Investigations of tectonomagnetic phenomena in China

Physics of the Earth and Planetary Interiors, 57 (1989) 11—22 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

11

Investigations of tectonomagnetic phenomena in China Zhan Zhijia Institute of Geophysics, State Seismological Bureau, Beijing (China) (Received September 28, 1987; revision accepted March 19, 1988)

Zhan Zhijia, 1989. Investigations of tectonomagnetic phenomena in China. Phys. Earth Planet. Inter., 57: 11—22. The Tangshan earthquake (M 7.8, 28 July 1976) occurred in North China, where there is relatively dense network of geomagnetic observations. Before the earthquake there were anomalies of — 8—10 nT in the total intensity of the geomagnetic field at Ninghe and Dongtietou sites, 50—90 km from the epicenter, and also in the Beijing survey area. Anomalous changes in the vertical component of the geomagnetic field before the earthquake amounted to — 12 nT at Tangshan site, and 10 nT at Changli station, which was — 80 km from the epicenter. There were also anomalous short-period geomagnetic variations before and after the earthquake. To investigate tectonomagnetic effects, we measured the magnetic field at 17 sites near an underground nuclear test in 1983 in West China. The total field changed by 2 nT at the time of the explosion, and there were changes of 1—2 nT before and after the explosion. We have also measured the magnetic field around the Miyun reservoir during 1983—1987 and observed a correlation between geomagnetic changes and variations of water level in the reservoir.

1. Introduction Tectonomagnetism is one of the most important subjects in solid Earth geomagnetism (Rikitake and Honkura, 1985). Tectonomagnetism is a term used to describe anomalies of geomagnetic variation caused by the general tectonic activity in the crust, such as earthquakes, volcanic activity, fault activity, and so on. It also includes experimental investigations of the geomagnetic effects caused by water storage in a reservoir, mine explosions or underground nuclear tests. Observations and research on tectonomagnetism have attracted much attention, not only because of its geophysical significance, but also because of its potential application in predicting natural hazards such as earthquakes and volcanic eruptions. Significant progress has been made in the study of tectonomagnetism since Nagata (1969) proposed it. For field observations, the United States (Johnston, 1978; Davis and Johnston, 1983; Zhan, 1986), the Soviet Union (Shapiro and Abdullabekov, 1982) and Japan (Rikitake et a!., 1980) have 0031-9201/89/$03.50

© 1989 Elsevier Science Publishers B.V.

set up dense geomagnetic observation networks for experimental predictions of earthquakes and have observed some seismomagnetic precursor phenomena. In addition, tectonomagnetic effects have been observed in volcanic areas (Johnston et a!., 1981; Davis et a!., 1984), in the vicinity of a reservoir (Davis and Stacey, 1972; Shapiro et al., 1978), and near an underground nuclear test (Hasbrouck and Allen, 1972) and an artificial explosion area (Kozlov et al., 1974). Johnston (1986) recently analyzed geomagnetic data from California and obtained a significant relationship between geomagnetic variation, uplift, gravity and dilational strain changes. In China, there has been much investigation of seismomagnetic phenomena which would assist in earthquake prediction, since the 1966 Xingtai earthquake (M = 7.2, Hebai Province) (Working Group of Geomagnetic Research, 1986; Zhan, 1988). Some seismomagnetic effects have been observed (Qi, 1978; He et al., 1983; Mobile Geomagnetic Group, Seismological Bureau of Yunnan Province, 1984; Peng and Xu, 1986; Zhan, 1987).

12

This paper briefly reports the seismomagnetic resuits associated with the Tangshan earthquake (M 7.8, 28 July 1976, Hebai Province) and the observational results of geomagnetic effects associated with underground nuclear explosion and water storage in the Miyun reservoir, =

2. Seismomagnetic studies related to the Tangshan earthquake A severe earthquake (M 7.8) occurred in Tangshan city, Hebai Province, on 28 July 1976. We investigated the deep conductivity structure for the Tangshan earthquake to determine whether a relationship existed between this and the earthquake. We analyzed data for short-period geomagnetic variations such as the sudden commencement (SC) and geomagnetic bay, the spectrum of geomagnetic variations of quiet days and disturbed days, and geomagnetic storms at 25 geomagnetic stations (locations shown in Fig. la) during 1973—1977. Figure lb shows the Wiese vectors at various stations around the Bohai Sea area; the solid lines denote the Wiese vectors for the SC events and the dotted ones the bay events. The Wiese vector method for electromagnetic induction is used to study roughly the trend and space distribution of a high-conductivity layer. Using many episodes of short-period geomagnetic variations, A and B coefficients (A and B generally depend on frequency and are called the electromagnetic transfer functions) are obtained by the leastsquares method, according to the formula ~ Z At~ H + Bz~ D ~l’ =

=

“

‘

where, for a particular event, z~H,~Z and 1~tD are respectively the variation amplitudes of horizontal and vertical components and the declinalion. Taking A and B as north and east components respectively, the Wiese vector is found. The physical meaning of the Wiese vector is that it is perpendicular to the trend of the anomalous conductivity belt and its direction is away from the high-conductivity belt (Untiedt, 1970). The space distribution of the Wiese vectors shown in Fig. lb thus means that Changli station (CL) lies in the

positive anomalous area of short-period geomagnetic variations and Yiantai station (YT) lies in the negative area, and that there exists an anomaly of underground conductivity in the Bohai Sea area and the trend of the high conductivity layer generally is east—west. Figure ic shows the frequency dependence of the anomaly in short-period geomagnetic variations around the Bohai Sea area. It can be seen from Fig. ic that the anomaly is most prominent at periods of 2 min—2 h, and vanishes at periods > 3 h. According to the above space distribution and frequency dependence of this anomaly, we found, by applying the numerical theory to the problem of three-dimensional electromagnetic induction (Qi et a!., 1981), an underground conductivity structure which could generate the anomalous variations. We found that there is local uplift of the high-conductivity layer in the upper mantle around the Bohai Sea area because the anomaly is most prominent at periods of 2 min—2 h and the depth of the Bohai Sea is <200 m. Its geographic location is shown by the dashed rectangle in Fig. lb. It can be seen from this figure that the Tangshan earthquake epicenter is near the northern margin of uplift of the high-conductivity layer (Qi et a!., 1982). Figure 2 shows the changes in the Wiese vector and its deviation vector with time. The deviation vector of the Wiese vector can be found by taking the difference of A from its mean as the north component and the difference of B from its mean as the east component (Rikitake, 1979). It is seen ~that changes in the amplitude and direction of the Wiese vector at the Changli station, which is near the Tangshan earthquake epicenter (about 80 km), are fairly obvious during the half-year before the earthquake occurrence. In June 1976, in particular, the amplitude was greatest 0.29), and the Wiese vector was to the east. During February— July 1976, the amplitude of the deviation vector at Changli station was greater than the mean value, and the direction was basically to the north during the former 3 months and tended to the east during the latter 3 months. However, the Wiese vector and its deviation vector at Beijing, Qingguang and Hongshan stations, which are far from the Tangshan epicenter, did not show a similar (—

)

13 (a)

(b I

ID

1150

120

115°

120°

SY.

CD

::

Sc .400

CY

MY

*

~

BX

CL

*

4~O

•NH

•

JHTG

I

~

DL

,.

/

Boha~ Sea

XZ

I

DZ HS

vi. I

WF

TA

I 2Okr,

0 I

350

I

350

Cc) I~Z/AH

4ZC//~ZB 9 SC

0

oStorm 0.3J S Disturbed day 0.21A Quiet day

3 0

20

0

o

Station

°

o

• 5

Station

‘Beijing °

S

Changlj

10

• 20

~T(min) 60

100

OO~

A

1.0I

0.2

I

0.5

2

~

I

I

4

6

~

~ l~(h)

8

10

18

28

48

Fig. 1. Anomalies of short-period geomagnetic variations in the vicinity of the Tangshan earthquake area. (a) Locations of the Tangshan earthquake epicenter (*) and geomagnetic Stations. The abbreviations of the Stations are as follows: DL, Dalian; SY, Shenyang; CY, Chaoyang; CD, Chengde; SC, Shacheng; XT, Xiaotangshan; BJ, Beijing; FS, Fangshan; MY, Miyun; TX, Tongxian; XJ, Xiji; BD, Baodi; CL, Changli; NH, Ninghe; TG, Tanggu; QG, Qingguang; JH, Jinghai; XZ, Xuzhuangzi; BX, Baxian; CZ, Cangzhou; HS, Hongshan; DZ, Dezhou; YT, Yiantai; WF, Weifang; TA, Taian. (b) Distribution of Wiese vectors (solid line is for SC events, dotted line is for bay events) and projection of the underground high-conductivity layer at the Earth’s surface (dotted line rectangle). (c) Frequency dependence of the anomaly. ~ Z~and ~ Z~,denote the amplitudes of vertical component variation at Changli and Beijing stations, respectively.

anomalous change (Zhan et al., 1987). Gong and Wu (1986) and Chen et a!. (1986) also obtained similar information on the short-period seismomagnetic effect for the Tangshan earthquake. With regard to the seismomagnetic effect of the Tangshan earthquake, Sun and Lu (1982) analyzed the geomagnetic data at the stations and sites around Tangshan (Fig. 3a) and obtained the re-

sults shown in Fig. 3b and c. The standard deviation of the geomagnetic data is 2.5 nT. The results show that the anomalous changes in the geomagnetic vertical component were about —12 nT at the Tangshan site (site 8 in Fig. 3) and 10 nT at the Changli station (CL in Fig. 3) before the Tangshan earthquake. The anomalous changes in the geomagnetic total intensity were 8 nT at the Ninghe site (site 3) and —10 nT at the Dongtitou —

—

14 0.2

(a)

~A/I /A I ~ \\\ /11 /////\ i//I 1\ I ti/It ~/1 \ ~MSO7

7/I/i

8

CL

/

BJ

A

,?

/~R

,~ \

\\\ t\

t

/

A

~

t~Ii

~

\\

QGI HS

tk

~

k

t+ ft I

~

t1~ft t\t fl M /~t\

INt\ /t 11th ft\\\\ 1973

1974

1\ ti/ti t~A\ 11\\ A Ii

t~

1975

1976

1977

Ms=7 .8 (b)

r

CL BJ

J

QG

L’’~~’

HS

~

‘~

‘

—i’

~----~~--~‘—‘-----‘

~

~

/

‘-“

~

~

~

~

~

~•-‘

.—--.‘..~

‘I~

~

~_1_

~•-

~

~

1973 1974 1975 1976 1977 Fig. 2. Changes in Wiese vector (top) and its deviation vector (bottom) at various stations. The arrow denotes the time of occurrence of the Tangshan earthquake.

site (site 5) before the earthquake. It can be seen that these changes are more than twice the standard deviation (2.5 nT), and such changes before the earthquake are significant anomalies. However, no obviously anomalous change occurred at other geomagnetic stations and sites. To study the seismomagnetic effect and earthquake prediction, we set up a dense geomagnetic survey network (108 sites) in the Beijing area in 1975 (Fig. 4a). There are 10 profiles altogether, which cross the main active faults—the Dachang and Babaoshan faults. The distance between adjacent profiles is 20—30 km, and that between adjacent sites is 5—10 km. We observed the —

—

total intensity of geomagnetic field several times a year by proton precession magnetometers. The Tangshan earthquake (M 7.8, 1976) occurred in the eastern part of the Beijing network. Because of noise in the survey data (the standard deviation of the survey data is 1.2 nT), the simple difference method cannot discern an observable influence of the Tangshan earthquake on geomagnetic total intensity in the Beijing area. However, to improve the signal-to-noise ratio we analyzed statistically the data from the densely distributed sites in Beijing area and the abundant geomagnetic data before and after the earthquake. This allows us to detect seismomagnetic information after eliminat=

15

(a)

(b)

-117°

Ms=7.8

‘2IIITTIIIIIIII1II

El)

2

•

913

7

6

3

9.

4

——

—_-f~~

—

~—

-

5 -

2OnT

6 ~—

7---=-=-----------------

A HE TA

0

100 ~ I

8

I

— — — —

10

~

—

— ~

— —

CL I

I I

1974

Cc)

I I

1976

I

I

I

1978

MS~7.8

—~

2

~

~

_4

2OnT

—

-

_7__

—

_!

~

—~--—

-~

-

-

---------------------------

—~--—--—-~--------~------

1975

--

1977

—

-

1979

Fig. 3. Changes in geomagnetic field at various stations and sites before and after the Tangshan earthquake. (a) Locations of the Tangshan epicenter (*), geomagnetic stations (A) and sites. The abbreviations of the stations and sites are as follows: 1, Majiadian; 2, Shengfang; 3, Ninghe; 4, Baigezhuang; 5, Dongtitou; 6, Tanggu; 7, Wenan; 8, Tangshan; 9, Qikou; 10, Cangzhou; CL, Changli; BD, Baodi; BJ, Beijing; HS, Hongshan; TA, Taian; QG, Qingguang. (b) Changes in the vertical component Z. (c) Changes in the total intensity F.

16 (a) 116°

117°

118°

40° —

Ms~7.8

I

I

(b)

0

I

*

2010’i

Cc)

Pls—7.8

_

1

_

33

2~ 9

I

I

I

1975

1976

9

11 I

1977 I

I

I 1977

1975 1978

4 nT~

_

7

4nT

1979

1980

I

1976

I

I

_I

1975

1976

1981

Cd) S

8

N=5

6

_

7

_____________

8

J1

0.2

—__-~-—---_...

I

1975

I

1111

1976

1975

1976

Fig. 4. Statistical information on the seismomagnetic effects of the Tangshan earthquake. (a) Locations of the Tangshan epicenter (*), and survey sites for S and R regions in the Beijing survey network. The rectangle denotes the location of Beijing, and the dashed lines denote the main active faults: 1, the Dachang fault; 2, the Babaoshan fault. (b) Changes of statistical parameter S,,,~,with time (the dotted line is for Feb. 1975—Feb. 1977; the solid line is for Jan. 1977—March 1981). (c) Changes of statistical parameter S,,,, with time in the S and R regions. (d) Changes of statistical parameter b 3~with time in the S and R regions.

17

ing more effectively the disturbances caused by random factors (Fan et a!., 1984). The results of

survey number 1 to n. b.~was computed by the least-squares method. Then b~0was obtained from

the statistical analysis are shown in Fig. 4b—d (Ren et al., 1984). The Smj and b50 in Fig. 4 are statistical variance parameters.

b5~ 1/M ~ (bin

Smj

=

l/M~

2

(Jim

I

—

b~)

(4)

1

where b~ l/M~ b

(2)

f1)

2

M =

=

1~

where M denotes the total number of survey sites, i denotes site number, and m and denote the survey number;

It is seen from eqns. (2) and (4) that Smj is a statistical variance parameter of geomagnetic at various sites in survey number m relative to

F~ F. Jim ‘~im Fm ~ Here F~and F~ are the geomagnetic total intensity at site i in survey number j and survey number m respectively;

in survey number j, and that ba,, is a statistical variance parameter of rates of change b. at various sites relative to the mean b~. Smj and b5~ could reflect statistical behavior of the change of time—space of the geomagnetic field. The statistical results and statistical test show that anomalous changes in the statistical parameter Smj (Fig. 4b) are obvious before the Tangshan earthquake. It can be seen in Fig. 4 that anomalies of the statistical parameter Smj (Fig. 4c) are more obvious in the S region, which is closer to the epicenter, than in the R region, which is further from the epicenter (Fig. 4a). Before the earthquake, the change

I

=

=

. —

—

M =

l/M ~ F,~

f,m

M

Fm

=

1/M ~

‘~im

i—i

here and Fm are the reference fields in survey number j and m respectively. If is a linear function of j, i.e., f = a +b (3) In]

I]

where b1~denotes the ratio of change of the geomagnetic field at site i during the period from

of b2~in the S region is obvious, but there is no obvious change of b~in the R region (Fig. 4d).

~ 0

1

20km

Fig. 5. Locations of the point of the underground nuclear explosion (*) and geomagnetic sites.

18

Therefore there was a greater anomaly in the region which is closer to the epicenter before the Tangshan earthquake.

3. Geomagnetic changes before and after an underground nuclear explosion To investigate the piezomagnetic effect of an underground nuclear explosion, we set up 17 geomagnetic sites in the underground nuclear test field in West China. During September—October 1983, we observed the total intensity of the geomagnetic field. Figure 5 shows the locations of geomagnetic sites. Site 1 is the nearest to the point of the underground nuclear explosion, at about 3.8 km. Site 17 is the furthest from this point, at about 140 km. The distance between adjacent sites for sites 3—12 is 3—5 km, and for sites 12—17 is 20 km. We used model G-826 and G-816 proton precession magnetometers from Geometrics (U.S.A.). At sites 1 and 2, model G-826 magnetometers were used, and were operated at sampling rates of once every 1 and 5 mm respectively from 19 September to 10 October 1983. At other sites, model G-816 magnetometers were used, and there were six surveys before and two surveys after the underground nuclear explosion. To investigate in detail the geomagnetic changes before and after the explosion, synchronous observations were made every 10 s, using G-826 magnetometers at sites 1 and 2, and G-816 magnetometers at sites 12 and 14. There was no significant electromagnetic noise at any of the sites. The geomagnetic field gradients around all sites were <1 nT m The magnetometers were reliable and stable. The statistical result of the observational data shows that the mean standard deviation is 0.46 nT. Table I shows changes in the differences of geomagnetic total intensity at various sites before and after an underground nuclear explosion. In Table I, t~F~ denotes the mean difference between site 2 and other sites before the explosion, ~F2 denotes the mean difference between site 2 and other sites after the explosion, and 8J ~F2—~F1. It is seen that the change 8f in mean differences of geomagnetic total intensity before and after the explosion increases regularly from 0.8 nT at site —

~.

TABLE I Changes in geomagnetic differences (in nT) at various sites before and after the underground nuclear explosion Site ~F 1 S.F2 6f No. 3 4 5 6 8 9 10 11 12 13 14 15 16 17

—13.8±0.52 18.6±0.44 —15.7±0.44 1.7±0.54 —9.9 ±0.47 —6.8 ±0.42 —10.4 ±0.56 17.6±0.53 44.0±0.59 56.6±0.49 94.5±0.52 18.4±0.60 —60.0±0.72 21.8±0.56 35.4±0.70

—14.6±0.49 19.0±0.29 —16.1±0.40 1.5±0.35 —10.1 ±0.39 —6.8 ±0.26 —10.7 ±0.47 17.4±0.46 44.2±0.43 57.0±0.41 95.0±0.36 18.8±0.42 —59.5 ±0.18 21.5±0.38 36.5±0.49

—0.8 0.4 —0.4 —0.2 —0.2 0.0 —0.3 0.2 0.2 0.4 0.5 0.4 0.5 0.3 1.1

___________________________________________ 3 to 1.1 nT at site 17, except for sites 8 and 16. Site 2 was the reference site because there was no distant station at which geomagnetic total intensity was synchronously recorded. Site 2 was only about 30 km from the explosion point, and may have been affected by the explosion. Therefore, the values in Table I may not represent the real effect of the explosion. On the whole, however, it is seen in Table I that the maximum anomaly in the changes before and after the explosion is 1.9 nT and the mean value is 1 nT. Comparing site locations in Fig. 5 and 6f in Table I, ~f at various sites and the distances between various sites and the explosion point show a relationship. Figure 6 shows the changes in the synchronous differences of the data with a 10-s sample (Fig. 6a) and a 5-mm sample (Fig. 6b) at sites 1, 12 and 14 relative to site 2. It can be seen that there is no obvious anomaly before and after the underground nuclear explosion, but an anomalous change of 2—3 nT occurs during the explosion time (Zhan et a!., 1985). —

—

.

.

4. Tectonomagnetic expenment at the Miyun re-

=

—

servoir In this experiment we studied the possible tectonomagnetic effect of changes in water level of

19

110 12 14

nT

(a)

~

....1.0I

~

~

~

~...

.0..

-

..

-~:-------~~-

~

.1,1

—30

.

...-.. .-.

.

-:

-I.-.

- ~

.-

.

jt(min) 30

0

1--:.

.........:..

.....,

.....~

-,:.

..,.

..

.,

...

.

------S

t(hour) —18

-12

,—6

6

0

12

18

Fig. 6. Changes in synchronous differences of geomagnetic total intensity for a 10-s sample (a) and a 5-mm sample (b) at various sites before and after the underground nuclear explosion.

the Miyun reservoir, which is in the northern part of Beijing. The northern part of the reservoir is shallow and the southern part is deep. The maximum depth is 63.5 m. The north bank of the reservoir has some electromagnetic noise, so we did not set up any sites there. Figure 7 shows the locations of geomagnetic sites around the Miyun reservoir. There were 21 sites. The distance be—

I 1

•

tween adjacent sites is 2—5 km. The distances between the sites and the reservoir bank are in range of tens of meters and 15 km. Using model G-816 and G-826 magnetometers, the total intensity of geomagnetic field was measured once each season from 1983 to 1987. Site 0 (Fig. 7) was taken as the reference site. A model G-826 magnetometer was set up at this site and operated at a recording rate of once each minute. The survey at sites 1—20 used model G-816 magnetometers. The standard deviation of the survey data is 0.81 nT. Figure 8a shows the geomagnetic changes of 6J between the adjacent surveys in March and July 1984 at 15 sites near the Miyun reservoir. The variation of the water level in the reservoir beTABLE II

13

7

•

~

Relation between geomagnetic changes and variations of water storage in the Miyun reservoir

12

Time

11

15 •1e

•

20

,

I

I

Fig. 7. Locations of geomagnetic sites around the Miyun reservoir.

Mar.—July 1984

June—Sept. 1985

Aug. 1986— Jan. 1987

8.1 (nT) —1.3 ±0.9 1.1 ±0.9 0.5 ±0.8 8h (m) 3.72 —5.30 —2.59 8 m3) 3.02 —3.62 —2.43 6v (nTm’) (1x10 flu —0.35±0.25 —0.20±0.17 —0.28±0.25 fv (1X108 nT m 3) —0.44 ±0.30 —0.30 ±0.23 —0.30 ±0.26 _________________________________________________________

20 (~)

(b)

~

~18

-2.5

—0.81

?

9.

.~‘

Fig. 8. (a) Changes in geomagnetic total intensity (in nT) at various sites from March to July 1984. (b) Relation coefficients fli (above 3) at various sites during March—July 1984. the line, nT m’) and fv (down the line, lx 108 nT m

tween the two survey periods is greater , 6J = 3.7 m, in March and July 1984, and the variation of the water volume is greater, ~v = 3.0 x 108 m3. Figure 8b shows the relation coefficients liz and fv between the geomagnetic changes 6f and vanations of the water level and the water volume in the reservoir dunng March and July 1984, i.e., Jli ôf/8h, fv = ~J/3v. Averaging the results at the 15 sites near the Miyun reservoir, the mean values are obtained: 8f= —(1.3±0.9)nT =

Jh

=

=

fl~=—(0.28+0.22)nTm —

and ~-

—8

Jv=—~0.35±0.3l)X10 nTm

—3

These results show that there is a negative relation between the geomagnetic changes and the vanations of the water level in the Miyun reservoir.

—(0.35 ±0.25)nT m~

5. Conclusion

—(0.44 ±0.30)

Anomalous seismomagnetic changes occurred before the Tangshan earthquake (M = 7.8, 1976)

and Jv

the relation coefficients are therefore obtained (Zhan et al., 1989):

X

10~nT m

Table II denotes the relation between the geomagnetic changes and the variations of the reservoir water in the Miyun reservoir during March—July 1984, June—September 1985 and August 1986—January 1987. The magnetic field changes could be correlated with water level changes during these periods. The mean values of

in the total intensity and vertical component of the geomagnetic field, and in long- and short-term variations of geomagnetic field both in the vicinity of the Tangshan earthquake and in the Beijing area. This suggests that there is probably more seismomagnetic information for a severe earthquake. For the Tangshan earthquake, the space

TABLE III Geomagnetic effect of underground nuclear explosions Place

Time

Magnitude

Cannikin, U.S.A.

1971

M 5 = 5.74

Medeo, U.S.S.R. West China

1983

M

=

4.7

Distance (km)

Geomagnetic change (nT)

Reference

3.0 0.7 3.8—140

9 8 2—3

Hasbrouck and Allen (1972) Abdullabekov et al. (1972) Zhan et al. (1985)

21 TABLE IV Observation results of tectonomagnetic experiments around several reservoirs Place Talbingo, Australia Charvak, U.S.S.R.

Maximum depth (m) 140

New Melones, U.S.A. Miyun, China

108 63.5

Sites

Time

16 35

1971—1972 1975 1976

6 21

1983—1987

Survey times 15

15

—

=

—

fv

=

—(0.72 ±0.30)

(0.103 ±0.125)nT m~

x

108

1) (nT m — 0.025

fv (108 nT m3) —1.09 —0.71

extent of the seismomagnetic effect is <200 1cm, and the magnitude is 10 nT. In addition, the above results suggest that the geomagnetic method perhaps has good potential as a tool in the search for long-, medium- and short-term earthquake prediction. Table III shows the results of observation of geomagnetic total intensity before and after underground nuclear explosions in the U.S.A., U.S.S.R. and China. It can be seen that the geomagnetic effect in West China is smaller. This is associated with the smaller explosion, the greater distance of the sites from the explosion point and weak magnetization of the underground rock in the test field. Table IV shows the results of observation of tectonomagnetic experiments near several reservoirs. It can be seen that the relation coefficient Jli between geomagnetic changes and the variation of the water level in the Miyun reservoir is greatest. The relation coefficient Jv between the geomagnetic changes and the variations of the water volume in this reservoir is nearly the same as the result reported by Shapiro et al. (1978). From Table IV the mean values are obtained: f/z

flu

nT m3

The negative relationship is in agreement with the piezomagnetic theory. For earthquake prediction by the geomagnetic method, it would be necessary to set up a dense geomagnetic network in a seismically active area and to use magnetometers of high accuracy and high stability. Observations should be made of the total intensity of the geomagnetic field, and of

—0.04±0.02 —0.28±0.22

—0.35±0.31

Reference Davis and Stacey (1972) Shapiro et al. (1978) Zhan (1986) Zhan et al. (1989)

each of its three components and of its short-period variations.

Acknowledgements We thank Dr. Malcolm Johnston and the two reviewers for their kind help and useful suggestions. We thank the Joint Seismological Science Foundation of China for its support.

References Abdullabekov, L.S., Golovkov, V.P. magand Skovorodkin,K.N., Yu.P.,Bezuglaya, 1972. On the possibility of using netic methods to study tectonic processes. Tectonophysics, 14: 257—262. Chen Shaoming, Xie Meijuan and Jia Yuzhi, 1986. Anomalous variation of short-term geomagnetic disturbance ~ Z/~S.H before and after 4(4): 147—150 (inearthquake. Chinese). North China Earthquake Sci., Davis, P.M. and Johnston, M.J.S., 1983. Localized geomagnetic field changes near active faults in California, 1974—1980. J. Geophys. Res., 88: 9452—9460. Davis, P.M. and Stacey, F.D., 1972. Geomagnetic anomalies caused by a man-made lake. Nature (London), 240: 348—349. Davis, P.M., Pierce, D.R., McPherron, R.L., Dzurisin, D., Murray, T., Johnston, M.J.S. and Mueller, R., 1984. A volcanomagnetic observation on Mount St. Helens, Washington. Geophys. Res. Lett., 11: 225—228. Fan Guohua, Hou Zouzhong, Zhan Zhijia and Huang Pingzhang, 1984. The influence of the Tangshan earthquake on the geomagnetic total field in the Beijing area. In: Earthquake Prediction. Terra Scientific Publishing, Tokyo, Japan, PP. 137 144. Gong Shaojing and Wu Zhanfeng, 1986. Possible changes of earth conductivity accompanying the Tangshan earthquake of 1976. Acts Seismol. Sin., 8: 28—36 (in Chinese, with English abstract).

22 Hasbrouck, W.P. and Allen, J.H., 1972. Quasi-static magnetic field changes associated with the Cannikin nuclear explosion. Bull. Seismol. Soc. Am., 62: 1479—1487. He Churn, Pen Chunyi, Zhang Siwei, Xu Guoming, Guo Juaxin, Zhang Zhitian and Lu Zhenfei, 1983. The geomagnetic anomaly and the characteristics of the rupture process in the fault plane during the Liyang earthquake (M 5 = 6.0). Seismol. Geol., 5(2): 52—58 (in Chinese, with English abstract). Johnston, M.J.S., 1978. Tectonomagnetic effects. Earthquake Inform. Bull., 10: 82—87. Johnston, M.J.S., 1986. Local magnetic fields, uplift, gravity, and dilational strain changes in southern California. J. Geomagn. Geoelectr., 38: 933—948. Johnston, M.J.S., Mueller, R.J. and Dvorak, J., 1981. Volcanomagnetic observations during eruptions, May— August 1980. U.S. Geol. Surv. Prof. Paper, 1250: 183—189. Kozlov, A.N., Pushkov, A.N., Rakhinatullin, R.Sh. and Skovorokin, Yu.P., 1974. Magnetic effect during explosions in rocks. Izv. Akad. Sci. S.S.S.R., Earth Phys., 66—71 (English edition).. Mobile Geomagnetic Group, Seismological Bureau-of Yunnan Province, 1984. Geomagnetic variation of the Jianchuang earthquake (M = 5.3). J. Seismol. Res., 7: 417—424 (in Chinese, with English abstract). Nagata, T., 1969. Tectonomagnetism. Int. Assoc. Geomag. Aeron., Bull., 27: 12—43. Peng Chunyi and Xu Guoming, 1986. Geomagnetic effect during the earthquake of Huanghai, near Jiangsu Province, Acts Seismol. Sin., 8: 309—316 (in Chinese, with English abstract). Qi Guizhong, 1978. On the dilatancy—magnetic effect. Acta Geophys. Sin., 21: 18—33 (in Chinese, with English abstract). Qi Guizhong, Hou Zuozhong, Fan Guohua and Zhan Zhijia, 1981. On the seismomagnetic induction effect (2). Acta Geophys. Sin. 24: 296—308 (in Chinese, with English abstract). Qi Guizhong, Zhan Zhijia, Hou Zuozhong, Fan Guohua, Wang Zhigang and Bai Tongxia, 1982. Anomalies of short-period geomagnetic variations and distribution of high conductivity layer in the Bohai Sea area. Sci. Sin., B, 25: 214—226. Ren Xixian, Qi Guizhong and Zhan Zhijia, 1984. The change of geomagnetic total field in Beijing area before and after the 1976 Tangshan earthquake. Acta Seismol. Sin., 6: 271—286 (in Chinese, with English abstract). Rikitake, T., 1979. Change in the direction of magnetic vector of short-period geomagnetic variations before the 1972 Sitka Alaska Earthquake. J. Geomagn. Geoelectr., 31: 441445. Rikitake, T. and Honkura, Y., 1985. Solid Earth Geomagnetism. Terra Scientific Publishing, Tokyo, Japan, pp. 171—191.

Rikitake, T., Honkura, Y., Tanaka, H., Ohshiman, N., Sasai, Y., Ishikawa, Y., Koyama, S., Kawamura, M. and Ohohi, K., 1980. Changes in the geomagnetic field associated with earthquakes in Izu Peninsula. Jpn. J. Geomagn. Geoelectr., 32: 721—739. Shapiro, V. and Abdullabekov, K., 1982. Anomalous variations of the geomagnetic field in East Fergake—magnetic precursor of the Alay earthquake with M = 7.0 (1978, November 2). Geophys. J. R. Astron. Soc., 68: 1—5. Shapiro, V.A., Pushkov, A.N., Abdullabekov, K.V., Berdeliev, E.B. and Mununov, M.Yu., 1978. Geomagnetic investigations in the seismoactive regions of Middle Asia. J. Geomagn. Geoelectr., 30: 503—509. Sun Romei and Lu Zhenye, 1982. Partial anomaly of the secular variations of the geomagnetic field before the 1976 Tangshan earthquake. J. Seismol. Res., 5: 397—408 (in Chinese, with English abstract). Untiedt, J., 1970. Conductivity anomalies in central and southem Europe. J. Geomagn. Geoelectr., 22: 131—149. Working Group of Geomagnetic Research, 1986. Observation and research of seismomagnetic phenomena and earthquake prediction in China. Earthquake Res. China, 2(2): 1—6 (in Chinese, with English abstract). Zhan Zhijia, 1986. Observation and research of tectonomagnetic effects in U.S.A. In: Song Shouquan, Zhang Hongyou, Mao Tongen and Zhan Zhijia (Editors), Seismomagnetic Research in the World. Scientific and Technical Paper Press, Beijing, China, pp. 18—27 (in Chinese, with English abstract). Zhan Zhijia, 1987. Seismomagnetic phenomena of the Tangshan earthquake. North China Earthquake Sci., 5(6) 307—309 (in Chinese). Zhan Zhijia, 1988. Seismomagnetic phenomena and their physical basis. In: Scientific and technical Department of State Seismological Bureau (Editor), The Results of Earthquake Monitoring and Prediction Method—Geomagnetism and Geoelectricity. Earthquake Press, Beijing, China, pp. 88—107 (in Chinese). Zhan Zhijia, Hu Rongsheng, Zhang Hongli, Zhao Congli, Shen Wenzhi and Gao Jintian, 1985. Test observations of geomagnetic total intensity before and after underground nuclear explosion. Seismol. Geomagn. Obs. Res., 6(4): 42—45 (in Chinese). Zhan Zhijia, Bai Tongxia, Gao Jintian, Feng Changhua and Zhang Yumin, 1987. Analysis on the change of geomagnetic short-period variations before and after Tangshan earthquake. J. Seismol. Res., 10: 311—322 (in Chinese, with English abstract). Zhan Zhijia, Gao Jintian, Hu Rongsheng, Zhang Hongli, Zhao Congli and Shen Wenahi, 1989. Tectonomagnetic experiment around Miyun reservoir Acta Seismol. Sin., 11(4) in press (in Chinese, with English abstract).