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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.
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-
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
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
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-
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
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
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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