

compared

4.30 to 4.45 km/set. region

structure

Pacific margin.

velocity determined

focal

(1) Honshu-Izu

in Kamchatka

4.60 km/set,

with

There is substantial

in the southern

island

Kurile Islands, (6) northern

both P- and S-wave velocity the northwestern

in various

of foci by using

Sea, (5) southern

km/set

mantle

112: 227-253.

structure

of 363 earthquakes

and (9) Taiwan-Luzon.

in the upper

and I.S. Sacks (Editors),

in detail to a depth of about 640 km from the analysis

at the depths

northwestern

variations

In; K Kobayashi

Japan

and the Okhotsk

the high heat-flow attenuation

low-velocity

sea

regions of the

zone extending

to 640

layer in the northwestern

INTRODUCTION

The normal

northwestern polarity.

according

Pacific

margin

comprises

This vast descending

to Isacks

Kurile-Kamchatka,

and

Molnar

the north

of several

island

edge of the northwest

(1971) Honshu,

can be divided the

Izu Bonin

into and

arc systems

Pacific

of

lithosphere.

four segments: the Marianas,

the each

characterized by distinct trends of the oceanic trench, the line of active volcanoes and the inclined seismic zones of earthquakes. A nearly continuous zone of deep and intermediate-depth to the Marianas.

earthquakes can be traced from Kamchatka through Japan and Although seismic activity between depths of 150 and 350 km is

relatively sparse in the Honshu and the Sea of Japan regions. the zone appears to be continuous in these regions down to 600 km depth. However. in the regions of Kamchatka,

Kurile

islands

and Hokkaido

in the north,

and Kyushu

and Shikoku

islands of Japan in the south, earthquakes occur only to a depth of about 200 km. Similarly, in the Ryukyu Islands chain, extending further south from Kyushu island to Taiwan, as well as in the Taiwan-Luzon arcs, earthquake activity extends to only about 250 km depth. The marginal sea regions of the Okhotsk Sea behind the Kurile islands chain and the Japan Sea are characterized by only deep earthquake activity between depths of 300-650 km. Island arcs in the western Pacific have attracted the attention of many geo-scientists all over the world, especially because of their anomalous structure as depicted by various geophysical features associated with them. Studies of seismic wave propagation in these island arcs have revealed significantly anomalous properties of the upper mantle in these regions. The presence of the relatively cold and dense subducting slab of the lithosphere causes more efficient propagation of seismic waves with less attenuation and considerably high velocity through it as compared to the surrounding normal mantle. A large number of studies during the past fifteen years has brought to light the lateral variations of seismic wave velocities attenuation in the upper mantle beneath most of the western Pacific island

and arcs

(Utsu, 1966, 1967, 1971a, b; Oliver and Isacks, 1967; Utsu and Okada, 1968; Kanamori, 1968, 1970; Mitronovas and Isacks, 1971; Hamada, 1973; Pascal et al.. 1973). However, most of the earlier studies were limited to only estimates of velocity contrasts between, rather than the actual velocity structure within, the subducting lithospheric slab associated with the inclined seismic zone (high Q, high V) and the aseismic surrounding mantle (low Q, low V). Further, S-wave studies are, in fact, comparatively much less due to inherent difficulties in dealing with them. We present here substantial evidence for large lateral variations of both the Pand S-wave velocity structure to about 250 km depth within the subducting lithospheric slab in the northwestern Pacific region. The evidence comes from an analysis of the P- and S-wave velocity-depth functions obtained to a depth of about 640 km (Kaila et al., 1971, 1974; Krishna, 1979; Krishna and Kaila, 1984; Kaila and

229

60’

IlO”

IZOI

130’ I

USSR

140’ I / _I

160’

IS@

OKHOTSK

170” I

170’ I

100’ I BERING

SEA

SEA

\ . 50

PACIFIC

PHILIPPINE SEA

LUZON

. 9,

OCEAN

of eOrihWaheS

Epicenters

Contours of hypocenlrol deplhs (Km) ( After lsocks ond Molnar,

----

I

e

AUSTRALIA

1971)

Axis of the oceanic trench

\

IASMAN SEA \

5001 IIO’E

I

I

I

I

I

IZO-

130’

140”

l5W

160’

epicenters

of earthquakes

Fig. 1. Map of the western regions.

Pacific

showing

I 170s

I

I

180”

170”

distributed

in various

island

arc

230

Krishna, 1983) by using Kaila’s (1969) analytical method, in several units of the island arc regions in the northwestern Pacific margin: (1) Honshu-Izu BoninJapan Sea, (2) Kyushu-Shikoku, (6) Northern Figure

Kurile

(3) Hokkaido,

(4) Okhotsk

Sea, (5) Southern

Is., (7) Kamchatka,

(8) Ryukyu

Is., and (9) Taiwan-Luzon.

1 shows the regions of the present

SOURCES

Kurile

Is..

study.

OF DATA

For determining the velocities at the depths of foci in various units of the northwestern Pacific margin, 363 earthquakes with their focal depths ranging from 35 to 640 km were selected for the period from 1957 to 1975. P- and S-wave travel times (T) and epicentral International Seismological

distances Summary

(A) were taken from the Bulletins of the (ISS) from 1957 to 1963 and from the Bulle-

tins of the International Seismological Centre (ISC) from 1964 to 1975. Travel times having residuals with respect to the J-B tables (1940) up to 10 set for P and up to 20 set for S waves, were used in the analysis. This is found to be justified when the travel-time curves are fitted to the observed data. The epicenters of all the earthquakes selected for this study are shown in Fig. 1. METHOD

OF ANALYSIS

Japan is characterized by three notable arc junctions; the Hokkaido corner between the Kurile and the north Honshu arcs, the central Japan located at the junction of the north Honshu and the Izu Bonin arcs, and the Kyushu corner between the Ryukyu arc and south Honshu. The earthquakes in these regions of Japan

were

central

Japan (comprising

treated

separately

for determining

Honshu

the velocity-depth

and the region transverse

functions

in

to it which includes

Izu

Bonin and the Japan Sea), southwest Japan (Kyushu-Shikoku), and northeast Japan (Hokkaido). Similarly, the earthquakes in the Kurile-Kamchatka region were treated separately for determining the velocity-depth functions in the Okhotsk Sea, southern Kurile

Is.-more

towards

the Kurile

trench

(between

44”N,

149”E and 47”N,

154”E), northern Kurile Is. (between 48”N, 154”E and 51”N, 157”E), and the Kamchatkain the Pacific coast. According to Gorai (1968) Taiwan belongs to the Cenozoic Alpine erogenic belt and seems to constitute together with the Philippine islands and the eastern deep sea region another geotectonic unit which can be distinguished from that of the Japanese islands region. Ho (1961) described the geologic framework between Taiwan and the Philippines and concluded that the tectonic divisions of Taiwan can be projected and extended southward to Luzon. Ludwig (1970) further concluded that Taiwan and north and central Luzon are part of a primary double arc system which is convex to the west rather than convex to the east as the Ryukyu arc in the north. From these considerations, the earthquakes in the Ryukyu arc and the Taiwan-Luzon arc were also treated separately for de-

231

termining the velocity-depth were obtained at the depths analytical

method,

functions in these regions. P- and S-wave velocities of foci of all the earthquakes by using Kaila’s (1969)

and the velocity-depth

functions

were then determined

in various

regions. For a deep focus earthquake, produces

a point of inflection

the ray which leaves the focus horizontally on the travel-time

curve at the epicentral

where it emerges on the surface of the earth. At the inflection maximum. distance between

However,

in the neighbourhood

of the inflection

(i, = 90”) distance

A*

point, p( = a7’/aA)

is

point (in the epicentral

range A, to A,), p can be considered to be almost constant. Therefore, A, and A, limits, the travel-time curve in the vicinity of the inflection point

can be fitted by a straight line (T =pA + a) having slope p and intercept a. However, beyond the limits A, and A, both p and a vary considerably. Figure 2 illustrates how effectively the epicentral distance limits A, and A, can be determined from the travel-time data of deep earthquakes. This property of the travel-time curve for a deep focus earthquake was utilized in the analytical method (Kaila, 1969) for determining the velocity at the depth of focus. As the foci of the earthquakes considered here lie within the subducting lithospheric slab, the velocities determined by this method indeed represent the true velocities at the corresponding depths .014r

I

I/PA oy

;072

;074

l/PA

;OCfS

;00,6

.002

,004

-

,006

.000

- O-6 L

Fig. 2. P-wave travel-time in various

data, from earthquakes

forms for determining

A, and A, limits.

with focal depth h in the central

Japan

region,

plotted

232

within

the slab, and

various

island arc units along the northwestern

here that

the presence

determination cally

of lateral

functions

have been

Pacific margin.

inhomogeneities

affect

of lateral

only velocity

in the case of a laterally inhomogeneities,

thus determined

in

It may be mentioned

the accuracy

to some extent by Kaila’s (1969) analytical

it is applicable

presence

the velocity-depth

method

of velocity

because

homogeneous

earth.

both p and a will not truely

theoretiIn the remain

constant over the epicentral distance range A, to A* to AZ. However, the range of A, to A, is generally not very large (only 6”-8”) for any focal depth. Therefore, the travel-time data-over such a small distance range of 6” to 8” in the neighbourhood of A* - can be safely used for estimating p and a values and the velocity value thus derived from the p estimate may not be distorted significantly but corresponds to the true velocity at the depth of focus. The presence of a subducting slab, with relatively higher velocity than the surrounding normal mantle, essentially increases the epicentral distance of the inflection point A*, relative to that for a homogeneous earth. The estimate of p in the neighbourhood of such a A*, however, still depends on the true velocity at the depth of focus in the slab. Therefore. it is possible to obtain a representative

value of the true velocity at the depth of focus by this method. to a focal depth, have The epicentral distance limits A, and A,, corresponding been taken from those determined by Kaila et al. (1971, 1974) by using the Japanese earthquake data. In our opinion this is quite justified because the limits A, and A2 are essentially functions of the focal depth under consideration and any variations in these limits in various regions may not be significant. It may be further mentioned here that no attempts have been made to relocate the earthquakes used in this study. Because, in our opinion, it is quite unlikely that the epicentral locations reported by the 1% and/or ISC are so much in error that they may lie outside the individual units considered here. Similarly, the focal depths reported are also not so much erroneous that they affect significantly the velocity-depth functions determined. Especially the deep focal depths are more accurately determined from pi’ and other phases.

Further,

a large error

in the focal depth

becomes

quite

evident

from an

anomalous value of the intercept (I, obtained for the T-A plot between A, and A ?, to which the focal depth is directly related (see eqn. 14. Kaila, 1969). When a plot of true velocity versus focal depth is made, a velocity-depth focal depth, deviates

considerably

point due to an erroneous

from the rest of the population

and contributes

a

large x2 value which can then be eliminated in the process of the x2 test, for the goodness of fit, which had always been applied to determine the acceptable velocity-depth models for various units of the subducting lithospheric slab in the northwestern Pacific margin. As will be shown further, confidence bounds for the slopes of all the velocity-depth models have also been determined, which allowed evaluating significant variations of different velocity-depth models in these regions. In fact it has been found that the observed lateral variation of velocities in various regions are significantly larger than the computed confidence bounds for the slopes of various individual velocity-depth functions.

233 TRAVEL-TIME

CURVES AND THE VELOCITY-DEPTH

The travel time data between which

yielded

deviations different

the

appropriate

for each focal depth. earthquake

FUNCTIONS

A, and A, were fitted by least squares p (apparent)

and

The travel-time

foci in various

regions

a values curves

with

shown

demonstrate

straight

their

in Figs.

the stationary

line,

standard 3-6,

for

p value

between A, and A, limits. Theoretical travel-time curves, computed for a set of P velocity models including a low-velocity layer (LVL), are shown in Fig. 7, with varying parameters of the LVL. The velocity model chosen here, but for the LVL, is the same as the P velocity model determined for the central Japan region which will be discussed further. The parameters of the LVL considered in these models are: (1) thickness-80 km (80-160 km depth) and 40 km (80-120 km depth) and (2) minimum velocity within the LVL7.4 km/set (= 8% decrease), 7.6 km/set (= 5% decrease) and 7.8 km/set 7, the travel times are computed

(= 3% decrease). In all these models shown in Fig. for a focal depth of 40 km, i.e., focus lying above

the LVL. It can be seen from this figure

that the presence

of the LVL causes

a

significant shift in the prograde travel-time curves in various models, of the order of 5-10 set reduced time. It may be mentioned that such a prominent shift in the travel-time curves has not been observed in any of the regions studied here. It has been well demonstrated in Fig. 8, reproduced from Krishna and Kaila (1984) that the observed travel-time data in the Taiwan-Luzon region do not fit to any velocity model consisting of a LVL in the northwestern Pacific. This is also quite evident from the travel-time curves fitting well, without any LVL, to the observed data from shallow focal depths, as shown in Figs. 4-6. Therefore, it can be inferred that there is no significant low velocity layer in the northwestern Pacific margin. The reciprocal of p, which is equivalent to the apparent velocity I/*, was then used to obtain the true velocity V at the focal depth h making use of the well known relation V= V*(r, - h)/r,, where r, is the radius of the Earth. The true velocities at depths

thus determined

northwestern

for P and S waves in various

Pacific margin

units

considered

along

the

are shown in Figs. 9-13.

Japan region

The velocity-depth functions for P- and S-waves as determined by Kaila et al. (1971, 1974), are shown in Figs. 9 and 10 respectively, which reveal significant lateral and vertical variations of velocities in this region. Although the velocities at 40 km depth are nearly the same, the velocity gradients are quite different in the central, southwest and northeast Japan regions, thus giving rise to laterally varying velocity structures for P and S waves. P velocity in the central Japan region increases linearly from 7.89 km/set at 40 km to 8.35 km/set at 170 km depth. There is a decrease in the velocity gradient at 170 km depth, but the velocity increases to 8.41 km/set at

234

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P-wave travel-time

including a low-velocity

(I’,,,,,) in LVL = 7.6 km/set decrease). depth).

curves for a focal depth of 40 km computed

layer (LVL). a. LVL thickness(r)

from velocity

km depth). mmimum

b. I = 40 km (80-120 km depth). ( = 8%decrease). k’,,,. = 7.4 km/xc

models velocity

I’,,,,, = 7.6 km/xc ( = 5% d. I = 80 km (80-160 km

( = 5% decrease).

c. I = 80 km (80-160 km depth), V,,,, = 7.8 km/set

= 80 km (80-160

( = 3% decrease).

365 km depth. As can be seen from Fig. 9, there is a sharp first-order velocity discontinuity for P waves at 365 km depth-the velocity abruptly increasing from 8.41 to 9.13 km/set which is about 8.6% increase across this discontinuity. Again, the velocity increases linearly from 9.13 km/set at 365 km to 9.96 km/set at 605 km depth. The above model of the P velocity-depth function for the central Japan gave a x2 value of about 68 on 61 degrees of freedom (d.f.) and therefore it is an acceptable model. The S velocity function in the central Japan region also reveals similar features as the P velocity function. S velocity in this region increases linearly from 4.30 km/set at 40 km to 4.62 km/set at 150 km depth. The velocity remains almost constant at 4.62 km/set between 150 to 345 km depth. As can be seen from Fig. 10, there is a first-order -the velocity increase across km/set at 345 velocity-depth and therefore

Fig. 8. Theoretical velocity

models

velocity discontinuity

for S waves also at 345 km depth

increasing from 4.62 to 4.84 km/set this discontinuity. Again, the velocity km to 5.18 km/set at 600 km depth. function for the central Japan gave a x2

it is an acceptable

P-wave travel-time including

which is only about 4.8% increases linearly from 4.84 The above model of the S value of about 66 on 61 d.f.

model.

curves, for a focal depth of 44 km (indicated

a low-velocity

layer (LVL)

in the Taiwan-Luzon

(TL)

travel time data is also shown in each model. Depth range of the LVL, minimum velocity reduction

in percentage

any LVL is not supported

within the LVL, are also indicated

by the observed

travel-time

by *). computed region.

from

The observed

velocity and associated

in each case. Note that the presence

data (after Krishna

and Kaila, 1984).

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In the southwest km/set

Japan

region

the velocities

remain

for P from 40 to 255 km and 4.37 km/set

depth.

On the other hand,

wave velocities

VELOCITY 7.6 C

almost

constant.

at 7.88

for S waves from 40 to 170 km

increase

linearly

in the northeast

Japan

(Km/Set)

8.0

8.4

8.8

7.2

7.6

8.0

8.4

8.0

8.4

8.8

9.2

9.6

IO.0

10.4

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8.4

8.8

I I.6

12.0

8C 120 160

2oc 240 280 5

320

;

360

a ,”

400 440 480 520 560 600 640 680 72c 7.6

Fig. 9. P-wave velocity versus depth plots for the central regions.

Straight

dashed

lines are the 95% confidence

standard another

lines are the least squares

deviations

in the velocity

velocity discontinuity.

fits obtained

limits values.

IO.8

II.2

(C), southwest (SW) and northeast for the velocity-depth

of their slopes.

Truncated

A is the velocity-depth

All these symbols

indicate

point

points

horizontal

( NE) Japan

shown by 0 and bars

represent

at 689 km depth.

the same in Figs. lo-13

the

indicating

shown further.

245

region,

from 7.89 km/set

km/set

at 40 km to 8.09 km/set

at 40 km to 4.60 km/set

Kurile Islands- KamchatkaThe velocity-depth

at 175 km for P and from 4.49

at 145 km depth for S waves.

Okhotsk

functions

Sea region

for P and S waves, as determined

by Krishna

(1979)

and Kaila and Krishna (1983), are shown in Figs. 11 and 12 which reveal significant lateral and vertical variations of velocities in this region. P velocity function in the Okhotsk linear

Sea region,

increase

determined

of velocity

in the depth

from 8.56 km/set

range

from 290 to 640 km, reveals

at 290 km to 8.71 km/set

depth. As can be seen from Fig. 11, there is a sharp first-order VELOCIT 4.0

4.4

4.0

5.2

5.6

Y

6.0

O1

4.0

velocity

at 390 km discontinuity

( KmlSec) 4.4

4.8

4.0

4.4

I

I

m

4.8

5.2

I NE

-1

40

160

240

P

g

360 t

440 t 460 520 560 600 I 640L

I

I

Fig. 10. S-wave velocity

I

I

I

I

I

I

I

I

versus depth plots for the central (C), southwest

Japan regions. For legend see Fig. 9.

I

(SW)

and northeast (NE)

246

S VELOCITY 4.0

4.4

( Km /‘SK,

4.8

5.2

5.6

6.0

56Ot

600

640

!

Fig. 11. P- and S-wave velocity versus depth plots for the Okhotsk P VELOCITY 7.2 0

7.6 I

8.0 I

6.4 I

8.8 1

9.2 I

9.6 I

I

I

1 Km/Set

sea region.

For legend see Fig. 9

) 7.2

7

7.6

0.0

8.4

El.8

SK 40: 5

I*

00-

z fii l200 160200

1

1

I

/

S VELOCITY 4.0 0

4.4 I

4.8 I

5.2 I

5.6 7

(Ym/Secl 4.0 r

4.4 I

4.8 I

5.2 I

5.6 0

NK

SK

40

160

200(

-200

Fig. 12. P- and S-wave velocity (NK)

and Kamchatka

(Kam)

versus depth plots for the southern regions.

For legend see Fig. 9.

Kurile

Is.

(SK ). northern

Kurile 1s.

247

at 390 km depth-the

velocity

is about

9.2% increase

increases

linearly

across

abruptly

from 9.51 km/set

This model of the P velocity-depth value

of about

function

8 on 12 d.f. and

in the Okhotsk

increasing

this discontinuity

at 390 km to 10.10 km/set function therefore

for the Okhotsk

features

which

the velocity

at 640 km depth.

Sea region gave a x2

it is an acceptable

Sea region, also determined

640 km, again reveals similar

from 8.71 to 9.51 km/set, for P waves. Again,

model.

S velocity

in the depth range from 290 to

as the P velocity

function.

S velocity

in this

region increases linearly from a very low value of 4.50 km/set at 290 km to 4.69 km/set at 390 km depth. There is a first-order velocity discontinuity for S waves also at 390 km depth; the velocity increasing from 4.69 to 4.95 km/set which is only about 5.6% increase across this discontinuity. Again, the velocity increases linearly from 4.95 km/set velocity-depth

at 390 km to 5.42 km/set

function

for the Okhotsk

at 640 km depth.

This model of the S

Sea region gave a x2 value of about

11 on

12 d.f. and therefore it is an acceptable model. The intermediate-depth velocity functions, shown in Fig. 12, reveal significant lateral variations in the Kurile islands - Kamchatka regions. In the southern Kurile region, P velocity increases linearly from 8.19 km/set at 40 km to 8.67 km/set at 185 km depth. On the other hand, in the northern Kurile region, P velocity increases linearly from 7.91 km/set at 40 km to 8.26 km/set at 185 km depth. In the Kamchatka region, P velocity increases linearly from 7.79 km/set at 40 km to 8.35 km/set at 170 km depth. The S velocity functions in these regions also show a consistent behaviour with the corresponding P velocity functions. S velocity in the southern Kurile region increases linearly from 4.61 km/set at 40 km to 4.82 km/set at 170 km depth. On the other hand, S velocity in the northern Kurile region increases linearly from 4.49 km/set at 40 km to 4.64 km/set at 155 km depth. The S velocity-depth function in the Kamchatka region could not be determined by the analytical method, due to lack of travel time data between A, and A, limits. The large magma chambers as delineated by Fedotov (1973) may be causing anomalous attenuation Ryukyu

of shear waves in this region.

and Taiwan-Luzon

region

The velocity-depth functions for P and S waves, as determined by Krishna (1979) and Krishna and Kaila (1984), are shown in Fig. 13. In the Ryukyu arc region, P velocity increases linearly from 8.05 km/set at 40 km to 8.47 km/set at 255 km depth, and S velocity also increases linearly from 4.34 km/set at 40 km to 4.55 km/set at 255 km depth. P velocity function in the Taiwan-Luzon region is also quite similar to that in the Ryukyu arc region, and reveals a linear increase of velocity from 8.04 km/set at 40 km to 8.42 km/set at 240 km depth. However, the S velocity function in this region reveals somewhat higher velocities, again showing a linear increase of velocity from 4.43 km/set at 40 km to 4.52 km/set at 180 km depth.

248

P 7.2 01

VELOCITY

i Km

/ Set

)

7.6

8.0

0,4

8.8

9.2

7.2

7.6

8.0

0.4

I

I

I

I

1

r

I

I

I

0.8

,o

-40

40-

^

9.2

I

-

BO-

80

z -

l20-

k o

-120

I

I

: 160-

200

+ -

1

280(

S (q..

R;4

,;a

VELOCITY

I

-

‘\

---x\i I

‘\

I

240-

\

I

I

I

- 160

- 200

- 240

‘280

(Km/SeC)

5;2

i

I60

240

Fig. 13. P- and S-wave velocity regions.

versus depth

plots for the Ryukyu

arc

(RA) and Taiwan-Luzon

(TL)

For legend see Fig. 9.

DISCUSSION

The P and S wave velocity-depth functions obtained in several units, of the subducting lithospheric slab in the northwestern Pacific margin, are shown in Fig. 14. It is quite evident from this figure that, there are remarkable lateral variations of the velocity structure in various regions. Lateral velocity variations of the order of 8-10% can be inferred, both for P and S waves, down to about 250 km depth. The P and S wave velocities in the southwest Japan and the southern Kurile islands regions

249

are found margin.

to be the lowest and the highest

A very conspicuous the velocity remains 4.37 km/set volcanic

feature of the velocity structure almost constant,

in the northwestern in the southwest

at 7.88 km/set

activity

in southwest

thus keeping

Japan,

chamber

dimensions

the P- and S- wave velocities

Japan is that

This may be related

which may be volcanically

of considerable

Pacific

for P from 40 to 255 km and at

for S waves from 40 to 170 km depth.

there may be a magma depth

respectively

nearly

to the

more active, because extending

constant.

to a great

Minakami

and

Mogi (1959) and Minakami (1962) have stated that, of the numerous active volcanoes in Japan, Sakurazima in southwest Japan is the most active one in respect to its frequent and abundant outflow of lava in historical times including the 1478, 1779, 1914 and 1946 lava flows. Recently, Ono et al. (1978), from the explosion seismic studies carried out during 1972 to 1977 in south Kyushu-especially around the Sakurazima volcano, found that attenuation of amplitude of the initial portion of the record for the ray path passing across the Sakurazima volcano was most conspicuous. They found a remarkable attenuation of seismic amplitude at the sites where the ray paths from shot points run just underneath the Sakurazima volcano. These findings are also quite consistent with the velocity structure for southwest Japan

S VELOCITY 4.0 O/P]

4.4

(Km/Set) 4.8

5.2

P VELOCITY 5.6

7.4 (

’

7.8 I

a.2 I

’

I

n

8.6 I

( Km/Set) m

9.0 I

9.4 I

m

9-8 I

!

IO.2 ,o

-

80

-

I60

-

240

-

320

Y E

-

400

k w n

-

480

-

560

, \

I 12

‘8

z 5

320

-

4

I: w n

\ 400

-

480

-

560

-

,

-

Central

2 3

-__ . ..

Southwest Northeast

4

-

Okhotsk

5 6

-.--_-

Southern Northern

7 640

-

8 9

7201

Japan Japan Jopon Sea

( Krishno,l979;

Kuriles Ku&s

Kaila

8 Krishna.

Kamchatko __--

Ryukyu T&on-

I

Arc Luzon

1

1983

I

‘“,~$~~~~~i~o,,g83,

- 640

I720

Fig. 14. P and S wave velocity-depth northwestern

\

Pacific margin.

functions

obtained

in several units of the island-arc

regions

in the

250

shown in Fig. 14, which shows that the attenuation as deep as nearly Several earlier variations frequency arrivals

studies

based on earthquake

of seismic properties

and the central

Japan

between

on the other.

waves beneath (nearly

zone in this region may extend to

255 km.

southwest

5% difference

observations

southwest

also found

Japan on one hand and northeast

They are: (1) more rapid Japan

(Matuzawa,

in the total travel

remarkable

attenuation

1933). (2) relatively

time at about

of high early S

800 km) with less

attenuation at stations in northeast Japan as compared to that at stations in southwest Japan and a ratio of initial amplitudes of about 2.5 between recordings at stations in northeast and southwest Japan (Morita, 1936). and (3) velocity differences of 6-7% for S and 5-6s for P waves between the high-velocity paths to the northeast Japan and the low-velocity paths to southwest Japan (Katsumata. 1970). It can be seen from Fig. 14 that similar variations of the velocity structure are prevailing to at least 250 km depth within the subducting slab in the northwestern Pacific margin. The P velocity functions in the Ryukyu arc and the Taiwan-Luzon regions are quite similar to each other and reveal velocities which are about 5% higher, on average, than those in southwest Japan. In contrast to the southwest Japan region, the velocity structure for P and S waves in the southern southern Kurile

Kurile region,

Islands region reveal considerably high velocities. In the P velocity increases linearly from 8.19 km/set at 40 km to

8.67 km/set at 185 km depth, and S velocity also increases linearly from 4.61 km/set at 40 km to 4.82 km/set at 170 km depth. These velocities for P and S waves are substantially higher than those found at comparable depths in various other regions along the northwestern Pacific margin. Kasahara and Harvey (1977) have also presented evidence for the high-velocity zone in the Kurile trench area from Ocean Bottom Seismometer (OBS) observations. They found that a shallow earthquake in the Kurile Islands reveals a typical oceanic mantle P velocity of 8.12 km/set and they attributed this velocity to the top of the descending slab of the lithosphere in this region. They have also further found that the deeper section shows a very high P velocity,

ranging

between

8.65 and 8.97 km/set

as indicated

by

earthquakes with foci between 50 and 230 km. Assuming I$/ V, = 1.795, they estimated the corresponding shear velocities as 4.52 km/set for the upper part of the slab and 4.80 km/set averaged over the upper 230 km. It can be seen that all these estimates of P and S velocities at the top of the slab as well as within the slab agree well with those shown in Fig. 14 for the southern Kurile region. The most prominent features of the deeper velocity structure in the northwestern Pacific margin, as can be seen from Fig. 14 are: (1) a substantial reduction to almost zero of the velocity gradients at 170-365 km for P and at 150-345 km for S waves and nearly constant P and S velocities in those depth ranges in the central Japan region; (2) the presence of a sharp first-order velocity discontinuity at 365 km for P and at 345 km for S waves in central Japan and at 390 km depth for P and S waves in the Okhotsk Sea regions; and (3) relatively very low S-wave velocities below this

251

discontinuity

in Japan

across the first-order 4.8% in the Japan Okhotsk

region)

than

is relatively

et al. (1975) studied

Sea regions.

region)

The S velocity jump

lower (5.6% in the Okhotsk

the corresponding

Sea and 8.6% in the Japan

Barazangi behind

as well as in the Okhotsk discontinuity

P velocity

jump

Sea and

(9.2% in the

across it.

the attenuation

characteristics

several island arcs of the earth by using teleseismic

of the upper mantle

P and Pp waves produced

by mantle earthquakes and recorded at the World Wide Standard Seismograph Network (WWSSN). They found that major zones of high attenuation exist beneath the marginal seas of Japan and the Okhotsk where the observed heat flow is also quite high (= 2.5 HFU). These high attenuation zones, according to them, are probably limited to 250-300 km depth. Barazangi et al. (1975) considered high temperatures and/or partial melting in the upper mantle as probably the main cause for the observed seismic wave attenuation. However, as shown in Fig. 14, the P- and S-wave velocities in the central Japan region are nearly constant in the depth range 150-365 km and the S velocities in the central Japan and the Okhotsk Sea regions are relatively much lower than normal down to even 640 km depth. This may be due to the existence Therefore,

of relatively

higher

the entire region under

than

normal

the marginal

temperatures

seas of Japan

at those

depths.

and the Okhotsk

may

be a high temperature zone as is also substantiated by the high heat flow observed there. The velocities of both P and S waves are strongly affected by changes in pressure, temperature and chemical and mineralogical composition, which may possibly extend to great depths. The temperature gradient may be highly variable from place to place, whereas the pressure gradient can be considered to be almost constant over various regions. The degree of compositional variation and its likely effect on the velocity structure in various island-arc regions in the northwestern Pacific may not be very significant. Therefore, the only possible explanation that can satisfactorily account for the observed lateral velocity variations, is the existence of lateral temperature

variations

northwestern

Pacific

convergence

velocities

beyond

down margin. might

the scope of the present

those lateral

temperature

to at least Probably cause

250 km depth differences

lateral

temperature

study to investigate

in various

units

of the

in the age of the plates variations.

However,

in detail the possible

and it is

causes for

variations.

CONCLUSIONS

(1) There are large lateral variations of both P- and S-wave velocity structures of the order of 8-lo%;, down to at least 250 km depth in several units of the subducting lithospheric slab in the northwestern Pacific margin. The P- and S-wave velocities are found to be the lowest in southwest Japan, whereas in the southern Kurile Islands region they are the highest. These observed velocity anomalies may reflect primarily the existence of large lateral temperature variations extending to those depths.

252

(2) There is no significant However, depth

for S waves,

(150-365

low-velocity

there is a decrease but

km depth)

the P and

in the central

S velocities

Japan

(3) The entire region comprising be a high temperature

layer in the northwestern

in the velocity gradient

remaining

nearly

constant

below

region.

the marginal

zone giving

Pacific margin.

at 170 km for P and at 150 km

seas of Japan and the Okhotsk

rise to relatively

low S-wave velocities

may

down to

about 640 km depth. (4) There is a sharp first-order velocity discontinuity at depths of 365 km for P and 345 km for S waves in Japan and at 390 km for both P and S waves in the Okhotsk sea regions. The S velocity jump across this discontinuity is relatively lower (5.6% in the Okhotsk Sea and 4.8% in the Japan region) than the corresponding P velocity jump

(9.2% in the Okhotsk

Sea and 8.6% in the Japan

region)

across it.

ACKNOWLEDGEMENTS

Our thanks are due to the Director, National his kind permission to publish this paper.

Geophysical

Research

institute,

for

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