Vol. 5 No. 4 6 9 9 - - 7 0 7 A C T A S E I S M O L O G I C A SINICA Nov. , 1992

Research on thermal structure

the crust in Bohai Sea and its

and thermal stress of

surroundings

Hu a Tian and Zhil i Z h a n g

Institute of Geophysics, State Seimtudogical Bureau, Beijiug 100081, China

Abstract

In this paper, the three-dimensional thermal structure and the two dimensional thermal stress caused by the horizontal inhomogeneity of

the thermal structure in the crust of the Bohai Sea and its surroundings were analyzed using the geothermal and the crust structural data

by means of the finite element method. As resulted, the horizontal distribution of the temperature in the upper crust is obviously differ-

ent from that in the lower crust in most part of the region. But the horizontal distribution of the heat flow is constant in the crust. There

is a belt where the thermal structure of the crust is intensely changed around Tangshan. There are some different characterist i~ of the

thermal stress of the crust around Tangshan, in the North-China plain, in the Bohai .Sea gul f , and in the middle part of the Bohai Sea.

Util izing those results, the distribution features of the seismic act ivi ty in the region were explained in this paper.

Key words: thermal structure of crust, horizontal inhomogeneity, thermal stre~,,.s, finite elements method, Bohai .Sea and its surround-

ings.

Introduction

The Bohai Sea and its su r round ings is the region where ea r thquakes occur f r equen t ly . The seis-

mic i ty in the region has the un ique character is t ics . It is found tha t the seismic force source in the re-

gion consists of two pa r t s : the stress resul ted by the plate m o v e m e n t and the t h e r m a l stress leaded by

the hor izon ta l i nhomogene i ty of the t he r m a l s t ruc tu re in the crust . M a n y people h a v e researched the

crust stress caused by the plate m o v e m e n t . It is still necessary to s tudy the t he rma l stress in order to

u n d e r s t a n d c lear ly the charac ter i s t ics of the seismic ac t iv i ty in the region. T h e r e f o r e , Zhil i Z h a n g

( Z h a n g e t a l . , 1 9 8 0 , 1 9 8 5 , 1 9 8 9 ) has made theore t ica l ana lys i s for this aspect. In this pape r , the

t h r ee -d imens iona l t he r m a l s t ruc tu re of the crust in the region was studied us ing a n u m b e r of the

geo the rmal measu red data and the crust s t ruc tu re data by means of the f ini te e lements method. The

two-d imens iona l t h e r m a l stress in two layers of the crust were also calcula ted. The effect of the hor i -

zonta l i nhomogene i ty of the t h e r m a l s t ruc tu re in the crust on the seismic a c t i v i t y , the seismic force

source and the crus ta l stress in the region are studied. The exclus ive d is t r ibut ion fea tures of the seismic

ac t iv i ty in the region are also expla ined. S i m u l t a n e o u s l y , the complete d is t r ibut ion of the t h r e e - d i m e n -

s ional t h e r m a l s t ruc tu re in the crust are g iven.

The calculation method of the thermal structure in the crust

To assure the t he r m a l s t ruc tu re of the crust is the key to ana lyze the t he rma l stress and the effect

of the t he rma l stress on the seismic ac t iv i ty . The ma in fac tors to decide the t he rma l s t ruc tu re are the

m a n t l e hea t f low ql ( z , y , z ) , the d is t r ibut ion of the crust m e d i u m , and the t h e r m o d y n a m i c a l p a r a m e

ters of the med iums . The last two fac tors h a v e two effects on the t he rma l s t ruc tu re of the crust . One

* The Chinese version.of this paper appeared in the Chinese edition of Acta Seismologk'a Sinica, 14, 2 9 - 35, 1992.

7 0 0 A C T A S E I S M O L O G I C A S 1 N I C A V o l . ,5

is the crust' s heat flow qz (x, y, z ) , which is generated by the decay of the radioactive elements in the

crust. The other is the "refracted" heat flow q3 (x, y, z ) , which is caused by reapportioning the ther-

mal energy from the deeper crust because of the speedy fluctuation of the surface between the crust

mediums in some area. The last one is especially apparent in the upper crust. Therefore, it can be con-

sidered that the heat flow q(x, y, z) in a point of the crust mainly consists of the three components

mentioned above, i .e . :

q(X, y, Z) = ql(X, y, Z) -~ q2(x, y, z) q- qa(x, y, z) (1)

The geothermal distribution in the crust satisfies the three-dimensional steady thermal conductive equa-

tion (2a) with isotropic and inhomogeneous mediums. Because the crust' s structure in the Bohai Sea

and its surroundings are much complicated and the three-dimensional thermal structure of the crust

can' t be decided from equation ( l ) . When equation (2a) and its boundary conditions (2b) and (2c)

are solved approximately, the three-dimensional thermal structure of the crust will be obtained. So,

the finite element method is adopted.

V • ( u V T ( x , y , z ) ) - } -A(x ,y , z ) = 0 (x ,y ,= ) E l (2a)

TIs ~ = To(z , y , z ) ( x , y , z ) E $1 (2b)

aT '~-~1% = gs(x ,y , z ) ( z , y , z ) E $2 (2c)

Where S : S I q - S 2 , V is the region enclosed by the boundary S; n(x , y , z ) is the thermal conductivity

of an isotropic and inhomogeneous medium; A(x , y , z ) is the heat-generated ratio of the radioactive el-

ements in a medium; To ( x , y , z ) is the temperature distribution on the section $1 of the bound-

ary ; gs(x ,y , z ) is the heat flow distribution on the section S~ of the boundary.

The whole region V is divided into some small elements l'e. Assuming N. is the. given shape func

tion of the i-th nodal in an element, the temperature function in the element will be approximated for

T (e) ( x , y , z ) : N~T~ e) , T/°) is the unknown temperature of the i - t h node in the element. Then ,

let R ( x , y , z , A ) equal the left of the equation (2a) . According to the Ritz approximation method, the

equation below is gained in every element:

f R ( x , y , z ) N , ( x , y , z ) d V e : f , [ V . ( u V T <e)) q-A~.V,dV~= 0 I' e I' e

Thirdly, the second derivatives in the equation above are integrated by parts with the Gauss' s theo-

rem.

f ,, N iV ° (~¢VT(e))dVe -}- f ANidl'e

~ - - - f uVTCe) 'VNidVe-} - [ AN,dVeq- [ V • (N0i~TT(e))dl'e v e d I e o I e

= - - V N , • (~¢VT('))dV~ q- AN,dI~ q-- , ~ : T 3 , , d S , , = 0 Ve l'e ' e

Substitute for T(~) = NjTJ ")

( V N ~ ) u • (VNj)Tj(~)dV~ = AN~dV~ -~ u ~ - N , d , % Ve ]'e S

No. 4 T ian ,H . et al. : T H E R M A L STRUCTURE AND THERMAL STRESS 701

Let

e V Nl)dV~

f r aT (') F} ~) = ANidV~ + / s . j - ~ - . Y ,dS~ I e ' "e

where , K (f)~) , F} ~ are the calculable coefficients of the equat ion.

F ina l ly , the discrete algebraic equation ( 3 ) in every element is given as.

[K] ( '>{T} ('> = {F}(~) (3)

Afte r assembling the discrete algebraic equation ( 3 ) to all e lements , the discrete algebraic equation

(4 ) in the whole region, which equal to equat ion ( 2 ) , is acquired.

[ K ] { T } = {F} (4 )

Because of the direction of n on the common boundary of two adjacent e lements , the area- integrated

i tem of F~ (e~ is zero on the boundaries between elements and only nonzero on the regional boundary S

after assembling {F~} (e). The i tem m u s t n ' t be calculated on the section St of the boundary because of

the reason ment ioned below and it is the effect of the boundary condition (2c) on the $2 section of the

boundary. The effect of the boundary condit ion ( 2 b ) on the equation is directly subject to equat ion

( 4 ) , made the nodal temperatures on the S~ section of the boundary equal to the related values of the

boundary condition (2b ) . T h e n , after solving equation ( 4 ) , the approximated temperature at every

node is obtained. In term of equat ion ( 5 ) , each direction of the heat f low in every node is given.

q~- - - - ndT/dx, ( i = 1 , 2 , 3 ) (5 )

In the ca lcula t ion , because complete boundary condition are impossible to g ive , the crust p s thermal

structure will be analyzed imi ta t ively in this paper. The values of the heat f low on the ground are the

imitated value. The crust ' s thermal structure and the heat f low at the upper surface will be certain si-

mul taneously . So , to decide the thermal structure of the crust needs the complete data on the crust ' s

s t ruc ture , the thermodynamica l parameters , and the measured geothermal data.

1. The distr ibution of the crust media

The geological structure is complex in the Bohai Sea and its surroundings (Xu et al. , 1 9 8 5 ) .

They can be roughly divided into two sorts of structure pat tern , i . e . , the rising and the s inking, and

have the characteristics that the rising area is al ternated with the sinking area. The north part in the re-

gion is one part of the Yanshan uplift block. The crust structure in the region except the north part is

approximate ly composed of four kinds of mediums , i. e. , the inat tent ive sediment , the sedimentary

rock , the metamorphic rock including the grani te , and the basalt. The surfaces between these four

med iums , especially in the upper crust , undulate apparent ly. According to the crust structure of the

region, the calculated model of the three-dimensional thermal structure in the crust was designed. The

upper surface of the model is 0. 3 km under the ground. Its lower surface is the Moho surface. The

model has four kinds of medium with 1883 nodes and 1448 elements. The studied region is f rom lon-

gitude l 1 6 ° E to 120°45 p E , and latitude 37030 ' N to 40°N.

702 A C T A SE1SMOLOGICA SINICA Vol. 5

2. The thermodynamieal parameters of the media

S imply , the thermodynamica l parameters of every medium in the model were assumed as isotropic

and homogeneous. See Table 1.

Table 1 The thermal conductivity and the heat-generated ratio of the mediums

Thermal conductivi ty Heat-generated ratio Number Medium

( W / m • *C) ( W / m a)

A Inat tent ive sediment 1 . 8 8 1, -I 7 >( 1 0 - r

B Sedimentary rock 2 . 9 3 1 . 8 8 )< 10 -7

C Metamorphic rock 3. 18 1 . 9 3 X 10 7

19 Basalt 2. 51 0. 2 1 X 1 0 - 7

In Table 1, the thermal conduct ivi ty of the medium A, B and C is the average of the measured

values of the related rocks respectively. The thermal conduct ivi ty of the medium D was obtained f rom

the reference. The heat-generated ratio of every medium was the average of those measured in related

rocks of the di f ferent geological era in the Nor th-China massif (Thermal Group , Insti tute of Geo logy ,

Academia Sin ica , 1978) .

3. The measured geothermal data and the calculation method

There were m a n y measured geothermal data in the Bohai Sea and its surroundings ( W a n g et al. ,

1988; Chen et al. , 1984; Thermal Group , Institute of Geology Academia Sin ica , 1978; Zhang et

al. , 1982; W u et al. , 1984) . The geotherm and its gradients distribution at the depth of 0. 3 km

were sorted out f rom those data. The gradients at the depth of 0. 3 km were multiplied by the thermal

conduct ivi ty of the related rocks in Table 1. The distribution of the heat f low q % ( x , y ) at the same

depth were gained. In the ca lcula t ion , the nodal temperatures of the upper surface at the model were

equal to those measured and supposing that all surfaces of the model except the upper and the lower

surfaces were adiabatic. Because after the model was exact the crustal heat f low q2 was certain and the

" r e f r ac t ed" heat f low q3 is a smaller va lue , the var iance of the surface heat f low q, was mainly related

to that of the mant le heat f low ql. In the w a y , f i rs t , the heat f low q~(z ,y ) of the upper surface in the

model was accounted out af ter the distribution of the initiated mant le heat f low q~o ( x , y ) was supposed

arbitrari ly. T h e n , the error funct ion ~ / ( x , y ) = (q,--q~o)/q~o between the measured heat f low q~0 ( z , y )

and the calculated heat f low at the upper surface was estimated. According to the e r ro r , the mantle

heat f low distribution was modified by q~ ( x , y ) = q l ( z , y ) ( 1 - - q ( x , y ) ) . The calculat ion was gone a-

gain. The step was repeated until the error funct ion ~/(z,y) at every node on the upper surface was less

than 2 0 % . The thermal structure of the crust in the Bohai Sea and its surroundings was achieved.

The three-dimensional thermal structure of the crust

As seen in the calculated results , the horizontal geothermal distribution of the upper crust is more

dif ferent f rom that of the lower crust in the region except the north part. The horizontal geothermal

distribution of the upper crust have many small h igh-geothermal areas (F igure l a ) . They get maxi -

m u m at about 5 km depth and disappeared at the depth of 10 km.

The reason to cause those is the significant undulat ion at the top surface of the sediment rock in

the upper crust and the di f ferent thermal conduct ivi ty between two mediums above and below the sur-

face. Wi th the depth increased, the undulat ion of the crystal base still exists but the thermodynamica l

parameters of the related mediums trend same. When the depth is more than 15 k m , the crust medium

trends homogeneous. S o t h e effect of the " r e f r ac t ed" heat f low can be neglected. The distribution of

the mant le heat f low ql decides the horizontal geothermal distribution at the deeper crust (Figures l b ,

l c ) . There are a string of h igh-geothermal small area along the northeast direction in the Bohai Sea

No, 4 Tiara,H. et e.I. :THERMAL STRUCTURE AND THERMAL STRESS 703

and a h i g h - g e o t h e r m a l a rea nea r T ian j in . The i r geo the rma l d i f fe rence increases wi th the depth and gets

above 100°C at last. In the no r th par t of the r eg ion , because the hor izon ta l d i s t r ibu t ion of the crust

m e d i u m is e v e n , there is a semi -c i r cu la r belt of the geo the rmal g rad ien t wi th in the whole c rus t , its

geo the rmal d i f fe rence is f r om a b o v e 60°C in the upper to above 120°C in the deeper. The d i f fe rence

be tween 10 km and 15 km is the most . There is a second h i g h - g e o t h e r m a l a rea nea r Ba igezhuang un -

der the dep th of 20 km. It e l imina ted the grad ien t belt . The ma in reason exis t ing the g rad ien t bel t is

tha t the d i f fe rence of the l i thosphere th ickness be tween the no r th par t and the Bohai Sea is m u c h large.

It makes the m a n t l e hea t f low v a r y ev iden t ly . The geo the rmal d is t r ibut ion on the Conrad sur face and

the Moho sur face a re s imi lar to tha t of the deeper crust .

40"

38"

117" 119"

117" 119" 40" ' \ ~ ' '°of

/ o Hoanglaum /

117" 40' f

3g" (e)

I

110"

oYf,00.<

Figure ] The horizontal geothermal distribution of the crust in the Bohai Sea and its surround-

ings. (a) at 5 km; (h) at 15 km; (c) at 25 km.

38*

117" 119"

2 ( 117" 119"

40" I ~ g s ~ y f l

r5%,

Figure 2 (a) The distribution of the crustal heat flow at the depth of 15 km (0. 4187X 10 -3

W / m 2 ) . (b) The distribution of the ratio between the mantle heat flow to the surface

heat flow.

The hor izon ta l d is t r ibut ion of the crusta l hea t f low isn ' t bas ical ly changed wi th in the ex ten t of the

crust . It is the same as the hor izon ta l d is t r ibut ion of the geo the rm in the deeper crust (F igu re 2 a ) .

The re is a g rad ien t bel t of the heat f low in the no r th par t . The va r i a t ion range is above 1 6 . 0 X 10 -3

( W / m 2 ) . There is a h i g h - v a l u e area of the hea t f low in the middle par t of the Bohai Sea and near

704 ACTA SEISMOLOGICA SINICA Vol. 5

Tian j in . Its d i f fe rence is above f rom 8. 0 X l 0 3 to 1 6 . 0 X 1 0 - 3 ( W / m ~ ) . The consis tence of the heat

f low dis t r ibut ion in the shal low and the deeper crust points out tha t the ef fec t of the " r e f r a c t e d " heat

f low on the hea t f low is smal ler t h a n on the geotherm. The rat io be tween the " r e f r a c t e d " heat f low

and the sur face hea t f low is genera l ly abou t ± 5 ~ , the m a x i m u m to - - 1 0 °//00. The nega t ive va lue

shows d i f fus ion of the energy. The rat io ' s d is t r ibut ion of the man t l e heat f low to the sur face hea t f low

can be seen in F igure 2b.

F i r s t , the d is t r ibut ion of the measured geo thermal da ta is ne i ther dense nor e v e n , and some areas

h a v e n r t a n y geo the rmal data yet. Second , the measured sect ions of the geo thermal grad ien t are shor t -

er. So the geo the rmal gradients on ly s tand for the local one w h e n the m e d i u m ' s d is t r ibut ion a r o u n d the

measured spot isn ' t even . T h i r d , the t h e r m o d y n a m i c a l pa ramete r s of the ac tual med iums var ied ap-

pa ren t ly but the pa ramete r s used in the ca lcula t ion neglected the va r i ance . Las t , due to the large size

of the m e s h , the l inear spl int of the func t ion in the e l e m e n t , and the negat ion of the inhomogene i ty

wi th the smal l s ize , the geo therm and its gradients g iven in the ca lcula t ion s tand for the ave rage of the

t empe ra tu r e ' s change a m o n g the longer section of the depth in a smal l area . The precise tectonic of the

t h e r m a l s t ruc tu re in the crust will be still researched advance . The qual i ty of the geo thermal data in

the Bohai Sea and its su r round ings is h i ghe r , so the rel iabi l i ty on the large-s ized d is t r ibut ion fea tures of

the crus ta l t h e r m a l s t ruc tu re in the region is bet ter . The size of the rel iable d is t r ibut ion is about 1 5' X

1 5 ' .

T h e thermal stress of the crust

1. Model

Because most of the ea r thquakes in the Bohai Sea and its su r round ings occurred be tween 10 km to

20 km at the d e p t h , the t h e r m a l stress at the two layers f rom 7 km to 10 km and f rom 10 km to 18 km

of the depth are ca lcula ted each other in this paper . Suppose the problem can be approx ima ted for the

p lane stress p rob lem with the homogeneous and l inear elastic med ium. The studied region of the ther -

mal s t ress over lapped tha t of the t he rma l s t ruc tu re in the crust . M a n y ex t ra e lements are added on the

f r inge of the studied region for e l imina t ing the effect of the b o u n d a r y condi t ions on the stress dis t r ibu-

t ion. The d is t r ibut ion of the geo therm in the studied region are the ave rage of the geotherrn a long some

segment of the d e p t h , i . e .

1 T--b--af, : T(z)dz ( 6 )

Those out of the studied region are given f rom the geo therm on the b o u n d a r y of the studied region. At

each l a y e r , the Young t s m o d u l u s , the Poisson r s r a t io , and the expans ive coeff ic ient of each layer is

9 . 4 X 1 0 1 ° ( P a ) , 0. 2 5 , 2 . 0 X 1 0 - 5 ( 1 / ° C ) respect ively . The mesh of each layer has 1147 nodes wi th

368 second-order e lements .

2. The b o u n d a r y cond i t ions

In the ca lcu la t ion , there are the same b o u n d a r y condi t ions on each of the model . Accord ing to the

ex t e rna l force s i tua t ion in the crust of the r eg ion , supposing the nodal d isp lacements wi th the y-direc-

t ion a long the b o u n d a r y of the no r th par t and the x-di rec t ional d i sp lacement at the no r thwes t corner are

zero a n d for o ther b o u n d a r y the d i sp lacement is free. The force caused by the plate m o v e m e n t in the

region i s n ' t added. Only it is considered tha t the inhornogene i ty of the crus t ' s t he rma l s t ruc tu re resul t -

ed in the t h e r m a l stress.

3. Results

Due to the charac ter i s t ics of the hor izon ta l change of the crust r s t h e r m a l s t ruc tu re in the Bohai

Sea and its s u r r o u n d i n g s , two pr inc ipa l stresses of the t h e r m a l stress are all tensi le in the no r th par t of

No. 4 T i a n , H . et a[. : T H E R M A L S T R U C T U R E AND T H E R M A L STRESS 705

the studied region (F igure 3 a ) . One of the pr incipal stresses is tensi le and the o ther is compress ive in

all the region except the nor th . The m e a n stress is compress ive in the N o r t h - C h i n a plain and the Bohai

Sea gulf (F igure 3 b ) . The tensi le area of the m e a n stress is en la rged wi th the depth increased in the

middle par t of the Bohai Sea. The m a x i m u m shear ing stress has the d is t r ibut ion as Figure 3c. The di-

rect ion of the m a x i m u m tensi le pr incipal stress is no r thwes t in the east h a l f - p a r t of the region and

no r theas t in the wes t ha l f -pa r t . As the geo thermal d is t r ibut ion at the deeper crust is r a re ly changed and

the geo the rmal change is s t r e n g t h e n e d , when the depth added , eve ry e l ement of the t he rma l stress in-

creased in va lue but hasn "t obvious change in the d is t r ibut ion t endency .

40'

38 ~

117" 119"

• A / / ~ . ~ c - 0 - y \ , ~ . / , ,~ / i

i

40' 117 ° 119"

iA\ ; .,,--+oo.

i

(b) 40"

38"

117" 119 +

Figure 3 ( a ) The distribution about the principal stresses of the thermal stress in the crust ( ] 0SPa) ;

( b ) The distribution about the mean stress of the thermal stress in the crust (1 0 ~ P a ) : ( c )

The distribution about the maximum shearing stress of the thermal stress in the crust

( 10SPa ) ( " - - " for Compressive and " ~ - " for tensile in the f igure ) .

4. The relation to the se ismic ac t iv i ty

The seismic ac t iv i ty in the Bohai Sea and its su r round ings has the fea tu re of belt (F igure 4 ) . The

ea r thquakes m a i n l y concen t ra t e near some large f r ac tu re belts. For e x a m p l e , the seismic belts f rom

Hej ian to T ian j in and f rom Tian j in to T a n g s h a n near the Cangdong fau l t z o n e , the seismic belt in the

middle par t of the Bohai Sea near the T a n l u faul t zone. The str ike of these seismic belts as well as tha t

of the f r ac tu re belts is in the no r theas t d i rect ion. The depth of the seismic ac t iv i ty is abou t f rom J 0 km

to 20 km in the N o r t h - C h i n a area and at 30 km in the middle par t of the Bohai Sea. The local f luc tua-

t ions of the crust stress and the geological s t ruc tu re de te rmined the f ea tu re of the seismic ac t iv i ty be-

cause the stress field resul ted by the plate m o v e m e n t are a lmost even . The ef fec t of the r ising of t em-

pe ra tu re on the crus t med ium are double . It not on ly makes the mate r ia l in the faul t sof tened so tha t

the faul t is easy to m o v e , but also genera tes the t he rma l stress tha t lets the crust stress var ied . So , the

changes of the t h e r m a l s t ruc tu re and the t h e r m a l stress in the crust are some re la ted to the d is t r ibut ion

fea tu re of the seismic ac t iv i ty and have the proper ef fec t to the occur rence of some ea r thquakes . E v e r y

e l ement of the t h e r m a l stress has a g rad ien t bel t wi th 108 ( P a ) stress va r ied f rom Tian j in to T a n g s h a n .

They make the crust p s stress field h a v e the local belt of va r i a t ion tha t the seismic ac t iv i ty a long the belt

is f r equen t . E v e r y e l ement of the t he rma l stress has lit t le change f rom Tianj in to the middle par t of the

706 ACTA SEISMO L O G I CA SIN]CA Vol. 5

Bohai Sea so tha t the stress d is t r ibut ion in the area are even and the seismic ac t iv i ty is re la t ive ly quiet.

The geo therm f rom Hej ian to T ian j in is a little higher t han those f rom Cangx ian to H u a n g h u a . Al-

t hough they couldn r t cause large va r i a t ion of the t he rma l s t ress , the sof ten func t ion to the faul t in the

area cou ldn p t be neglected. O b v i o u s l y , the sof tened section in the faul t is easier to move in the same

stress condi t ion. So , the seismic ac t iv i ty f rom Hej ian to Tian j in is more f r equen t than f rom Cangxian

to H u a n g h u a . As s h o w n by the research on the T a n g s h a n e a r t h q u a k e , the force induced the ea r th -

quake h a v e an ex t ra tensi le stress besides the regional hor izon ta l tectonic stress. The the rma l stress m a y

h a v e some cer ta in ef fec t on the ea r thquake . As seen f rom the the rma l s t ruc tu re of the c rus t , the

geo the rm in the Bohai Sea and its su r round ings are lower t han 550°C above the depth of 20 km and the

geo therm in the region except the no r th par t are higher t han 550°C under 25 km. It is genera l ly

t hough t tha t the rock t rans fe r s the br i t t leness to the pl iabi l i ty tha t the crack can ' t occur easily when the

geo therm is above 550°C. The geo thermal d is t r ibut ion is closely related to focal depth. M o r e o v e r , the

tensi le a rea of the m i n i m u m pr incipal stress in the the rmal stress and the s inking area in the geological

s t ruc tu re are rough ly over lapp ing in the pos i t ion , and the compress ive area and the r is ing area are at

the same posit ion. A l though the the rmal s t r e ~ leaded by the hor izonta l inhomogene i ty of the t he rma l

s t ruc tu re in the crust are on ly 1 0 7 ( P a ) in v a l u e , they have the par t icular effect to the dis t r ibut ion of

the seismic a c t i v i t y , the large ea r thquake ' s genera t ion and the geological s t ruc tu re in the region. The

ef fec t is more i m p o r t a n t for some local a r eas , especial ly at the f au l t , at the concen t ra ted stress area

and at the area of large t he r m a l stress va r ia t ion .

38*

117 ° 119" I o 0

0 0 0

a~Ti cO o o 0 0 0 0

o o 0

-Oooo 8 ooO oo o°- %o 0

oo ooo o o o °

O O

o o o ° O O O

I I IO ~ 0 ~ 1 oOOO

Figure 4 The distribution of the seismic act ivi ty in the Bohai Sea and its surroundings.

This project is suppor ted by the Chinese Jo in t Seismological Science Founda t ion .

References

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