Three-dimensional seismic attenuation structure beneath the Japanese islands and its tectonic and thermal implications

Tecfonophysics, Elsevier

163

159 (1989) 163-180

Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Three-dimensional seismic attenuation structure beneath the Japanese Islands and its tectonic and thermal implications TOSHIHIKO

HASHIDA

**

Earthquake Research Institute, University of Tokyo, Bunkyo-ku, (Revised version accepted

Tokyo I13 (Japan)

March 15, 1987)

Abstract Hashida,

T., 1989. Three-dimensional

thermal

implications.

Magmatism

intensity

lateral

variation

structure

reported

dominant

district,

features

generally

attenuation

continue

down

on the crustal

Third, in the upper mantle, forming

low-Q

these loci do not exist beneath controlled

by the existence high-Q

Honshu

arc and along Ryukyu

the subducting although

zones in the upper cold oceanic

the thickness

structure

thermal

subducting

The thermal

plate, overriding

** Present address:

diapirs,

Second,

which is assumed

to estimate

exists

the obtained

for about

to the distribution This suggests

that

to lie beneath

of the lithosphere

structure

side of the arc are except for the

coast.

These crustal shows

volcanic

materials,

and high-Q

Since the thermal reflects

to be about 30 km or so. In SW Japan,

that

structure

the thermal

structure.

of active and other Quatemary side of the volcanic

the upper front

mantle

front

attenuation

is

or a group of volcanoes.

along

the Kurile-northeast

slab seems to reflect a cooling

may reach

of

structure

a volcano

side of the volcanic

lying above the sinking

800

crustal

are distributed. structure

to be

3-D structures

in the crustal

on the Pacific

of Quatemary

rocks

which are considered mantle

of the

the attenuation

readings

on the continental

that even on the continental

exists.

effect of

60 km or more in such outer where the high-Q

uppermost

arcs,

mantle

60 km may exist. are proposed

structures

lithosphere

for NE and

In the uppermost reflect

the difference

and volcanic

and Volcanological

0 1989 Elsevier Science Publishers

SW Japan,

based

mantle of NE Japan of tectonic

settings

on the obtained

3-D attenuation

a higher temperature

is expected

in the two regions,

such as the

activity.

Department,

(Japan). 0040-1951/89/$03.50

and reflects

to a depth of 90 km.

difference

areas

areas exist mainly

exist on the Pacific

considerations.

Seismological

be noted

The thickness

of about sections

and other geophysical

than in SW Japan.

* (1984);

cross

by inversion

structure

15,000 intensity

that the attenuation

no volcano

arc. The high-Q plates.

of about

to the distribution

suggests

It should

mantle

point is inverted

a distinct

where pre-Cenozoic

where

has been considered

also ties, a thick lithosphere Possible

areas

is determined

1951 and 1983. By combining

loci are found, which correspond

of upwelling

Fourth,

first,

mantle.

areas correspond

chain.

and

of Tectonics,

areas exist on the Pacific side of the arc. In SW Japan,

age, this feature

the arc-volcanic

and its tectonic

Aspects

* is used: the intensity

(high-attenuation)

and small low-Q

provinces

Islands

in the attenuation

of three layers are constructed

to the uppermost

Low-Q

between

features:

(low-attenuation)

Islands

Thermal

Islands.

Agency

low-Q

areas are dominant

to the geological

depends

volcanoes

the following

the Japanese (Editors),

the Japanese

contrast

and Shimazaki

maps consisting

In NE Japan,

is age dependent.

correspond

strongly

by Hashida

Meteorological

reveals

some high-Q high-Q

beneath

a remarkable

of the S-wave at the observation

3-D attenuation

SW Japan.

whereas

Kyushu

&art

at the seismic source. The data consist

structure

NE and

beneath

H.J.

159: 163-180.

the Japanese

developed

by the Japan

in Japan,

The obtained between

beneath

and

structure

reveals

acceleration

and the acceleration

eight districts

areas

structure

the method

of the maximum

earthquakes

Tectonophysics,

The result clearly

of thermal

structure

D. Chapman

(3-D) seismic attenuation

data.

For the inversion, a measure

seismic attenuation

S. Uyeda,

and Metamorphism.

A three-dimensional seismic

In:

B.V.

Japan

Meteorological

Agency,

Chiyoda-ku,

Tokyo

100

164

Introduction Three-dimensional neath

the Japanese

investigated veloped Lee

by applying

(1976)

(e.g.,

the inversion

velocity

in the wedge structures

analyses

descending

1977,

1980,

revealed

above

the sinking which

often

et al.,

the

plate and low-velocity

part

high-

I

SEA

OF

JAPAN

I

regions

plate.

3-D

provide

information

on thermal

however,

have not been

than velocity,

de-

1981;

1984; Hirahara

have

of attenuation

with more critical

method

1980; Horie and Aki, 1982;

et al., 1984; Ishida, These

be-

recently

1977) and Aki and

Hirahara,

and Mikumo,

Hasemi

tions

structures been

have

by Aki et al. (1976)

Hirahara 1989).

(3-D) velocity Islands

us

condifully

investigated. In this paper, a 3-D seismic attenuation structure beneath the Japanese Islands is investigated by inversion of seismic intensity data. The Japanese Islands belong to two arc-trench systems (Fig. l), NE Japan island arc and SW ___I_ __ __---I-.

Japan island arc. The region of the Japanese Islands is one of the most active areas on the earth;

13UE

!

--

/

\

140’E

the shallow great earthquakes occur along the trenches, deep seismic activities are observed, and many Quaternary and active volcanoes are distrib-

Fig. 1. Index map showing the Japanese Islands and island arc systems consisting of trenches and volcanoes. Dots and circles

uted in belts parallel

(after Aramaki

Sugimura

to the oceanic

and Uyeda,

1973; Yoshii,

trenches

(e.g.,

1979a). These

features are summarized in Fig. 1. The NE Japan island arc consisting of the Kurile, the northeast Honshu

and

the

Izu-Mariana

acterized

by subduction

SW Japan island arc Honshu arc and the by subduction of the Inverse estimates structure have been

arcs,

of the Pacific

is plate.

charThe

consisting of the southwest Ryukyu arc, is characterized Philippine Sea plate. of the depth-dependent Q usually made by a spectral

technique (e.g., Anderson and Archambeau, 1964; Anderson et al., 1965; Teng, 1968). Recently Ward and Young (1980) and Young and Ward (1980) attempted to estimate 3-D Q structures of geothermal areas by inversion of the differential attenuation data derived from a reduced spectral ratio. A 3-D Q structure beneath the Tohoku district, northeast Honshu, was estimated by the inversion technique using spectral data (Umino and Hasegawa, 1984). Here, the seismic intensity data are used to estimate the 3-D attenuation structure beneath the Japanese Islands.

denote

active

and

other

Quatemary

volcanoes,

respectively

and Ui, 1979). VF and AF show the volcanic

front and the aseismic

front, respectively.

There have been several papers in which seismic intensity data were used to investigate the attenuation structure (e.g., Utsu, 1966; Ikami, 1975; Nakanishi and Horie, 1980). These studies indicate that the seismic intensity data contain important information on the attenuation structure. As a natural extension of these studies, a method to estimate the 3-D attenuation structure by inversion of seismic intensity data was developed by Hashida and Shimazaki (1984). An advantage of using seismic intensity data is that there is a large number of observational data for a single earthquake because the seismic intensity is measured without the use of instruments. The method has been already applied to several regions and the validity and usefulness of it has been demonstrated (Hashida and Shimazaki, 1985, 1987; Hashida et al., 1988; Satake and Hashida, this issue). In this study, the method is applied to the Japanese Islands.

165

First,

the method

described.

of analysis

After construction

tion map, the obtained

will be briefly

of the 3-D attenua-

structure

will be discussed

TABLE

1

Block configuration District

in comparison with other geophysical and geological data, particularly from the thermal point of

Hokkaido

view.

South Hokkaido-

Method of analysis

Tohoku

north Tohoku

used in this study Block size (km3)

Number

XYZ

blocks

80x90~30

8X6X8

65x60~30

8x8x9

60~70x30

8x8x9

of

South Tohoku-

In this Japanese

study Islands

3-D structures islands.

the

3-D

structure

is constructed

of eight districts

The method

beneath

the

by combining

the

which cover all the

of estimating

the 3-D struc-

ture of each district is the same as that used by Hashida and Shimazaki (1984) who invert the seismic

intensity

coefficients

data

to estimate

in a number

of blocks,

50x60~30

9x6x8

Kanto

north Kanto

50x60~30

9x6x6

Chubu-Kinki

50X50X30

8x9x3

Chugoku-Shikoku

60~55x30

9X9X4

Kyushu

60~70x30

7x7x6

Z = depth.

the attenuation and the accel-

intensity data quantitatively, the intensity is assumed to be a measure of the maximum ground

eration

at the seismic source, simultaneously.

The

reason

why the Japanese

into

acceleration

eight districts is that the number of unknown parameters, i.e., the sum of the number of blocks

The JMA maximum

penetrated by rays and the number of earthquakes, has its limit because of the limited memory of the available computer. As the resolution of peripheral blocks in each district is usually poor,

(1943) formula:

Islands

are divided

of S-waves

at the observation

point.

intensity I is then converted to the acceleration a (gal) by Kawasumi’s

a = 101/2-0.35 The observed maximum formulated as follows;

acceleration

a can

where S is the acceleration

which is assumed

be

districts are chosen so that they mutually overlap. Eight districts are shown together with the earthquakes used in this study in Fig. 2. These districts, from the northeast to the southwest are as follows: Hokkaido, south Hokkaido-north Tohoku, Tohoku, south Tohoku-north Kanto, Kanto,

Chubu-Kinki,

Kyushu.

The block

Chugoku-Shikoku configurations

tricts are shown in Table

and

for these dis-

1. The horizontal

block

size varies from 50 to 90 km, but the vertical size is 30 km for all districts. We construct the 3-D structure beneath the Japanese Islands by combining the 3-D structures of the eight districts for each layer.

isotropically

radiated

from the point

source,

to be G is

the geometrical spreading factor, g is the amplifying effect at the earth’s surface. D, and Tk are the attenuation

coefficient

(s-i)

and

the travel

time

(s) in the k-th block, respectively. Based on the assumptions

which will be men-

tioned below, this formulation termine D, for each block and

is used to deS for each event

Method

by a damped least-squares inversion of intensity data. The damping factor is given from the ratio of the variance in the data to that in the model. We use the variance of 0.34*, 0.172 and 0.012 s-*

Here, we briefly review the method developed by Hashida and Shimazaki (1984). The seismic intensity data, which measure the degree of ground shaking, contain information on the “size” of the earthquake source and attenuation along the path from the event to the stations. In order to treat the

for the natural logarithm of observed acceleration data, In a, for the natural logarithm of source acceleration, In S, and for attenuation coefficients of blocks, D, respectively. The surface amplifying effect g is assumed to be fixed at 2, by only taking into account a free surface effect. The seismic intensity differs from

166

500 km 0

30 60 90 UIoA.o+X+X

I20

150 160 210

DEPTH Fig. 2. Map showing the Japanese Islands divided into eight districts for inversion, and the distribution of epicenters (depth in kilometers) used in this study.

point

to point

even

in a small

area

where

the

ground condition changes locally. As a result, attenuation structure, particularly that of the first (i.e., the uppermost) layer, might be influenced by local site conditions. The attenuation structure of interest here, however, is not a local but a regional one. Thus, we take the block configuration so that each block includes several stations and try to estimate the regional structure free from the local site conditions. The variation

in S-wave velocity

V, is assumed

to be much smaller than that of S-wave attenuation D (6Vs/ V, -CCSD/D). Then we use a Iayered velocity structure to calculate the ray path, geometrical spreading factor G, and travel time T in each block. Focussing and defoeussing of seismic energy in a laterally heterogeneous medium produce a large-amplitude variation. Thus, this assumption causes underestimation of a spatial variation of attenuation and makes areas of high attenuation smaller, whenever high attenuation corresponds to low velocity.

167

It is also important

for the inverse

structure

attenuating

regions do not exist. We cannot

ve any

reliable

contains

to assume

problem

attenuation

information

r,)

along

a very highly attenuating

words, the attenuation

expressed

is not appropriate

of

that very highly a path region.

retrie-

Japan

which

Usami,

In other

by exp( - c D, .

in such a case of itrong

spatial

variation

in any case underestimate

of attenuation.

ary note, we should results

only

tenuation

show

a

Thus as a caution-

emphasize the minimum

that the obtained estimate

Sea coast, and at intermediate 1975;

earthquakes

of at-

variation.

Utsu,

1982).

depths

The

of seismicity

almost

in Japan.

all events

larger

(e.g.,

distribution

in the Fig. 2 well represents

eral features includes

non-linearity. These assumptions

tive earthquakes, have often occurred along the Pacific coast and sometimes inland, and along the of

the gen-

The data than

set

5.9 from

1951 to 1983. The data

set also includes

number

than 6.0. The total num-

of events smaller

ber of earthquakes

is about

In the following, the earthquakes cause

800.

attention

is paid

which occurred

the location

to selecting

in the ocean,

of earthquakes

larly the focal depth,

a large

here,

be-

particu-

is not well constrained.

The

focal depth of shallow earthquakes which occurred in the Japan Sea is changed to 20 km if the

Data The data consist

of seismic intensities

reported

by the Japan Meteorological Agency (JMA) (1951-1983) in the JMA eight-degree scale (O-7). The seismic intensity has been measured basis of human perception and movement jects observed might appear

on the of ob-

by experts without instruments. It to be highly subjective, but inde-

pendent estimates point never differ

by two experts at the same by as much as 1 on the JMA

intensity scale. Furthermore, tually the same hypocenters

earthquakes with virlead to nearly identi-

cal intensity patterns. These facts indicate that the intensity data are useful in retrieving information on the earth’s structure. The intensity 0, which has been listed in the seismological bulletins of the JMA since 1961,

reported depth is deeper than 30 km, because precise determination of hypocenters based on the microearthquake network (e.g., Sato, 1984) suggests that shallow occur

at depths

earthquakes shallower

than

in the Japan 30 km,

Sea

namely

within the uppermost layer in this study. Additionally, we do not use the earthquakes which occurred depths

near

the Japan

are deeper

ficult to judge correct or not.

than

trench

whose

reported

30 km, because

whether

the reported

it is difdepths

are

The distribution of the stations used in this study is shown in Fig. 3. In the pre-1961 seismological bulletins of the JMA the intensities at weather stations and climate stations are listed (“ kunai-kansokusho” in Japanese), while in the post-1961

bulletins,

only

the

intensities

at

the

means that the shaking is not felt by people but recorded by seismometer. There is no lower limit

weather stations have been listed. We used the intensities not only at weather stations but also at

of shaking

climate

for intensity

records the shaking. necessarily consistent lationship mentioned

0 as far as the seismometer The intensity 0 then, is not with Kawasumi’s (1943) reabove. In order to overcome

this problem we only use intensity 0 data at stations located at an epicentral distance shorter than the maximum felt distance of the earthquake. This procedure in selecting intensity 0 is the same as that of Utsu (1984). The distribution of earthquakes used in this study is shown in Fig. 2. We employ the hypocenters determined by the JMA. In the Japanese Islands, felt earthquakes, including large destruc-

stations,

in order

to increase

of intensity data for each earthquake. racy of our solutions and the number

the quantity The accuof reason-

able solutions increase as the number of data increases (Hashida and Shimazaki, 1984). Inversion and results The initial attenuation coefficients are 7.85 x lop3 s-l for the uppermost layer and 3.14 x lo-’ s-i for the deeper layers, which correspond to the crust and the upper mantle, respectively. This initial attenuation model will be used as the stan-

168

500

km

. Fig. 3. The distribution

TABLE Numbers variance

of stations

used in this study.

2 of earthquakes, improvements

stations

District

Tohoku

Tohoku South Tohoku-north Kanto

Kanto

data

used in this study.

The number

of unknown

parameters

Stations

Intensity

data

Unknown

parameters

Variance

114

67

1726

268

37.9

122

65

1994

355

46.6

101

66

1630

317

33.5

98

66

1943

273

38.2

171

74

4358

346

23.8

Chubu-Kinki

69

74

1336

155

22.9

Chugoku-Shikoku

68

77

1352

202

25.7

Kyushu

65

60

831

185

37.9

808

549

15170

2101

Total

of inversion

and

are also listed

Earthquakes

Hokkaido South Hokkaido-north

and intensity

after inversion

improvement

(W)

169

dard model. In the next section we will discuss the deviations estimated cients

from

these

by inversion.

are converted

attenuation These

mantle,

that a representative

to the intensity spond

attenuation

to those

who investigated

is 1 Hz. These of Umino

and

Qs values

based

frequency

Hart,

Hasegawa

beneath

by longer

period

waves (Anderson

1978). The S-wave velocities

be 3.8 km/s

on an

in the crust

and

and

are assumed 4.3 km/s

to

in the

upper mantle.

related

Qs values

The assumed Qs structure is also similar of the standard earth model SL8 de-

termined

coeffi-

to the Q, values, 400 for the

crust and 100 for the upper assumption

Honshu. to that

coefficients

Initial

corre(1977)

northeastern

values

for

calculated

from

the initial

attenuation

source

the observed model,

accelerations intensity

data

are and

and then the inten-

140IE

135’E

(a) LAYER

1 (O-30km)

Fig. 4. The obtained attenuation structure of the Japanese Islands for (a) layer 1 (depth = O-30 km), (b) layer 2 (depth = 30-60 km), and (c) layer 3 (depth = 60-90

km). In areas where the broken lines (contour interval = 1.5 X 10d2 s-‘)

coefficients are larger than the standard value of 0.785 and 3.14 x 10m2 s-’ where the thick contour lines (contour interval = 1.0 X lo-*

s-t)

are drawn, attenuation

for layer 1, and layers 2 and 3, respectively. In areas

are drawn, attenuation coefficients are smaller than the standard

value. H and L indicate high Q and low Q, respectively.

170

sity data for each district the

3-D

attenuation

The number tions

from

are inverted

structure

of unknown the standard

the events) and intensity

attenuation

coefficients

source accelerations is listed

are the number data,

after inversion. by half a block and obtain

another

2.

(e.g. Hashida,

districts

the 3-D

for each layer

tenuation

of data

which the corresponding

diagonal

structure

matrix

is higher

and Shimazaki,

only

solution.

45-N-f

135’E 40*N -t

130’~

&J

t

(b1 LAYER

2 (30~60km)

Fig. 4 (continued).

‘0

n

Islands.

In

the solution

of

element

of the

than 0.35 as the relia-

ble one (Hashida the reliable

the 3-D at-

the Japanese we regard

resolution

of the eight

to construct

images beneath

140’E

30’N

1986). structures

the case of combination

in the southwest

attenuation

other papers

stations

We shift the block configlength

to resolve lateral variation on a finer scale. Detailed structures for the eight districts are given in We combine

of

in Table

of events,

and the improvement

variance

district.

(i.e., devia-

for each district

uration direction

for each

parameters

and those from the initial Also tabulated

to estimate

We

1984)

and

sometimes

use have

171

inconsistent because gions

structures

in the overlapping

these areas are situated where

solutions

are sometimes

poorly

solved. This shows that the solutions

the solution struct not cause

of

laterally

for the uppermost

combine

the results

the reliable

re-

regions

are

In those cases we choose

with higher resolution.

a map

structure

correct.

re-

with resolu-

tion higher than 0.35 in the peripheral not necessarily

areas

in peripheral

Thus, we con-

varying

attenuation

three layers. We could of the lower

solutions

layers

be-

with high resolution

are not mutually overlapped and almost ble solution exists in a few districts. The 3-D attenuation attenuation

areas

areas are shown tour

interval

tenuation interval

correspond

= 1.5 X 10e2 high

40’N

t

3 (60-90km)

s-l). Q and

of 90

to high-Q

by the thick contour

by the broken

indicate

135’E

Fig. 4 (continued).

to a depth

In these figures lower

s-l),

areas which correspond

are shown “L”

which

= 1.0 X lo-*

140’E

(c) LAYER

images

km are shown in Figs. 4a-c.

no relia-

contour The low

and

lines (conhigher

to low-Q

atareas

lines (contour letters

“H”

and

Q, respectively.

172

Prominent features for each layer are described in the following by using the Q value based on the assumption that a representative frequency is 1

in the Kanto district, where two sinking slabs are inferred (e.g., Shimazaki et al., 1982; Nakamura et al., 1984). The attenuation structure of each region

Hz.

can often

In the top crustal remarkable between

layer

difference

(Fig.

in

4a), we note

attenuation

NE and SW Japan.

(inner

arc)

are dominant

structures

In NE Japan

(Q = 45) areas on the continental

a

some

be correlated

low-Q high-Q

These

with the seismic

or the distribution

parameters

side of the arc

whereas

structure

such as the heat

correlations

structures observed

of other

of

those

may

flow of the region.

be used

parameters

data are sparsely

velocity

geophysical

to predict

the

for

the

which

distributed.

(Q > 400) areas exist on the Pacific side of the arc (outer district,

arc). In SW Japan, high-Q

except

for the Kyushu

(Q > 400) areas

are dominant and several exist on the Pacific coast. In the uppermost

in the inner

low-Q

mantle,

Comparison

(Q = 60) areas

at depths

of 30-60

km (Fig. 4b), regions of small attenuation coefficient (high-Q) appear more distinct than those in the crustal layer, as the standard attenuation coefficient of this layer is larger than that of the top crustal layer. However, the general characteristic of the attenuation structure is similar to that of the crustal actually

layer.

The contrast

of attenuation

is

larger in this layer than that in the crustal

layer. The highest Q is more than 1000 and the lowest Q is 30, and the ratio reaches almost two orders of magnitude,

although

estimate the highest Q because of Q-~’ may be larger than

we cannot

precisely

the standard 1.0

X

with the previous velocity structures

arc

error

10. ‘. Some

remarkable features are the low-Q (Q = 30-50) areas lying in the inner arc of NE Japan and of Kyushu, and the high-Q (Q > 300) areas lying in the outer arc of NE Japan. In SW Japan, except for Kyushu, high-Q (Q > 300) regions are dominant, although several low-Q (Q = 70) zones are found. Layer 3, in a depth range of 60-90 km (Fig. 4c), shows a similar attenuation structure pattern to that of layer 2, but the area with good estimates becomes smaller because of poor resolution or the lack of observations. Discussion We will discuss the obtained attenuation structure of the Japanese Islands in comparison with other geophysical and geological data. We do not discuss the structure in detail here, as Hashida and Shimazaki (1985) discussed the detailed structure

Three-dimensional the Japanese Islands

velocity structures beneath have been investigated by

many authors, who are referred to in the first section of this paper. The 3-D attenuation structure estimated here by inversion of seismic intensity data should be compared with these previous velocity studies to check its reliability and to clarify its characteristics. The comparisons have been already made for the Kanto and Tohoku regions (Hashida and Shimazaki, 1985, 1987) and consistent attenuation

structural patterns and velocity in

are found between the upper mantle.

Namely, high- and low-Q regions correspond well to high- and low-velocity regions, respectively. Similarly,

correlations

between

these characteris-

tics are found for Central Japan (velocity structure obtained by Hirahara et al., 1989) and Kyushu (velocity structure obtained by Hirahara, 1981) but are not found for SW Japan (except for Kyushu). The attenuation structure does not show the subducting Philippine Sea plate, with high Q. along the Nankai trough, although Hirahara (1981) clearly showed the subducting plate as having high velocity. This difference might be caused by the fact that the subducting Philippine Sea plate is a relatively “hot” plate as suggested by Yamano et al. (1984), because attenuation structure strongly depends on the thermal condition. The young “hot” plate in the Nankai trough might show relatively high velocity, and yet standard Q. Crustal attenuation structures do not always correlate with velocity structures. Seismic intensities, which are the data source of this study, generally reflect local site conditions. Therefore, we took the block configuration in the inversion process so that each block includes several sta-

173

lions,

expecting

relatively other

hand,

velocity

that solutions

crustal

inversion

structures are sensitive

ness and the thickness (e.g., Ashiya

same method

et al., 1963) and the crustal

to the crustal

structure

thick-

ively. The dist~bution

cover

material

of North

with the attenuation

by Satake

attenuation

crustal

by 3-D

of the sedimentary

the 3-D velocity

New Zealand

ure obtained The

Figures 5a and b show the distribution of Quatemary volcanic material in Japan (Sugimura

et al., 1986). Kuge and Satake (1986)

have compared Island,

determined

are

On the

of those blocks

free from local site conditions.

and Hashida

structure

struct-

(this issue).

was estimated

by the

as is used here. They show that the

attenuation

with geophysical

structure data (Stern,

is more

correlative

1985) than the velo-

corresponds that

respect-

of the Quatemary

volcanic

to the low-Q

for in the Chugoku suggests

attenuation,

district.

areas,

except

This correspondence

the temperature

of the crust

in the

low-Q areas is higher than that in the surrounding areas.

Another

interpretation

is that

shows low-Q because

it consists

als cont~ning

The attenuation

fluid.

waves can be expected

to be large

with

and/or

high

temperature

the

of porous

crust

materi-

of seismic in materials

containing

fluid

city structure. Thus, in this study, the obtained attenuation structure is considered as yielding reli-

(e.g., Jackson and Anderson, 1970; Winkler and Nur, 1979). The low Q related to Quatemary

able information,

volcanic

even if the attenuation

is not fully correlated

with velocity

Crustal age dependence It is interesting crust correspond tive or other

of attenttation structure

to note that low-Q areas in the to the volcanic

Quatemary

(a) Quaternary

structure

structure.

areas, where ac-

volcanoes

Volcanic

exist {Fig. 1).

material

in the Chugoku

masked by the surrounding nant in the region. The crustal outer

high Q is mainly

arc of northeast

zone of SW Japan. to the geological

Honshu,

These high-Q

provinces

district

might be

high Q which is domidistributed

in the

and in the inner areas correspond

where rocks older than

Materials

(b) LAYER 1 (O-30km) Fig. 5. a. Distribution of Quatemary volcanic

material (after Sugimura et af., 1963). The contours show the thickness of the material

(contour interval = 20 m (solid line) and 10 m (broken line)). b. The attenuation structure for layer 1. The contours and symbols are the same as those in Fig. 4.

(b)

Mesozoic-Cenozoic

Fig. 6. Summarized Distribution

geological

and JC denote

respectively.

b. Distribution

and M denote respectively.

maps (after Kanmera

of Paleozoic-Mesozoic

ages. P-M

igneous

sedimentary

late Paleozoic

rocks

(c)

et al., 1980) and the estimated and metamorphic

- early Mesozoic

of Mesozoic-Cenozoic

igneous

attenuation

which are efassified

rocks corresponding

to M are mainly and symbols

exposed

neous rocks are shown in Figs. 6a and b, respectively (Kanmera et al., 1980). The uppermost mantthe areas

also shows

granitic

of the Japanese

into two groups -’ Cretaceous

into three groups according -- early Cenozoic

rocks. c. The attenuation

according

Islands.

a.

to their

time (60-200

Ma),

to their ages. Q. N

time (older structure

than 50 Ma).

for layer 1. The

are the same as those in Fig. 4.

Cenozoic are distributed (Fig. 6a). The distributions of Paleozoic-Mesozoic sedimentary and metamorphic rocks, and Mesozoic-Cenozoic ig-

below

structure

time (older than 200 Ma) and Jurassic rocks, which are classified

Quaternary (younger than 2 Ma), Neogene (2-24 Ma) and late Mesozoic

The igneous

contours

le just

rocks,

LAYER 1 (O-30km)

high Q (Fig.

4b). Judging from the fact that the crustal age where these older rocks are exposed is more than 60 Ma, the areas may have “cool” and “hard” roots down to a depth of about 60 km. These correspondences show a correlation in the seismic attenuation becoming weaker with crustal age. The correlation is clearly seen by comparison of the crustal attenuation (Fig. 66) with the distribution of igneous rocks which are classified into three groups according to their ages (Fig. 6b). There have been several papers which have shown the correlation of seismic Q with

crustal age (e.g. Solomon and Jordan, 1980). These the correlation of mantle with the crustal age. This

and Toksiiz, 1970; Sipkin latter two studies showed (2 for long-period waves study shows the correla-

tion of crustal Q as well as the uppermost mantle Q for short-period waves with the crustal age (Figs. 4b and 6). This is supported by the studies of the coda Q of local earthquakes which indicate lower Q (both scattering and intrinsic) in regions having recent tectonic activity (e.g., Aki, 1980a, b; Singh and Herrmann, 1983). The correlation may indicate that the obtained attenuation structure reflects the thermal condition of the crust and the upper mantle, because there is evidence that the thermal structure is generally controlled by the crustal age (e.g., Polyak and Smirnov, 1968; Sclater and Francheteau, 1970; Chapman and Pollack, 1975; Water et al., 1980).

175

the outer arcs of the Kurile, the northeast Honshu and the Kyushu regions. The crustal layer just

Upwelling mantle diapirs One of the remarkable

features found in the

above

the high-Q

mantle

also shows high Q.

upper mantle (Figs. 4b and c) is low Q along the

Therefore,

volcanic chain. The low Q is considered from its

the thickness of the lithosphere in those outer arc

distribution

regions is not 30 km but about 60 km or so. These

to be related to the arc volcanism.

The location

of volcanoes

is shown in Fig. 1.

it is more reasonable

thicker lithospheres

to consider that

in the outer arcs appear to

it is worth noting that the low-Q

show a cooling effect of the subducting cold oce-

areas lying on the Japan Sea side of the volcanic

anic plates. In SW Japan where high-Q mantle is

front

the

dominant, the lithosphere thicker than 30 km also

volcanic arcs. The gaps in the low-Q areas in the

appears. On the other hand, the region with low-Q

Hokkaido

Furthermore, do

not

necessarily

continue

along

to

upper mantle such as the volcanic area where the

areas where the volcanoes are sparsely distributed or are not found (Fig. 1). Similar observations showing correlative locations with volcanoes are

crustal layer also shows low Q, may have litho-

reported for 3-D velocity structures (e.g., Hasemi et al., 1984; Hirahara et al., 1989) and for aniso-

renewed igneous activity. Further studies, such as

and Tohoku

tropic body-splitting

districts

correspond

S-waves (Ando et al., 1983).

sphere thinner than 30 km. The lithosphere tends to thicken with crustal age except for areas with on the lateral variation of P, velocity, are needed to clarify the problem of the lithospheric thick-

Therefore, the low Q corresponding to active and other Quatemary volcanoes is interpreted as the

ness.

result of upwelling diapirs which generate the arc

Possible thermal structure

volcanism (e.g., Tatsumi et al., 1983). This may also suggest that the uppermost mantle under the volcanic arc is not everywhere in a state of partial melting, but that only the upwelling hot diapirs corresponding to a volcano or a group of volcanoes contain partially molten bodies. Thickness

of the lithosphere

in the Japanese

Islam&

It is well known that the attenuation structure beneath the Japanese Islands is characterized by the high-Q (and high-velocity and -density) slabs

There have been many studies on the thermal structures beneath island arcs (e.g., Hasebe et al., 1970;

Minear and Toksiiz,

1970;

Sugimura and

Uyeda, 1973; Andrews and Sleep, 1974; Bodri and Bodri, 1978; Tatsumi et al., 1983). More recently, Honda (1985) proposed a thermal model of the Tohoku (NE Japan) subduction zone by combining geophysical observations with petrological considerations. In the following, we will show the cross sections of the Q structure in the Tohoku and Chugoku-Shikoku districts which are consid-

subducting beneath the low-Q (and low-velocity

ered to reflect the possible thermal structures in

and -density) mantle wedge (e.g., Utsu, 1971; Yoshii, 1972). The low-Q mantle wedge is believed to be in direct contact with the overlying high-Q

NE and SW Japan, respectively.

crust, typically in NE Japan (e.g., Yoshii, 1972, 1979b). Thus, the high-Q crust as the thin, = 30km thick lithosphere, and the low-Q mantle wedge are considered as important features of the Japanese Islands. The low P, velocity of about 7.5 km/s (Yoshii and Asano, 1972) supports this idea. The 3-D structure obtained in this study, however, reveals lateral variation of attenuation in the uppermost mantle (Figs. 4b and c), and does not show low Q everywhere. In particular, it should be noted in Fig. 4b that high-Q mantle lies beneath

First, we take the averages of the attenuation coefficients along the arcs of the Tohoku and Chugoku-Shikoku districts to make the cross sections across the arcs. The averaged attenuation coefficients are converted to Q values by assuming that the representative frequency related to the intensity data is 1 Hz. The estimated cross sections of the Q str&ture are shown in Fig. 7c. Second, we convert the Q values to temperatures. To do so, the attenuation mechanism should be known. Several possible mechanisms have been proposed (e.g., Jackson and Anderson, 1970; Solomon, 1972; Anderson and Minster, 1981). The

(a)

mW/m*

Northeast

f-

NSO’W

0 Structure ____-.

Japan

(Tohoku) ____~

Southwest

-

c

N20’W

0

Japan

Structure(Chuaoku-Shikoku)

L

100

km

Fig. 7. a. Gross patterns of heat flow data compiled by Yamano and Uyeda (1988) for NE and SW Japan. b. Two schematic cross sections of thermal structure for NE and SW Japan inferred from the obtained Q structure shown in (c). Inferred isothermal lines in Fig. 7b are contoured, and the bottom of the lithosphere is assumed to represent an isotherm. Two cross sections in the Tohoku and Chugoku-Shikoku districts show typical features for NE and SW Japan, respectively. The ticked lines correspond to the upper planes of the Pacific slab (NE Japan) and the Philippine Sea slab (SW Japan). The solid triangles indicate the location of the volcanic front VF. AF and Tr show the aseismic front and the Japan trench or the Nankai trough, respectively. Upwelling diapirs causing the arc volcanism and seismic coupling zones between the subducting and the overriding plates are also shown. The aseismic belt in NE Japan (Yamashina et al., 1978) is also shown. c. Two cross sections of the Q structure for the Tohoku and Chugoku-Shikoku districts. These are estimated from the averaged attenuation coefficients along the arcs. Details are explained in the text.

relationship between attenuation, Q-i, and temperature, T (OK), is usually given as follows: Q’

a exp( -c/T)

where c is a material constant depending on the pressure. The actual mechanism in the real earth

In Fig. 7b, we refer to the hypocentral distributions (Hasegawa et al., 1979; Mizoue, 1976) to show the upper planes of subducting plates. Overriding lithosphere is shown with the stipple. The

of the

bottom of the lithosphere is assumed to be the isotherm which corresponds to a Q value of 180. The Q value is inferred from the cross section of NE Japan by Yoshii (1979b) in which the lithosphere is 30 km thick in the area of the Japan Sea

Japanese Islands. Nevertheless, here we simply assume that an equal Q value shows an isotherm in the uppermost mantle above the subducting slab, where nearly identical material is considered to exist. We do not discuss the absolute temperature because of poor knowledge of the material constant c, and restrict our discussion to relative thermal structure. The inferred isotherms in the mantle of NE and SW Japan, from Q structures, are shown by the broken lines in Fig. 7b.

coast and the subducting Pacific slab contacts with overriding lithosphere down to a depth of 60 km at the aseismic front. The crustal low Q beneath the Pacific Ocean may reflect soft sediments rather than high temperature. We do not draw any isotherms in the crust because the crystal Q sometimes reflects such soft sediments rather than thermal condition. Gross patterns of heat flow data compiled by Yamano and Uyeda (1988) are shown in Fig. 7a for comparison. Higher heat-flow areas

might change from place to place, because the material changes with depth and laterally, particularly

in a subduction

zone

such

as that

177

nearly

to the areas where the isother-

correspond

mal lines become The difference and SW Japan is expected rather

shallower. in thermal

ducting

by the thermal plate,

mantle

lithosphere.

In NE Japan

while in SW Japan

Philippine

Sea plate angle.

Neogene

the

Quaternary

Sea coast.

may be related

volcanism

and

relatively

to hot

Philippine

Sea plate. The ~thosphere

Philippine

Sea coast

subducting

plate down to a depth of about 40 km.

the relatively and

arc volcanism while

of the lithosphere

should be

the cold Pacific plate is

in NE Japan,

Sea coast and the Philippine

of the subthe overriding

is subducting,

Explosive

appears

and

the Japan

The thinning

of NE Japan

conditions

arc volcanism

NE

temperature

The difference

subducting, shallow

between

Higher

in the uppermost

than in SW Japan

influenced

structure

is remarkable.

as shown in Fig. 7b. In this figure the lithospheric thickness gradually becomes thinner towards both

hot a

generated Nankaido

only

sub-

causes

great

a seismic

earthquakes

earthquakes,

the

with

the

model

that

coupling

such

the estimated

try of the great earthquakes

beneath

contacts

in the lithospheric

plane

the

with since

If we assume contact

directly

the

which

as the

1946

fault geome-

(e.g., Ando,

1975) is

dued volcanism occurs in SW Japan. Therefore, upwelling diapirs are schematically shown only in

explained.

the NE Japan cross section. Owing to the effects of the subducting plates and arc volcanism, colder

Conclusion

lithosphere occupies the outer arc of NE Japan and the middle part of SW Japan.

The 3-D seismic attenuation structure beneath the Japanese Islands is estimated by inversion of

In NE Japan a strong thermal gradient between the aseismic front and the volcanic front is found at the corner of the mantle wedge. The location of this strong thermal gradient in the uppermost mantle is nearly coincident with the ancient (20

seismic intensity data. The method developed by Hashida and Shimazaki (1984) is used in this

M.y.

old)

volcanic

front

1971). The mantle-wedge

(Matsuda

and

Uyeda,

feature differs somewhat

from that in the structural model suggested by Yoshii (1975, 1979b). It is inferred from this study

study. The data consist of about 15,000 JMA intensity reports from 800 earthquakes. By combining Japan,

the 3-D structures of eight districts in 3-D attenuation maps consisting of three

layers are constructed The obtained difference Japan

to a depth of 90 km.

3-D structure

between

NE

and

high Q is dominant

reveals SW Japan.

a distinct In NE

in the outer arc, while

that the aseismic front is not a seaward edge of the aseismic mantle wedge at a depth range of 40-60

low Q dominates

km, but only a landward edge of a seismic interface between two convergent plates. Furthermore, it is worth noting that the “aseismic belt” which is

except for Kyushu, high Q is dominant and small low-Q areas exist mainly in the outer arc. Comparison with velocity studies shows that the ob-

defined

by Yamashina

in the inner

et al. (1978) as a belt where

tained

attenuation

structure

no seismic activity appears in the crust, corresponds to the low-temperature lithosphere of the outer arc. The cause of the “aseismic belt” should be explained by a model considering the structural features presented here.

dence

of crustal

attenuation

In SW Japan it may be possible to interpret the thickness of the lithosphere as being 30 km or so and the aseismic continuation of the relatively high-Q subducted Philippine Sea slab as lying just below the lithosphere. Hirahara (1981) pointed out the possible existence of the aseismic Philippine Sea slab beneath the Chugoku districts from 3-D velocity structure. Here, however, we interpret the high Q in layer 2 as the root of the lithosphere,

arc. In SW Japan,

is reliable. A depenupon age is found.

Crustal low Q corresponds well to the distribution of Quaternary volcanic material, and high Q corresponds to the geological province where preCenozoic rocks are distributed. Low-Q loci corresponding to the location of volcanoes are found in the uppermost mantle. These loci may indicate the upwelling diapirs which generate the arc volcanism. The thickness of the lithosphere in the Japanese Islands is discussed, based on the 3-D attenuation structure: The outer arc of NE Japan and the main part of SW Japan, where high-Q zones appear in the uppermost mantle, seem to have thick lithosphere of about 60 km. A possible

17x

thermal

structure

for NE

ferred from average tion structure. between

and

The difference

involve

settings

could be explained

plates,

overriding

Andrews,

by

which

lithospheres

Aramaki,

Profs. T. Utsu, K.

Prof.

K. Shimazaki

S. Uyeda

Yamashina,

Yamano

throughout

T.

and

Miyatake,

K. Satake

K.

provided

are also thanked.

and Drs.

Hirahara, helpful

ments.

They

stitutes

part of my Ph.D. dissertation

the University

for his

this study.

and T. Usami,

This

M. com-

paper

con-

accepted

by

of Tokyo.

from

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