Earthquake-related ground motion and groundwater pressure change at the Kamaishi Mine

EGEqEERNG GEOLQGV ELSEVIER

Engineering Geology 43 (1996) 107-118

Earthquake-related ground motion and groundwater pressure change at the Kamaishi Mine Isao Shimizu a,,, Hideaki Osawa a'l, Toshihiro Seo b, Shinji Yasuike c, Syunji Sasaki c a Kamaishi Site Office, Power Reactor and Nuclear Fuel Development Corporation (PNC), Kamaishg Iwate, Japan b Tono Geoscience Center, Power Reactor andNuclear Fuel Development Corporation (PNC), TokL Gifu, Japan c Abiko Research Laboratory, Central Research Institute of Electric Power Industry, Abiko, Chiba, Japan Received 30 June 1995; accepted 20 March 1996

Abstract

The Power Reactor and Nuclear Fuel Development Corporation (PNC) has been conducting studies on earthquake effects at the Kamaishi Mine. The aims of the studies are to observe the attenuation characteristics of ground motion with depth, and to understand the influences of earthquakes on the deep groundwater. Seven seismographs have been installed at four different levels of the mine with depths varying from 0 to 600 m. According to the observations since January 1991, maximum acceleration recorded deeper than 150 m has a tendency to decrease to 1/4 to 1/2 of that at the ground surface. We also monitored water pressure in boreholes, inflow rate and electric conductivity of groundwater from drift wall, and the water chemistry before and after earthquakes. Twenty cases of earthquake-related changes in water pressure have been observed during the period from November 1991 to December 1994. The range of groundwater pressure changes are generally less than 0.1 kgf/cm2 with a maximum of 0.35 kgf/cm2. Almost all these changes tend to recover slowly to the original state within about one week. In these twenty cases the static crust strain calculated after Dobrovolsky et al. (1979) (Estimation of the size of earthquake preparation zones. Pure Appl. Geophys., 117: 1025-1044) from the magnitudes and epicentral distances are larger than 10 -s. As interseismic variation, the annual groundwater pressure change is less than 1 kgf/cm2, which corresponds well with the rainfall record.

1. Introduction

The P N C is conducting a comprehensive geoscientific research p r o g r a m to build a firm scientific basis for the safe disposal of high level radioactive waste in deep geological formations.

* Corresponding author. 1Present address: Nuclear Materials Division, Nuclear Safety Bureau, Science and Technology Agency, Chiyoda, Tokyo, Japan. 0013-7952/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0013-7952(96) 00054-3

Since 1988, in situ experiments in fractured rock have been performed at the Kamaishi Mine to try to understand the deep geological environment. Earthquake-related studies at the Kamaishi Mine have been carried out since 1990. The main objectives are to study model attenuation of ground motion due to earthquakes and to evaluate the influence of earthquakes on groundwater. In this paper we discuss some of the results concerning the attenuation of ground motions with depth and the groundwater pressure changes in boreholes in the Kamaishi Mine.

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2. Geological setting The Kamaishi Mine is located approximately 600 km north of Tokyo. The bedrock in the area consists mainly of Paleozoic to Mesozoic sedimentary formations, and early Cretaceous granitic rocks (Fig. 1). The Ganidake igneous complex is mainly composed of granodiorite with minor amount of diorites and monzonites. The Kurihashi granodiorite is mainly composed of granodiorite and is distributed near the Ganidake igneous complexes. Skarns associated with the Ganidake granodiorite

host the Fe-Cu ore bodies of the Kamaishi Mine along its contact with the sedimentary rocks. The main experimental area is situated at the northern end of the EL. 550 m drift (550 m above sea level) located at a depth of about 300 m from groundsurface, and the drift branches off to the northeast and northwest directions (NE and NW drifts). The NE drift runs mainly in diorite and the NW drift in granodiorite. Both areas have fractures developed mainly in the east-west direction. The KO-10 point of the NE drift is the main site for earthquake-related studies.

Table 1 Monitored parameters Items

Location

Rock type

Remarks

Water pressure

Borehole: drilled at end of 550 m drift KWP-I: length 458 m, horizontal KWP-2: length 393 m, - 2 0 N W KWP-3: length 540 m, horizontal Drift wall: 550 m NE drift KO-10: fractures of E--W direction Drift wall: 550 m NE drift KO-10: fractures of E W direction Drift wall: 550 m NE drift KO-10: fractures of E - W direction

Granodiorite/ sedimentary rock

Strain type, water pressure meters

Diorite

Weighing inflow by load cell

Diorite

Electrical conductivity (converted to 25°C) Na, K, Ca, Mg, C1, SO4 H C O 3, SiO 2

Inflow rate Electrical conductivity Chemical components

Diorite

I. Shimizu et al./Engineering Geology 43 (1996) 107-118

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3. Observation methodology

3.1. Seismological observations Seismological observations have been conducted since February 1990 to measure the attenuation characteristics of the ground motion at different levels in the Kamaishi Mine drifts (Fig. 2). Seismographs K-l, K-2, K-3 and K-4 were installed on February 1990, K-5 and K-6 were installed in January 1991. K-I, K-5, K-2 and K-6 seismographs are set on a vertical line. The K-6 seismograph is set at the deepest drift, approximately 650 m below the ground surface. The trigger level of seismographs K-1 to K-4 is 0.3 gal while that of K-5 to K-7 is 0.5 gal.

3.2. Monitoring of hydrology and chemistry of groundwater The hydraulic and chemical characteristics of the groundwater have been observed at the locations indicated in Fig. 2. The monitored parameters are as listed in Table 1.

3.2.1. Monitoring of waterpressure Water pressure is monitored in three boreholes, which were drilled for mineral exploration during 1971 to 1973. These boreholes, located in the EL. 550 m drift approximately 300 m below ground surface, encounter sedimentary rocks at about 120 m from the drift wall. The monitoring was started in February 1990 by installing water pressure meters (resolution of approximating 0.001 kgf/cm2) of a strain guage type with measurement range of 0 to 50 kgf/cm2. Measurement interval is set at 10 s.

3.2.2. Monitoring of inflow The monitoring point of inflow is located at KO-10 in the EL. 550 m drift. The inflow from a drift ceiling dripped onto a vinyl sheet of about 2 m E size, and was introduced into a measuring tank (the size of the vinyl sheet was changed to about 1.5 m 2 from May 1993). The weight of the tank is measured by a load cell with a measurement range of 0-20 kg and a resolution of approximately 0.1 g. The tank is equipped with a siphon to drain

1. Shimizu et aL/Engineering Geology 43 (1996) 107-118

110

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Fig. 3. Epicentral distribution of 46 earthquakes observedduring the period from January 1994to December 1994. The earthquakes larger than ~0.5 gal on the K-2 seismograph are plotted. excess water automatically. The measurement interval is set at 10 min.

3.2.3. Monitoring of electrical conductivity of groundwater The monitoring point of electrical conductivity of groundwater is located at KO-10. The measurement error of the electrical conductivity is approximately 0.2 llS/cm. The data are converted automatically into those for water temperature of 25°C. The degree of electrical conductivity is highly dependent on water temperature, but the temperature of the inflow is stable at about 10°C. The measurement interval is set at 30 min.

two 100 cc bottles), which rotate in a cycle of 120 h. An earthquake motion of over 4 gal is detected in the drift. The sampling system is set up to automatically stop after 96 h, thus we can get a rotation of the samples for the 24 h preceding the earthquake and the 96 h following the earthquake. The collected samples were analyzed in the laboratory for the chemical components listed in Table 1.

4. Results and discussion

4.1. Seismological observations 4.1.1. Seismic activity

3.2.4. Sampling and chemical analysis of groundwater An automatic sampling system collects groundwater every 4 h. This system is equipped with 30 pairs of sampling containers (one pair consists of

Two hundred and eleven earthquakes with an acceleration of 0.5 gal and over were observed during the period from February 1990 to December 1994. The epicenters of these earthquakes, according to the Japan Meteorological

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Agency, were plotted on the Pacific Ocean side of Aomori to Fukushima prefectures, especially in the east off Iwate prefecture (epicentral distance of 20-120 km). The earthquakes were mostly of a magnitude smaller than 5. Maximum acceleration observed at the surface level of the Kamaishi Mine ( K - l ) were mostly below 4 gal. The distribution of epicenters for the 46 earthquakes over 0.5 gal which occurred between January and December

1994, is shown in Fig. 3. Table2 shows major earthquakes observed at the Kamaishi Mine.

4.1.2. Comparison o f acceleration observed at surface and underground levels The ratios of maximum acceleration observed at seismographs K-l, K-5, K-2 and K-6 are shown in Fig. 4. Fig. 4 includes 41 ground motion data observed simultaneously by all four seismographs.

Table 2 Major earthquakes observed at the Kamaishi Mine Earthquake name and location

Data

Major earthquakes by magnitude East off Hokkaido 10/4/94 South off Kushiro 1/15/93 Southeast off Hokkaido 7/12/93 Major earthquakes by acceleration North of Miyagi prefecture 11/27/93 East off lwate prefecture 5/6/93 East off Hokkaido 10/4/94

Magnitude

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Epicentraldistance (km)

Maximumacceleration (gal)

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36.9 24.5 1.4

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l 12

1. Shimizu et al. ,,Engineering Geology 43 (1996) 107-I 18

The maximum accelerations at K-6 are about l to 1/2 times as large as those at K-2 and K-5, while accelerations at K-6 are only 1/2 to 1/4 times as large as those at K-I. A numerical analysis was conducted using the superposed S-wave refraction theory ( S H A K E code) to model the attenuation of earthquake motion. The "South off Kushiro" earthquake (Table 2) was quoted as the input seismic wave for S H A K E code. The results are given on the right-hand half of Fig. 5. The acceleration response obtained from the calculations corresponded closely with the data obtained from observations of both the east-west and the north-south components. It is thus possible to estimate the acceleration deep underground by using the " S H A K E " code.

4.2. Monitoring o f hydrology and chemistry o f groundwater 4.2.1. Variation

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I. Shimizu et al./Engineering Geology 43 (1996) 107-118

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of water pressure for KWP-1 which is considered to show an influence of the earth tide. In order to examine the relationship between the earth tides and water pressure fluctuation, the data for July 18-25, 1993, are shown in a greater detail. Tidal cycles shows a close correspondence to the water pressure fluctuation. The response of the water pressure spectrum to earth tides was examined using Fourier analysis. The results show that the amplitude of the water pressure fluctuation corresponding to the M2 earth tide is 6.2 cm for KWP-1,

3.5 cm for KWP-2, and 3.5 cm for KWP-3, as equivalent to water head value. According to Kawabe ( 1991), the calculated amplitude of water head corresponding to M2 earth tide using the equator conversion rate is 1-10 cm. The data for KWP-1 are also within this range. The boreholes for water pressure monitoring have been found to have a sensitive response to the earth tides. As coseismic variation, Fig. 7 also shows the change in water pressure related to an earthquake which occurred at 22:19, July 12, 1993.

L Shimizu et al...'Engineering Geology 43 (1996) 107 118

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115

I. Shimizu et aL/EngineeringGeology43 (1996) 107-118 138.0

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results indicate that the range of inflow change caused by earth tide is 0.6 cc/min or 0.3% of the total inflow of 226 cc/min. So far, no clear change has been observed in the inflow with occurrence of earthquakes. 4.2.3. Variation of electrical conductivity of groundwater The long-term variation of electrical conductivity monitored at location KO-10 is shown in Fig. 14. A comparison between electrical conductivity and the water pressure for KWP-I is also shown in Fig. 14. Electrical conductivity is low (153 IxS/cm: the measurement error is 0.2 IxS/cm) during spring to summer when the KWP-1 water

pressure is high. Likewise, electrical conductivity tends to become higher (156 BS/cm) during the winter season when KWP-1 water pressure is low. KWP-1 water pressure and electrical conductivity have a negative correlation. The value of seasonal variation of electrical conductivity is about 2%. So far, no clear change has been observed in the electrical conductivity due to occurrence of earthquakes.

4.2.4. Variation of water chemistry With regard to seismogeochemical studies before and after earthquakes, King et al. (1981) have reported on changes in ion concentrations of

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L Shimizu et aL/'Engineering Geology 43 (1996) 107-118

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N a + and SO,I- in water taken from a well. In the Kamaishi Mine, the automatic water sampling system worked with 10 earthquakes as o f December 1994. O f the 10 earthquakes, four have resulted in an apparent decrease in the ion concen-

tration o f SO42- (maximum 4 rag/l) and an increasing trend for that o f H C O 3, (Fig. 15). But it is not certain whether these change are related to the earthquake. It is necessary that further monitoring be continued•

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I. Shimizu et aL /Engineering Geology 43 (1996) 107-118

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~ o o 230 ....~: 19

.....................

i i i ~ i i i i ~ iiii ~ i i i i t ii:: . . .:~. . . . . i -• .~....t....'....~.....-....i....~.....: i i : : ~.....-....~....J.........~....-....i....L....'....i....~ ~ ! Ii :: i :: i I :: :: .... ~ .~.!....::,. i....~..., i :~ ::.....

~ 228 . c.: ...i...~ o 226 ....i.....i-..•:,•...~-....i....i.....i....:,-...4.....i....~.....i....i....+...-.i....i.....!.....i....+.....i....i....... : M :: i i :: i i i :: i i i :: i i i i i i i i ~ : ~ 224

5

10

15

20

25

Date (February 1991 ) Fig. 12. An example of a groundwater inflow record from fractures at point KO-10. The record shows a periodic fluctuation of groundwater inflow. The inflow was gathered by using a 2-m2 vinyl sheet.

o 0.5 o 19 03 0

..................... i .......................... i .......................... !...........................i ..............................................

~ -0.5 o

.....................

vvvv'v~-v

v V v vvV'VvV'~] V'VV'V'VVVV

i ......................................................

°~

~V'V'V"

. ...........................................................................

i

5

10

15

20

25

Date (February 1991) Fig. 13. Result of inverse Fourier transform of inflow data shown in Fig. 12.

~'~ 34.0 I_ E I:

.~

1140 .p~ .|120

,~,..A., ...~q,KO-10 . . . . .

I:KWP-1

I~,

4 80 o

A

-J

= 32.5

I

tD ¢O .m

n

157 E3 A

K~O~01~#~~,~

f~

•

1

156155 E

~ ~

154n =k

+

I

153 ¢D I

2

I

I

4

I

I

I

6 8 1993

|

I

I

|

I

i

10 12 2

i

i

i

i

=

I

4 6

19948

+

,

i

*

152

kU

10 12

Month (1993-1994)

Fig. 14. Long-term variations of inflow and electrical conductivity in groundwater from fractures at point KO-10 and the water pressure at KWP-1. The inflow was gathered by using a 1.5-mz vinyl sheet•

5. Conclusions F r o m the earthquake observation and hydraulic studies conducted within the Kamaishi Mine, the

following conclusions can be drawn. (1) Accelerations vary with depth, where accelerations at 650 and 150 m below ground surface are 1/2-1/4 times and 1-1/2 times, respectively, the surface values. (2) From the monitoring data of underground water pressure at depths below 300 m, water pressure changes about 1 kgf/cm 2 due to seasonal variation of rainfall and about 0.006 kgf/crn 2 due to the influence of earth tides. (3) The changes in water pressure related to earthquakes follow a step-wise change and these changes recover slowly to their original state: within about one week. These changes were mainly around 0.1 kgf/cm 2, with a maximum recording of 0.35 kgf/cm 2. (4) Earthquakes which cause changes in water pressure have theoretical earth crustal strain levels larger than 10 -8. (5) With regard to inflow and electrical conductivity, no significant changes have been observed upon occurrences of earthquakes. (6) An apparent change in the chemistry of inflowing water occurred during earthquakes. However, further study is required to ascertain whether these changes are related to the earthquakes.

118

L Shimizu et aLiEngineering Geology 43 (1996) 107-118

s0 i

the Kamaishi Mine". We thank members of the Committee. The seismic information was kindly provided by the Japan Meteorological Agency.

i' i Na

--.40-

E ~'30-

K (_.+0.01)

.... o----

C a (-t-0.1)

.... ~".... --Ew--

O

~_~..L__.I-~_.J._.Y._.

i!

t-Q20

i

....

i i

tO

O

10-

', i -32-16 0

Earthqu~ake

i

(-I-0.1)

.......e ........

Mg (±0.001) C1(_+0.02)

.... • ....

SO4 (+0.2)

___A.-.

H C O 3 (__+0.5)

--v--

SiO2 (~0.1)

( ): show

mea-

surement

errors

(mg/£

)

16 32 48 64 80 96

Hour

Fig. 15. Variations of ion concentrations of groundwater samples taken from fractures at point KO-10. Horizontal lines indicate average value, and horizontal dotted lines indicate one standard deviation. Hour "0" shows the exact time that the earthquake occurred (September 6, 1993, M=4.0, depth= 22 km, epicentral distance = 12.4 km).

Acknowledgements This study has been partially conducted under the guidance of an Advisory Committee of "The study on earthquakes in the in-situ experiments at

References Dobrovolsky, I.P., Zubkov, S.I. and Miachin, V.I., 1979. Estimation of the size of earthquake preparation zones. Pure Appl. Geophys., 117: 1025-1044. Igarashi, G. and Wakita, H., 199l. Tidal responses and earthquake-related changes in the water level of deep wells. J. Geophys. Res., 96(B3): 4269-4278. Kawabe. I., 1984. Anomalous changes of CH4/Ar ratio in subsurface gas bubbles as seismogeochemical precursors at Matuyama Japan. Pure Appl. Geophys,, 122: 194-214. Kawabe, I., 1987. Identification of seismogeochemical anomalies in subsurface gas CH4/Ar ratio: Geochemical filtering of earthquakes. Geochem. J., 21: 105-117. Kawabe, I., 1991. Hydro-geochemical anomalies associated with earthquakes. Earthquakes, 2 (44): 341-364 (in Japanese with English abstract). King, C.Y., Evance, W.C., Presser, T. and Husk, R.H., 198l. Anomalous chemical changes in well waters and possible relation to earthquakes. Geophys. Res. Lett., 8(5): 425 428. Muir Wood, R., 1994. Earthquakes, strain-cycling and the mobilization of fluids.In: Geofluids: Origin, Migration and Evolution of Fluids in Sedimentary Basins. Geol. Soc. Spec. Publ., 78: 85-98. Shigetomi, K., Yamada, M. and Fujii, S., 1992. Coseismic changes in groundwater level at Osakayama. Annu. Dis. Prev. Inst. Kyoto Univ., 35(B-1 ): 359 370 (in Japanese with English abstract).