8
Vol. 5 No. 4 683--690 ACTA SEISMOLOGICA SINICA Nov. ,1992 p values of continental aftershock activity in China" Zhixin Zhao t~ , Kazuo Qike 2~ , Kazuo Matsumura 3~ and Jiren Xu ~ 1 ) Institute of Geophysics, 6"tale Sei.sTnologiz'al Bureau, Bei.)in 9 100081, Chi~a 2 ) Geophysical Institute, f'aculty of S'c/mwe, Ryolo ~nicer.s'ity. Japan 3) Disaster Prevention Research Institute, Kyoto Unlrewsity. ,la~tn Abstract The temporal attenuation characteristics of the frequency of aftershock activity for 32 earthquakes ( II being more than or equal to 5) were quantitatively analyzed by using the maximum likelihood estimation for point process. The results indicated that the average value of the attenuation coefficient of aftershock activity (p value in the modified Omori's formula) of the East China and the North--South Seismic Belt of China (NSB) was 0.91, being les~s than the p value of the western China. Comparing the p values of aftershock activi ties in the continental part of China with those in the Japanese islands, it was found that the p value in the continental part of China was less than that in the Japanese islands. Wherewith the p value is not related to the magnitude of main shock. Key words: modified OmorFs formula, maximum likelihood estimation for point process, p value, aftershock sequence, secondary af tershock. Introduction Studying the characteristics of aftershock sequence of large earthquakes is very significant for un derstanding the proceeding of earthquakes. The characteristics of aftershock activity in various ways were studied. The results of earthquake focal mechanism of aftershock were used to the study on the seismotectonics and the earthquake-generating stress field (Langer et al.. 1987; Shedlock et al. , 1987). The relation between the aftershock distribution and the fault of main shock, and study on the density of aftershocks in the fault interface were also paid attention to (Seeber and Armbruster. t987; Yamanaka et al. , 1989). The precursory changes before large aftershocks in the continental part of China were analyzed by the authors of this paper (Zhao et al. , 1989). But the study on attenuation features of aftershock frequency in the continental part of China has not been carried much. The temporal attenuation characteristics of the frequency of 32 aftershock sequences recorded well in the continental part of China were mainly analyzed in this paper. The two aftershock sequences recorded well in the Taiwan region were ~lso analyzed quantitatively here. No one of the main shocks of these aftershock sequences analyzed in the paper was less than M = 5. 0. The main shocks of M~6.0 are 1 6, including the 1 9 7 4 Shiwe Dam M = 5. 2 event. The events between M = 6. 0 and M=7. 0 are 4, and events of M~7. 0 are 14. The spatial distribution of attenuation coefficients of aftershock frequencies and some factors of after, shock activity related to the attenuation coefficient were analyzed using the statistical method. Method The temporal variation of aftershock frequency is expressed as followings (Utsu, 1991) -" * The Chinese version of this paper appeared in the Chinese edition of Acta Sei.wnoloqica Sinu'a, 14, 9--16, 1992.

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Vol. 5 No. 4 6 8 3 - - 6 9 0 A C T A S E I S M O L O G I C A SINICA Nov. , 1 9 9 2

p values of continental aftershock activity in China" Z h i x i n Z h a o t~ , Kazuo Qike 2~ , Kazuo M a t s u m u r a 3~ and J i ren Xu ~

1 ) Institute of Geophysics, 6"tale Sei.sTnologiz'al Bureau, Bei.)in 9 100081 , Chi~a

2 ) Geophysical Institute, f'aculty of S'c/mwe, Ryolo ~nicer.s'ity. Japan

3) Disaster Prevention Research Institute, Kyoto Unlrewsity. ,la~tn

Abstract

The temporal at tenuation characteristics of the f requency of aftershock act iv i ty for 32 earthquakes ( II being more than or equal to 5)

were quant i ta t ively analyzed by using the m a x i m u m likelihood estimation for point process. The results indicated that the average value

of the at tenuation coefficient of af tershock act iv i ty (p value in the modified O mor i ' s fo rmula ) of the East China and the N o r t h - - S o u t h

Seismic Belt of China ( N S B ) was 0 . 9 1 , being les~s than the p value of the western China. Compar ing the p values of aftershock activi

ties in the continental part of China with those in the Japanese islands, it was found that the p value in the continental part of China was

less than that in the Japanese islands. Wherewi th the p value is not related to the magni tude of main shock.

Key words: modified O m o r F s f o r m u l a , m a x i m u m likelihood est imation for point process, p va lue , aftershock sequence, secondary a f

tershock.

Introduction

Study ing the charac ter is t ics of a f te r shock sequence of large ea r thquakes is ve ry s igni f icant for un

de rs tand ing the proceeding of ea r thquakes . The charac ter is t ics of a f te rshock ac t iv i ty in var ious ways

were studied. The results of ea r thquake focal m e c h a n i s m of a f te rshock were used to the s tudy on the

se ismotectonics and the e a r t h q u a k e - g e n e r a t i n g stress field ( L a n g e r et a l . . 1 9 8 7 ; Shedlock et a l . ,

1 9 8 7 ) . The re la t ion be tween the a f te rshock d is t r ibut ion and the fau l t of ma in shock , and s tudy on the

dens i ty of a f te r shocks in the faul t in te r face were also paid a t t en t ion to (Seeber and A r m b r u s t e r . t 9 8 7 ;

Y a m a n a k a et a l . , 1 9 8 9 ) . The p recurso ry changes before large a f te rshocks in the con t inen ta l par t of

Ch ina were ana lyzed by the au t ho r s of this paper ( Z h a o et a l . , 1 9 8 9 ) . But the s tudy on a t t enua t ion

fea tu res of a f te r shock f r e q u e n c y in the con t inen t a l par t of Ch ina has not been carr ied much .

The t empora l a t t enua t i on charac ter is t ics of the f r e q u e n c y of 32 a f te r shock sequences recorded well

in the con t inen t a l par t of Ch ina were m a i n l y ana lyzed in this paper . The two af te rshock sequences

recorded well in the T a i w a n region were ~lso ana lyzed quan t i t a t i ve ly here. No one of the ma in shocks

of these a f te r shock sequences ana lyzed in the paper was less than M = 5 . 0. The ma in shocks of

M ~ 6 . 0 are 1 6 , inc luding the 1 9 7 4 Shiwe Dam M = 5. 2 e v e n t . The even ts be tween M = 6 . 0

and M = 7 . 0 are 4 , and even t s of M ~ 7 . 0 are 14. The spat ial d i s t r ibut ion of a t t enua t i on coeff ic ients

of a f tershock f requencies and some factors of after, shock ac t iv i ty related to the a t t enua t ion coeff ic ient

were ana lyzed us ing the stat ist ical method .

Method

The t empora l va r i a t i on of a f te rshock f r e q u e n c y is expressed as fo l lowings ( U t s u , 1991) -"

* The Chinese version of this paper appeared in the Chinese edition of Acta Sei.wnoloqica Sinu'a, 14, 9 - -16 , 1992.

Page 2: pvalues of continental aftershock activity in China

6 8 4 A C T A S E I b M O L O G 1 C A S 1 N I C A Vat . S

n(t) = K / ( t q- c) -~ ( I )

where n(t) being t ime f requency of af tershocks, t being time length starting from main shock, K , c

and p being parameters model ing the decay of the aftershock act ivi ty, p is the at tenuation coefficient of

aftershock f requency. The max imum likelihood estimation introduced by Ogata ( 1 9 8 3 ) were used to

determine the values of K , c and p in the present investigation. The max imum likelihood nature loga-

r i thmic expression of (1 ) is

7 T

l n ( L ( K , c , p , t ) ) = ~ ( l n ( n ( t , ) ) - - | n ( s ) d s (2 ) i 1 J '~

Where n(t , ) is the aftershock f requency at t;(t, is the origin t ime of the i- th af te rshock) . X is number

of aftershock occurr ing in the duration between time T and 55. As the left side of (2 ) get the max imum

values of K , c and p are optimal ones. The obvious advantages of this method are that the parameters

K , c and p can directly be calculated by using the origin times of aftershock (t~) and that the calculat-

ed points are increased. The method makes it possible to est imate quant i ta t ively aftershock sequence

without long record.

The modified Omor i ' s formula with the fol lowing formation is adopted to fit the aftershock se-

quence includes one or more large aftershocks with the secondary aftershock.

M

K, (3 ) ~.~.aH(t -- T,) (t - - T, q- e,)'. n(t) i = 0

where H is the Heaviside funct ion. M is the number of large aftershock with the secondary aftershock.

To and T~ are the origin t ime of the main shock and the i- th large af tershock, respectively. In this case,

the total number of parameters in ( 3 ) becomes 4M-~-3. The number of parameters K , e and p be-

comes M % - 1 , respect ively , and that for T, becomes M. The most appropriate model for the sequence is

selected among the models (1 ) and (3 ) by Akaike ' s Informat ion Criterion (AIC) (Aka ike , 1974) .

The model with the smallest AIC is the best model among the examined ones. AIC is expressed by

AIC = - 2 max (ln ( l ike l ihood)) q- 2 (number of parameters) (4)

Data

The data in the present analysis mainly comes from the fol lowing catalogues. ( I ) Seismological

Catalogue of China ( B. C. 1 8 3 1 - - A . D. 1969) (Science Press , 1983) ; ( 2 ) Seismological Catalogue of

China ( 1 9 7 0 - - 1 9 7 9 ) (Seismological Press, 1 9 8 3 ) ; (3 ) Sei6"mological Catalogue of Tanqshan earthquake

( from duly, 1976 to December, 1979) (Seismological Press, 1 9 8 0 ) ; (4 ) Seismological Catalogue of

Fast China (from 1970 to 1979) (Seismological Press , 1980) . It is necessary to explain the homo-

geneity of these data f rom the catalogues. Gu tenberg -Rich te r ' s magni tude- f requency plots ( G - R

graph) were used to cheek the reliability of data by means of the same method as that of Zhao el al.

( 1 9 8 9 ) in the present analysis. These catalogues were compiled on the basis of many records of obser-

vatories in C h i n a . It is impossible that the aftershocks with magnitude larger than or equal to

3 . 0 usually were not recorded. Therefore the threshold of magni tude of aftershock was adopted M =

3. 0 or M = 4. 0 as analyzing the p value of aftershock sequence which main shock was larger than or

equal to 7. 0 in this study. The fol lowing way was employed to determine the large aftershock for

main shock with M = 7. 0. Aftershock which was accompanied by a large number of secondary af ter-

Page 3: pvalues of continental aftershock activity in China

No. ~ Zhao ,Z . X. et al. : p VALUES OF CONTINENTAL AFTERSHOCK ACTIVITY IN CHINA 685

shocks and the d i f fe rence of whose magn i tude f rom m a i n shock was less t han 1 . 2 was regarded as large

a f te rshock . Of course , p va lue can also be calcula ted employ ing f o r m u l a ( 3 ) for the a f te rshock se-

quence which accompan ied a large n u m b e r of the secondary a f te rshocks but the d i f fe rence be tween the

magn i tudes of itself and the ma in shock was not less t han 1 . 2 , for e x a m p l e , the T a n g s h a n M = 6 . 5

a f te r shock in Table 1. In some cases , a f te rshocks were not well observed dur ing abou t severa l tens of

minu te s a f te r large shock. W e excluded the data in the severa l tens of minu te s a f te r large shock. M a n y

observa tor ies were set up in East Ch ina since 1970s. The capaci ty mon i to r ing mic roea r t hquake in this

region becomes s t ronger . They make it possible to ana lyze quan t i t a t i ve ly a f te r shock sequences whose

m a i n shocks are larger t h a n or equal to M : 5 . 0 in East China . The af te rshock sequences whose m a i n

shocks are less t h a n M : 6 . 0 in the present s tudy were selected f rom the eas te rn region of China . The

thresholds of magn i tude of a f te rshock sequences were taken as 1. 0 on basis of the Seismological Ca ta -

logue of East China .

Factors related to p value

The a f te r shock sequences ana lyzed in the present paper and the p a r a m e t e r s , va lues of K , c and p

de te rmined f rom the stat ist ical me thod were shown in Table 1 and Tab le 2. Af te r shock sequences in

Table 1 and Table 2 were ones inc luding the secondary a f te rshocks and not inc lud ing those , respect ive-

ly.

Table 1 p values of aftershock sequences ( I )

T~ Lat. Long. Depth .~ts M,h K, c, pl Days .\" Location

a. mo. d. h. rain. (kin)

1975 Feb. 4 19 : 36 49°42 ' 122°42 ' 16 7 . 4 4 . 9 9 . 9 2 9. 9 0. 87 1199 139 Haicheng

1978 May I8 2 0 : 3 3 40°42 ' 122°36 ' 13 6 . 3

1976 May 29 22 : 99 2 C 3 3 ' 98"45' 20 7 .4 3 . 7 34 .4 9. 904 0. 85 53 348 Longling

May 31 13 : 08 24°15 ' 98°38 ' 16 6. 5 3. 7 2 * *

Jul . 21 23 : 10 24°49 s 98°36 ~ 5 6 . 6

1976 Jul . 28 3 : 42 39°38 ' 118"11 r ii 7 . 8 4 . 6 1 5 . 2 8 9 .911 0 . 9 9 110 185 Tangshan

1976 Jul. 28 7 : 17 39027 ' 1 1 7 0 4 7 ' 19 6 . 5 1 .75 *

1976 Jul. 28 18 : 45 39%0' 118°39 ' 10 7. 1 5. 18 ~

1976 Nov. 15 21 : 53 39°17 ' 117°59 ' 17 6 . 9

1976 Jul. 28 3 : 42 39038 ' 118°11 ' 11 7 . 8 4 . 6 1 5 . 5 9 0. 091 0 . 9 7 288 2 0 4 Tangshan

1976 Jul. 28 18 : 45 39°50 ' 118"39 ~ 10 7 .1 5 .51 *

1976 Nov. 15 2 1 : 5 3 39°17 ' 117050 ' 17 6 . 9 0 . 0 ~

1 9 7 7 May 12 19 : 17 39021 ' 117°48 ~ 18 6 . 5

1976 Aug. 16 22 : 06 32042 ' 104°96 ~ 15 7 . 2 3 . 3 22. 1 9. 997 0. 71 7 197 Songpan

1976 Aug. 22 05 ~ 49 32°36 j 104°98 ' 10 6 . 7 10. 1 ~

1976 Aug. 23 11 : 39 32°391 104"98 ~ 22 7 . 2

1974 Aug. 11 99 ~ 14 39°24 ' 73°48 ~ 39 7 . 3 4. 9 31. 3 0. 50 1 . 2 0 1 27 Wuqia

1974 Aug. 12 05 : 21 39o24 , 73°39 ' 33 6. 4

1976 Nov. 07 92 : 04 27°36 ' 101006 ' 21 6 . 7 4 . 0 4 . 6 5 0. 0975 0. 94 36 34 Yanyuan

1976 Dec. 13 1 4 : 3 6 27o24 ' 101°0ff 21 6 . 4

The asterisk shows that cl or p, is the same as c, or p~ of last line.

1. Relat ionship between p and lasting t ime of aftershock

There is not a n y quantitative method to determine the end t ime of aftershock act ivity at present.

If the aftershock with magnitude larger than or equal to the threshold did not occur during more than

one m o n t h , the present investigation thought that the aftershock sequence was ended. The lasting t ime

Page 4: pvalues of continental aftershock activity in China

686 ACTA SEISMOLOGICA SINICA Vol. 5

o f a f t e r s h o e k is r e l a t e d t o t h e m a g n i t u d e o f m a i n s h o c k g e n e r a l l y . D e t e r m i n i n g t h e e n d e d t i m e o f a f t e r -

s h o c k s e q u e n c e u s u a l l y w a s l i m i t e d f r o m t h e o b s e r v a t i o n o f a f t e r s h o e k a n d t h e m a g n i t u d e t h r e s h o l d s e -

l e c t e d i n t h e p r a c t i c e . F o r e x a m p l e , t h e M a r c h , 1 9 6 6 X i n g t a i M = 7 . 2 e a r t h q u a k e , i t w a s a m u l t i p l e

e v e n t t y p e s e q u e n c e . A l t h o u g h i t s m a g n i t u d e w a s b i g g e r , t h e l a s t i n g t i m e s e l e c t e d w a s n o t l o n g , o n l y

t w o d a y s . A l l t h e l a s t i n g t i m e s o f a f t e r s h o c k s e q u e n c e s a n a l y z e d i n t h e p r e s e n t s t u d y w e r e s h o w n in

T a b l e 1 a n d T a b l e 2 . W h e r e T i s t h e o r i g i n t i m e o f m a i n s h o c k o r l a r g e a f t e r s h o c k , N is t h e n u m b e r o f

a f t e r s h o c k u s e d i n t h e a n a l y s i s .

Table 2 p values of aftershock sequences ( II )

T~ Lat. Long. Depth Ms Mth K, c, pl Days A- Location

a. mo. d. h. min. (km)

1950 Aug. 15 22 : 09 28°24 ' 96"42' 8 .6 4 .0 31, 3 1.22 1.29 656 86 Chayu

1951 Oct. 22 05 : 34 23042 ' 121°181 7 .3 4 .0 55. 4 0 .63 1.54 359 127 Taiwan

1954 Jul. 31 09 : 00 38°48 ' 104°12 p 7. 0 4 .0 1, 54 0. 003 O. 75 337 25 Minqin

1966 Mar. 13 0 0 : 3 1 24012' 122°42 ' 42 7 .8 4 .0 4 .3 0.06 0.91 253 42 Taiwan

1966 Mar. 22 16 ." 19 37030 ' 115°06 ' 7 .2 4 .0 5. 19 0. 0 1.07 2 28 Xingtai

1969 Jul. 18 13 : 24 38°12 ' 119°24 ~ 35 7. -i 4 .0 4.04 0. 036 1. 10 108 31 Bohai

1970 Jan. 05 01 : 00 24°12 r 102°41 ' 13 7 .8 4 .0 11.07 0.17 1.14 185 63 Tonghai

1970 Aug. 10 02 : 29 35°42 ' 116°53 f 34 5. 0 1. 0 7. 56 0. 006 l . 08 11 61 Qufu

1971 Jun. 5 18 : 21 37026 ' 113°25 ' 5 .2 1 .0 9 .26 0.011 0 .62 100 136 Huoshan

1971 Jun. 28 13 : 01 37°49 r 106013 ' 5 .2 1 .0 12.75 0.28 1.12 84 61 Wuzhong

1971 Aug, 16 12 : 57 28°5T 103°47 ' 24 5, 9 4 .0 12, 12 0. 58 l . 06 30 -14 Mabian

1973 Feb. 6 1 8 : 3 7 33"18' 100"42' ]] 7.6 -1.0 t . 6 0 .003 0 .78 :194 27 Luhuo

1973 Nov. 30 04 : 17 32"52 r 111°31 ' 5. 0 1 .0 13.4 0. 52 1.30 39 39 Zhechuan

1974 Apr. 22 08 : 29 31027 ' 119"19' 18 5 .8 1 .0 24.84 0.23 0.74 66 217 Liyang

1974 May 11 03 : 25 28012 ' 104006 ' 14 7. 1 4 .0 2. 1 0. 007 0. 79 300 30 Daguan

1974 Dec. 22 12 : 46 41012 ' 123"36' 5. 2 l. 0 21.34 0. 01 0. 67 41 202 Shiwe

1975 Sept. 02 20 : 10 32*65' 121°48 ' 5 .7 1 .0 1, 96 0. 007 0. 88 119 20 Huanghai

1975 Dec. 4 22 : 16 28034 ' 105°02 ' 5. 1 1 .0 2 .22 0. 018 0 .89 122 24 Changning

1976 Apr. 6 0 0 : 5 4 40"14 r 112012 ' 18 6 .5 1 .0 3 .55 0.003 0 .90 77 35 Helinger

1976 Sept. 23 04 : 07 40°5 ' 106"21' 49 6.4 1.0 '1.20 0. 068 0. 82 218 47 Bayinmuren

1976 Oct. 14 22 : 35 40"31' 112°31 ' 5 .2 1 .0 6. 15 0. 046 0. 87 72 51 Liangcheng

1977 Mar. 14 06 ." 16 40"30' 112030 ' 5 .2 1. 0 3. 74 0. 0 0. 63 76 49 Liangcheng

1977 Jun. 5 1 2 : 3 7 41057 ' 121°18 ' 18 5.1 1.0 2. 39 0.004 1 .0 40 22 Fuxin

1977 Oct. 19 10 : 44 23023 ' 107"32' 12 5. I I. 0 83.51 3.87 1.27 57 113 Pingguo

1979 Feb. 6 22 : 45 48"54 r 116°45 ' 5. 1 1 .0 3. 43 0. 05 0. 84 90 31 Xinbarihu

1979 Mar. 2 15 ." 20 33"11' 117025 ' 11 5. I 1 .0 9. 11 0. 063 0. 97 140 73 Guzhen

1979 Jun, 19 12 : 15 37006 ' 111"52' 5 .5 1 .0 6. 12 0. 008 0.81 191 74 Jiexiu

1979 JuL 9 18 : 57 31°27 ' 119°15 ' 12 6 .3 1.0 27.6 0. 015 0 .92 172 274 Liyang

T h e i n f l u e n c e o v e r t h e a t t e n u a t i o n c o e f f i c i e n t o f t h e a f t e r s h o c k s e q u e n c e d u e t o t a k i n g d i f f e r e n t

l e n g t h o f t h e s e q u e n c e w a s a n a l y z e d in t h e p r e s e n t p a p e r . T h e p r a c t i c e f o r t h e 2 8 t h J u l y , 1 9 7 6 T a n g -

s h a n M = 7 . 8 e a r t h q u a k e w a s s h o w n in T a b l e 1 a s a n e x a m p l e . T h e a t t e n u a t i o n c o e f f i c i e n t o f t h e a f -

t e r s h o c k a c t i v i t y ( p v a l u e ) w a s 0 . 9 9 w h e n t h e e n d t i m e w a s t a k e n a s t h e 1 5 t h N o v e m b e r , 1 9 7 6 . p

v a l u e w a s 0. 9 7 w h e n t h e e n d t i m e w a s t a k e n a s t h e 1 2 t h M a y , 1 9 7 7 . I t i n d i c a t e d t h a t a s t h e l a s t i n g

t i m e o f a f t e r s h o e k a c t i v i t y w a s s e l e c t e d a p p r o p r i a t e l y , t h e i n f l u e n c e o v e r p v a l u e f r o m t h e s e l e c t i n g

d i f f e r e n t l a s t i n g t i m e o f a f t e r s h o c k s e q u e n c e w a s n o t m u c h i m p o r t a n t . T h e p r a c t i c e s f o r o t h e r a f t e r -

s h o c k s e q u e n c e s w e r e s i m i l a r l y c a r r i e d o u t i n t h e p r e s e n t s t u d y t o o . W h e n p v a l u e s f o r a a f t e r s h o c k s e -

q u e n c e h a d l a r g e d i f f e r e n c e d u e t o t a k i n g d i f f e r e n t e n d t i m e , t h e e n d t i m e o f a f t e r s h o c k a c t i v i t y o u g h t

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No. 4 Z h a o , Z . X. et al. :p V A L U E S OF C O N T I N E N T A L A F T E R S H O C K A C T I V I T Y IN C H I N A fi87

to select aga in unt i l l0 tended to a s table va lue ( the d i f fe rence of p va lue due to d i f f e ren t end t ime gen-

e ra l ly was not large t han 0. 3 ) .

2. Re la t ionsh ip b e t w e e n p and magn i tude of ma in shock

Last ing days of a f te r shock ac t iv i ty were possibly related to the magn i tude of ma in shock , whi le p

va lue for d i f f e ren t m a i n shock was not closed to the las t ing days of a f te rshock ac t iv i ty and the m a g n i -

tude of ma in shock. As s h o w n in Tables 1 and 2 , las t ing days had large d i f fe rence a m o n g those a f t e r -

shock s e q u e n c e s , f rom one or two days to three yea r s . But p va lues of all sequences approached

to 1. 0 no t be ing re la ted to the las t ing t ime. As men t ioned a b o v e , for the sequence of the 22 th M a r c h ,

] 9 6 6 Xing ta i ' e a r thquake , its las t ing t ime was t aken on ly two days , the p va lue was 1. 07. It was not

large. The re la t ion be tween p va lue in the two tables and the magn i tude of ma in shock was s h o w n in

F igure l b . The p va lue was not re lated to the magn i tude of main shock f rom these results. The a t t enu -

a t ion coeff ic ient of a f te r shock s equence , p, possibly ref lected the fea tures of the med ium in the a f t e r -

shock area .

1.6

1.2

0.8

0.4

(a)

I i I I I L L I ~4 5 6 7 8

1.6

1.2

0.8

0.4

| L 6

(b) •

• • O •

I I I k I l I - M 6 7 8

Figure I p value var ia t ion with magni tude of main shock. ( a ) Japanese islands (a f te r Matsu ' u ra , 1986) ; (b ) China.

Results

The stat ist ical d i s t r ibut ion of the a t t enua t ion coeff ic ients of a f te r shock sequences (p v a l u e s ) in

Ch ina was s h o w n in F igure 2. Most of 34 t9 va lues d is t r ibuted be tween 0. 7 and 1 . 3 . The n u m b e r of

j0 va lues be tween 0 . 8 and 1 . 0 was 15. The n u m b e r of p va lues larger t han 1 . 3 or less t han 0. 7 were

few. The ave rage va lue of all i0 was 0 . 9 5 . The n u m b e r of p va lues larger t han 1. 0 was 10 in the con-

t inen ta l par t of C h i n a , most of t hem was less than l . 0. The i r ave rage va lue was 0. 93.

The spat ia l d i s t r ibu t ion of p va lue in Ch ina deduced f rom the present s tudy was s h o w n in F igure

3. It is seen tha t all the regions in which p va lues of a f te r shock ac t iv i ty can be de t e rmined a lmos t lo-

cated in Eas t Ch ina and the N o r t h - - S o u t h Seismic Belt of Ch ina ( N S B ) where there were more obser-

Page 6: pvalues of continental aftershock activity in China

688 ACTA SEISMOLOG1CA S1NICA Vol. 5

vator ies . Wes t Ch ina is a seismical ly ac t ive region. But the a f te rshock ac t iv i ty was not observed well .

N

0 I I I , 0.6 0.8 1.0 1.4 1.6 1.2

Figure 2 Statistical distribution of p value of aftershock sequence in China.

80 ° 100" 120"

~' o.g,o / \

I 0 82 0.90 ~ ' ~ "... 172 / ~ - - 1 "_~9-~f~ o.99c;~,~o ½

o.75('~ ~ o.62,,., "~.,/'--'.'.9 ".,

0.78 1 0 7 1.300 0.74

1.29(9ooJ¢~ t ~ (°"8° ~( 0.91

0 M=5 0"8501 O O127 J ~ . 5 4

o n : o -I ' - "

Figure 3 Spatial distribution of p value of aftershock activity in China.

Only one af te rshock sequence can be de te rmined of p va lue in X izang (T ibe t ) or X in j i ang r eg ion , re-

spect ively , p va lues of these two af te rshock sequences were all larger t han 1 . 2 . The i r ma in shocks all

occurred in or near the b o u n d a r y be tween the I n d o - A u s t r a l i a n plate and Euras i an plate. These two

large p va lues were possibly re la ted to the plate b o u n d a r y . In the eas te rn par t of C h i n a , the a f te r shock

sequences wi th p va lues larger t han 1. 0 were re la t ive more in the con t inen t a l side of the Bohai bay. p

va lues of a f te rshock sequence in Nor th Ch ina and some regions in East Ch ina were a lmost less t h a n

1 . 0 . Most of p va lues in NSB was less t han 1 . 0 . Exc lud ing the two p va lues in X izang ( T i b e t ) and

X in j i ang reg ions , the ave rage p va lue of a f te rshock ac t iv i ty in the eas te rn par t of Ch ina inc luding NSB

Page 7: pvalues of continental aftershock activity in China

No. I Zhao,Z. X. et al. :p VALUES OF CONTINENTAL AFTERSHOCK ACTIVITY IN CHINA 689

was 0 . 9 1 . It is somewha t smal l . On ly two sequences in T a i w a n were ana lyzed in the present sttrdy,

The i r resul ts also were shown in Figure 3.

D i s c u s s i o n

/) va lues obta ined in this ana lys is seem to be less than those in the Japanese is lands obta ined by

Matsu r u ra ( 1 9 8 6 ) as a whole. Matsu ' u ra ' s resul t was shown in F igure l a for conven ience of corn

par ing wi th the results of China . F rom Figure 1, 1 ) va lues of the J apanese is lands were not closely re-

lated to magn i tude of ma in shock , p va lues more t han 1. 0 were more. The ave rage va lue of p was

1. 07. Seven p va lues of e leven ones were larger than 1. 0. p values of China were mos t ly less

than 1. 0 (F igure l b ) . The ave rage va lue was 0. 93 as men t ioned in above section.

Some a t t enua t ion coeff ic ients of a f te rshock sequences (h ) in the eas tern par t of Ch ina were deter -

mined by W a n g and W a n g ( 1 9 8 3 ) us ing the same exponen t ia l decay model as tha t used by Mogi

( 1 9 6 2 ) . The results for the Japanese is lands obta ined by Mogi ( 1 9 6 2 ) and the resul ts in the eas te rn

par t of Ch ina obta ined by W a n g and W a n g ( 1 9 8 3 ) were shown in F igure 4a and Figure 4b , respec-

t ively . Most of h values of af tershock sequences in the J apanese is lands were larger than 1. 0. h va lues

less t han 1. 0 were on ly three in the 35 h va lues of af tershock sequences . In the con t inen ta l par t of

C h i n a , most of h va lues were less than 1. 0. h va lue lager than 1. 0 was on ly one in 9 values , h va lues

of the con t inen t a l par t of Ch ina shown in F igure 4 were less than those of the J apanese is lands obv ious-

ly. This resul t is cons is tent wi th the resul t ob ta ined in this analysis . These results suggests tha t the f re-

quency of a f te rshock sequence occur red in Ch ina decreased wi th t ime s lower t han tha t in J apan .

1.40 [h]

1.91

1.13 1.5.4

[ a.'.~6

1.08

i o l . 2

i.28 o

[.35 L25

~1.07 ~.~.6g 1.29

1.02 (a)

100 ° 120"

/ o0 ,

/ o0 2 1/

Figure 4 The attenuation coefficient of aftershock frequency h. (a) The Japanese islands; (b) China.

The a t t enua t i on coeff ic ient of a f tershock ac t iv i ty p (or h) possibly related to the physical charac-

terist ics of rocks in the a f te r shock region and he te rogene i ty of spatial d is t r ibut ion of e a r t h q u a k e - g e n e r -

a t ing stress field. It depends on the recovery rate of the f r ic t ional s t reng th re la t ive to the re laxa t ion

t ime of the shear stress. M i k u m o and Miya take ( 1 9 7 9 ) showed tha t the a t t enua t i on coeff ic ient p of af-

tershock sequence decreased wi th increas ing he te rogene i ty of the f r ic t iona l s t r eng th d is t r ibut ion of dif-

f e r en t asperi t ies on a faul t . This is because it takes longer t ime to complete r ead jus tmen t s of the shear

stress d is t r ibut ion over the fau l t wi th more he te rogeneous fr ic t ion s t rength . Accord ing to this hypo the -

sis , the f r ic t iona l s t r eng th d is t r ibut ion of the faul ted rocks in the a f te rshock regions in the con t inen ta l

par t of China seems to be more he te rogeneous t han tha t in the J apanese islands. The con t inen ta l par t of

Page 8: pvalues of continental aftershock activity in China

690 A C T A S E I S M O L O G I C A S I N I C A Vol. 5

China has the characteristics of typical continental crustal structure. All of the major events and their

aftershocks in the continental part of China analyzed through the present study are shallow events.

Their hypocenters distribute almost within the depth from 5 to 30 km in the granitic layer of the conti-

nental crust. The events and their after.shocks in Japan occurred in the island arc region. The differ-

ence of the geological structure is the possible reason that p value in the continental part of China dif-

fers from that in the Japanese islands. As for the spatial distributional heterogeneity of iv value in the

continental part of China, it might be due to the difference of the frictional strength distribution of the

faulted rocks in the aftershock regions from the difference of medium distribution in China. Other-

wise , the frictional strength distribution of faulted rocks in the aftershock regions of two aftershock se-

quences in West China analyzed in the present study possibly are more homogeneous than that in East

China.

It is one of the developing seismological subjects to analyze quantitatively the stress field and

medium of the source region using seismic parameters. The features of medium in the continental part

of China have been investigated using the variation of p values in the modified Omori's formula in the

present study. Such study will be developed and solidified with the increase of the observation.

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