Coseismic strain steps observed by three-component borehole strainmeters

Tecronophyms. Elsevier

144 (1987) 207-214

Science Publishers

207

B.V., Amsterdam

- Printed

in The Netherlands

Coseismic strain steps observed by three-component borehole strainmeters SEIICHI SHIMADA, SHOJI SAKATA and SHIN’ICHI NOGUCHI National Research Center/or

Disaster Preoention,

Tennodm 3. Chome, Sakura-mura,

Ibaraki-ken. (Received

January

Niihuri-gun,

Tsukubu Science City.

305 (Japan)

2, 1986: revised version accepted

May 8, 1986)

Abstract Shimada,

S., Sakata.

strainmeters. Thirteen Japan.

S. and

coseismic

strain

motions

or long-period

amplitudes

theoretical seismic

demonstrate

the

high

borehole

to determine

Coseismic

M&hanics

were‘installed

by 3-component

60 m apart,

waves.

The calculated

reliability

strainmeters

of the

strain

including

aseismic

event.

by long-period

Even mechanism surface

strain

in the Kanto

area,

steps were in good agreement

at Yasato

with

determined

steps agree with observations strainmeter

by P-wave initial

within

a factor

of 2 for

can detect steps over 0.3 nanostrain.

strainmeter.

deformation

slight

de-

When

the network

steps are observed

at several

These

of stations

stations,

of

it will be

of the earthquake.

difference

step amplitudes investigated

can only evaluate the “total including aseismic deformation

Publishers

strainmeters

borehole

144: 207-214.

existed

in the direction of printo calculated

cipal axes, and the ratios of observed

or body waves

0 1987 Elsevier Science

three-component

of less than 1000 km agree fairly well with those calculated based on the fault model. However, a

Comparisons of observed coseismic strain steps to the deformation calculated from focal mechanisms have been done for several earthquakes (for instance, Kasahara, 1974; Okada, 1980; McGarr et al., 1982; Furuya, 1984). Kasahara (1974) studied strain steps from the Off Nemuro 0040-1951/87/$03.50

by

Tectonophysics,

Peninsula earthquake of 1973 (M = 7.4) (Japan Meteorological Agency (JMA)) and concluded that the strain steps observed at the epicentral distance

cannot detect aseismic motion, if it occurs, accompanying seismic faulting. The measurement of static deformation fault mechanism” of an earthquake.

observed

from local mechanisms

and coseismic

The focal mechanism of an earthquake is determined by two methods: P-wave initial motions and long-period seismic waves. When the source process is complicated, the mechanism determined by P-wave initial motions represents only the motermination

borehole

three-component

is constructed

fault mechanisms,

steps Faulting.

and the observed

The three-component

Introduction

tion of the triggering

strain

of Earthquake

strain steps were calculated

and within 10 o for directions.

three-component possible

S.. 1987.

(Editor),

steps were observed

Two sets of strainmeters

each other. Corresponding

results

Noguchi,

In: R.L. Wesson

strain

and

from 2 to 5. Okada tilt steps

(1980)

from six earth-

quakes in and around Japan and found no empirical relationships between the observed and the calculated steps, and that the polarities of the steps were nearly random. He concluded that there remain unknown miscellaneous factors involved in strain or tilt measurements. McGarr et al. (1982) studied the coseismic strain changes ranging from 5 x lo-‘” to values exceeding 5 X lo-’ which were observed with three Sacks-Evertson borehole dilatometers for hundreds of mine tremors in the magnitude

B.V.

ranged

range

-1-3.7

and at hypocentral

dis-

208

tances

of 50 m to about

that the amplitudes strain

2 km. They

and polarities

steps were generally

with theoretical dislocation

Furuya

(1984)

with borehole

belonging

to the Japan

that the polarities

those calculated were in proportion

studied

volume

the absolute

strain

strainmeters

Meteorological

Agency differential

did not agree with

from the source

at some stations,

agreement

based on point-source

and

found

of the coseismic

in excellent

expectations

theory.

steps observed

concluded

model,

values

to the theoretically

transformer

but that

of the steps predicted

values. The biggest problem in coseismic strain step measurements is the coupling between the crust and instruments. There are many troublesome factors involved with the observations in vaults (Shichi

and Okada,

1979). Weathered

a vault loosens the coupling tensometers used for strain

section of detection

port

rock around

with the crust. Exmeasurements in a

vault have a mechanically complex detector and supporters, and this complexity also affects the coupling with the ground. In contrast, borehole measurements have some advantages concerning

Fig. 1. Schematic illustration of the three-component strainmeter.

the coupling with the crust. It is easier to find a site and to reach the depth of fresh and massive rocks. The three-component borehole strainmeters

method used to detect section area change is essentially the same as that in the Sacks-Evertson volumetric strainmeter (Sacks et al., 1971). The observed section area increases AS,, AS, and AS, are obtained from the horizontal prin-

have a very simple structure and are well coupled to the surrounding rocks. In this paper we present the strain steps observed with the three-compo-

transmitted

to the differential

cipal strains

extension B-see al., 1982a):

the first time.

ei + ez = (AS, + AS, + A&)/3/f

Observation Instrument

transformer.

c,, e2 and the direction

nent borehole strainmeter at Yasato, central Japan, where this kind of strainmeter was installed for

following

borehole

equations

The

of maximum (Sakata

et

C, - C2 -+[(A&-AS?)z+(AQA&)* + (AS, - AS,)*] li2/3B tan 2B = a(

AS, - AS,)/(2AS,

- AS, - AS,)

The concept of the instrument was introduced by Sakata et al. (1982a). Figure 1 shows a schematic illustration of the three-component strainmeter. The detection part of the strainmeter is equally divided into three independent sections by

(1) In this formulation, the three sections are numbered counterclockwise in the topview cross section, and B is the angle, defined counterclockwise, from the axis of c1 to the boundary axis between

rigid partition walls. Each section is filled with silicon oil and is connected to the bellows with a pipe. When the crustal strain field around the strainmeter changes, the volumetric change induced by the section area change is detected as the displacement of a bellows, and the displacement is

section 1 and section 3. A and B are the constants determined by elastic constants and the thicknesses of the detection cylinder metal, expansive mortar and surrounding rock. Detailed descriptions of the instrument (1987).

are given by Sakata

et al.

209

Kanto

Asian

observed,

District

the

original

shows an example

of the strain

earthquake

occurred

area

Plate

including which

was

observation steps.

For

Figure

about

35 km

point.

The record

the thirteen

3

step caused by the

in the central

Kanto

27, 1983 (M = 6.0). The

on February

center

case.

to the

south

shows

coseismic

epi-

of the

very sharp

steps

that

we

observed,

there seems to be no case in which the

coseismic

step

was followed

by creeping

move-

ments. Table Fig. 2. Location Research

Center

of installed

strainmeters

for Disaster

and

Prevention

the National

1 lists the observed

steps at Yasato,

(NRCDP).

where extension

tive. The absolute strains In 1982, two sets of strainmeters 60 m apart at Yasato

station

(Fig.

of the boreholes

2). The depths

principal

0.1 and 100 nanostrain. changes

The

are in good agreement

with each other, mostly within a factor of 2 for the amplitude and within 10” for the direction. Table 2 shows epicentral coordinates and magnitudes of the earthquakes in Table 1. The epicentral coordinates are determined by the JMA and the National Research Center for Disaster (NRCDP), except for the Off Akita (No. 5 in Table 2). For this earthquake two-fault models determined Mori (1983) and coordinates

Coseismic strain steps were originally detected from the Off Ibaraki earthquake (Sakata et al.,

Observed

are between

Tsukuba

Strain steps

TABLE

is taken as posi-

are 160 m

near Mount

(No. 1) and 165 m (No. 2) respectively. The surrounding rock is massive granite. The data were telemetered to the National Research Center for Disaster Prevention (Shimada et al., 1983).

1982b). So far, 13 coseismic

of the strain

values of the observed

two sets of strain

were installed

value

Prevention earthquake we use the

by Shimazaki and are given at the

central points of the deeper horizontal edges of the faults in Table 2. Shimazaki and Mori (1983)

strain steps have been

1 values of coseismic

No.

Origin

strain

steps at Yasato,

time (JST)

Observed

with origin principal

times of the earthquakes

strain (nanostrain) No. 2

No. 1 max.

direct.

min.

max. 68

- 35

93”

-0.5

0.7

60”

-0.5

1.2

3.7

82’=

- 3.2

2.6

-0.2 -0.7

0.3

0.1 -0.3 -0.5 - 0.6

0.2

loo

- 0.4

0.9

64O

-0.4

- 1.0 - 1.3

1.4

75O

- 1.0

1.0

45”

-1.4

84” 99O

-38 - 0.5

Aug. 14 1982

6:14

0.6

60”

-0.3

4

Feb. 27 1983

21:14

6.1

82”

5 6

May 26 1983 Jul. 2 1983

11:59 7:03

3.3 0.6

2O 23”

7

Aug. 8 1983

12:48

0.1

-15”

8

Oct. 28 1983

lo:50

0.3

-30°

9

Dec. 30 1983

11:30

0.3

-20”

10

Jan. 1 1984

18:03

0.3

I0

11

Jan. 17 1984

20:13

1.1

61°

12

Jan. 18 1984

0:31

1.5

7o”

13

Sep. 19 1984

2:02

1.1

39O

3

* “Max.” maximum

and

“min.”

principal

indicate strain

values

as measured

of the maximum clockwise

and

minimum

from the north.

min.

85”

84 1.4

Jul. 23 1982 Jul. 24 1982

direct.

1.3

23:24 2154

1 2

*

principal

-5O 20”

1.5 - 3.1 0.0

-I0

-0.6

0.5

-23”

-0.3

0.4

-30°

- 0.5

0.2

strain

and

“direct.”

indicates

the direction

of

210

Fig. 3. The records occurring

of coseismic

in the central

Kanto

strain

step observed

by the three-component

area (M = 6.0) on February

borehole

strainmeter

(No.

1) for the earthquake

27, 1983.

did not determine the depths of the faults, SO we presume reasonable values for these depths. Fig-

Focal

mechanisms

reported

by Dziewonski

et al.

earthquakes whose coseismic strain steps are observed. The epicentral distances ranged from 20 to

(1983a, b; 1984a, b, c; 1985) were determined by the centroid-moment tensor method, using the records of long-period seismometers. The mechanism of the Off Ibaraki earthquake (No. 1 in

480 km.

Table

Theoretical

termined using long-period body waves. Shimazaki and Mori (1983) used P-wave initial motions,

ure

4 shows

the

epicentral

distribution

of the

calculation

3) by Kikuchi

and

Sudo

(1985)

Focul mechanisms

long-period

Table 3 shows the focal mechanisms and the seismic moments of the earthquakes in Table 1.

termine the mechanisms of the Off Akita earthquake (No. 5). They did not determine dip angle and slip angle of the second event in the earth-

TABLE Origin

No.

body waves and surface

was de-

waves to de-

2 times. empirical Origin

region,

time (JST)

1

Jul. 23 1982

2

Jul. 24 1982

23:24 2154

epicentral

coordinates

Region

Epicentral

according

to the JMA of the earthquakes

listed in Table 1

coordinate

M

lat.

long.

depth

(north)

(east)

(km)

Off Ibaraki

36.18”

141.95 o

30

JMA

7.0

Off Ibaraki

36.12”

142.05 o

30

JMA

6.2

3

Aug. 14 1982

6:14

4

Feb. 27 1983

21:14

Southwest

5

May 26 1983 11:59

Off Akita

7:03

and magnitudes

Off Ibaraki

reference

36.48 o

141.22O

40

JMA

5.4

35.94O

140.16 o

72

JMA

6.0

40.4O

139.3O

15

Shimazaki

7.1

40.9 o

139.4 o

15

Off Fukushima

36.91 o

141.19O

54

JMA

5.8

Southeast

35.54O

139.05 o

18.3

NRCDP

6.0 5.1

Ibaraki

6

Jul. 2 1983

7

Aug. 8 1983

12:48

8

Oct. 28 1983

lo:50

9

Dec. 30 1983 11:30

Near Choshi

10

Jan. 1 1984

18:03

Off Kii Penin.

Southwest

Yamanashi Ibaraki City

and Mori (1983)

36.23”

139.99O

51.6

NRCDP

35.73O

140.73O

45.5

NRCDP

5.3

33.62O

136.84”

388

JMA

7.3 5.6

11

Jan. 17 1984

20:13

Off Ibaraki

36.45 o

141.25 o

43

JMA

12

Jan. 18 1984

0:31

Off Ibaraki

36.45 o

141.27”

43

JMA

5.9

13

Sep. 19 1984

2:02

Off Boso Penin.

34.05 o

141.55O

13

JMA

6.6

211

TABLE

3

Focal mechanisms No.

and seismic moments

of the earthquakes

listed in Table 1 *

Focal mechanism strike

dip

Seismic moment slip

( x

reference

Reference

70 26 dyne-cm)

1

190”

9”

80”

2.8

Kikuchi

2

231°

10”

110”

Dziewonski

et al. (1983a)

0.15

Dziewonski

et al. (1983a)

3

18X0

26O

84”

Dziewonski

et al. (1983a)

0.011

Dzrewonski

et al. (1983a)

4

191.1”

26.4”

0.078

Dziewonski

et al. (lY83b)

5

15O

25”

Shimazaki

and Mori (1983)

Kikuchi

100.0 o

and Sudo (1984)

EPID. JMA (1983)

85O

Shimazaki

and Mori (1983)

25

and Sudo (1985)

~ IO0

25”

85”

6

33O

85”

119O

Dziewonski

0.034

Dziewonski

et al. (1984a)

7

268O

76”

165”

lmoto et al. (1984)

0.037

Dziewonski

et al.

8

70”

14O

lmoto (1984)

0.0045

9

0.018

Dziewonskr

et al. (1984b)

6.0

Dziewonski

et al. (1984~) et al. (1984~)

33

YOO

(1984a)

231.4O

30.3O

10

163”

74”

73”

11

206O

43”

112”

EPID, JMA (1984)

0.028

Dziewonski

12

217O

48”

119”

EPID, JMA (1984)

0.045

Dziewonski

et al. (1984~)

13

137O

73O

Dziewonski

2.1

Dziewonski

et al. (1985)

* Azimuth

quake,

132.7 o

et al. (1984a)

-69’

of the strike is measured

so we suppose

Imoto (1984) Dziewonski

that

clockwise

these

et al. (1984~)

et al. (1985)

from the north

angles

had

the

same values as those in the first event. The seismic moment of the Off Akita earthquake was recalculated using the value of 6 x 10” dyne/cm2

for the

and slip angle is positive

rigidity

for reverse faulting.

of the crust instead

of 3 X 10” dyne/cm2,

as adopted by Shimazaki and Mori (1983) because we calculated theoretical strain steps at Yasato station using the value of 6 X 1O1’ dyne/cm2. Also, we assumed that the fault lengths and widths of the first and the second events of the Off Akita earthquake to be 35 x 30 km and 60 x 35 km respectively, based on the distribution of its aftershocks.

Focal

mechanisms

from Imoto

(1984), Imoto et al. (1984) and the Earthquake Prediction Information Division, JMA (1983, 1984) were determined by P-wave initial motions recorded with short period seismometers. We used the seismic moments calculated with the focal mechanisms in the same reports for the abovementioned earthquakes. For the earthquakes with no estimation of the seismic moments in the reports, we used the seismic moments determined by centroid-moment tensor method (1983b, 1984a, b, c). For the southwestern Ibaraki earthquake (No. 8) the seismic moment has not yet been determined, so we estimated the moment using the empirical relationship between magnitude M and seismic moment M,,, given by Kasahara (1975): log M,, = 1.5M + 16.0 Fig.

4.

coseismic

Epicentral strain

distribution steps

(YST) is also plotted.

of

are observed.

the

earthquakes

Location

whose

of strainmeter

Here, M as determined the calculation.

(2) by the JMA

is used for

212

82.

off

7.23. lbaraki

82.

off

7.24. lbaraki

82.

off

8.14. lbaraki

83. SW

83.

2.27.

off

lbaraki

5.26.

83.

kit.3

7.

2.

Fukushima SE

off

83.

8.

8.

Yamanashl

I No.1

-

Fig. 5. Comparison

Calculated

between

the observed

strain step

and the calculated

strain

with the observed of

the

From the focal mechanisms and seismic moments shown in Table 3, we calculated the strain steps at the Yasato station. For the calculation we

calculate

assumed a point source except for the Off Akita earthquake (No. 5) and a uniform semi-infinite body for the medium. For the Off Akita earthquake, we used the formula for a finite fault given

servation

steps.

steps, we discuss

measurements

and

the theoretical

the instrumental sensitivity

the

models

steps. Firstly,

constants,

of the instrument,

the reliability used

to

we examine

the azimuth

and

the

which vary with ob-

point.

by Sato and Matsu’ura (1974). For both cases, the P- and S-wave velocities were assumed to be 6

The azimuths of the instruments are determined during installation using a magnetic compass. The measurement error is within 3”. The difference in the directions of the maximum extensions between the observed and the calculated

km/s and 3.5 km/s for the medium, respectively. Figure 5 shows the comparison between the ob-

steps shows the mean values and standard deviations of 1” + 93 and lo JI 12” for No. 1 and No. 2

served and the calculated strain steps. There is no case in which the observations are not in harmony with the calculation, although the agreements between the observed steps of instrument No. 1 and No. 2 are better than those between the calculated and observed steps. Discussion

Reliability

of meusurements

Based on the observations of strain steps, and the comparison of the theoretically predicted steps

respectively.

From

these values,

it seems that the

azimuth measurements using a magnetic compass do not have any systematic errors larger than 10 O. On the other hand, the sensitivity of the instruments is a function of the elastic constants of the surrounding rock, the mortar and the instrument, and the geometry of the detection part of the instrument. Although physical constants are measured by well logging and core sample tests, an error might arise from estimations of the elastic constants of surrounding rock, because the surrounding rock usually has 3-dimensional inhomogeneity. The detection part of the strainmeter is

213

very slender

and the sensitivity

two-dimensional

by this geometrical to or less than sensitivity. and

From

strains ments

percent

of the observed

may be equal of the value

the comparison steps,

on a

The error caused

approximation

several

the calculated

ratios

is calculated

approximation.

of

of the observed

the mean

values

to the calculated

of the

principal

are 1.08 i 0.61 and 1.00 f 0.87 for instruNo. 1 and No. 2 respectively.

indicate

that the estimation

These values

amplitude

those steps,

and

the

are

observed

probably

earthquake,

if coseismic

nanostrain

are available

the three-component observation a network several

points

of stations

1 x 1O’4 dyne-cm,

the

inho-

instruments or azimuths

of the instruments. of models

In the theoretical calculation of strain steps, we assumed the medium to be a uniform semi-infinite body, neglecting the effect of layered structure and sphericity. Those effects should increase in

depth

although

occurring of more

the resolution

on the mechanism

and

the

of the earthquake.

(1) These instruments

simultaneously

strain steps for 13 earthquakes, from 5.1 to 7.7. The coseismic tained

from

the

two

straincentral recorded

with M ranging strain steps ob-

independent

strainmeters

showed a good agreement for all the earthquakes. The differences in magnitude and direction of the horizontal maximum strain are mostly within a factor of 2’ and 10” respectively. (2) The coseismic strain steps observed were found to be in satisfactory agreement with those theoretically computed from the focal mechanism and

served and the calculated steps does not seem to become larger for the earthquakes with long epi-

bility

for instance the southeast Yamanashi earthquake (Imoto et al., 1984) the discrepancy between the seismic and strain observations seems to become larger than in other earthquakes (Fig. 5). The comparison between the observed and the calculated strain steps suggests that it would be possi-

on a scale of

the focal mechanisms

Two sets of three-component borehole meters were installed 60 m apart at Yasato, Japan in 1982.

gated.

that caused by other uncertainties. Although it is difficult to evaluate the focal models from the strain steps observed at only 1 point, the observed steps are reasonably consistent with the focal mechanisms as shown in Fig. 5. For the earthquakes with a complicated focal process,

When

Conclusion

proportion to the epicentral distances for shallow earthquakes, but the discrepancy between the ob-

central distances. So it seems that the error caused by the approximation of the medium is less than

of

positioned.

with seismic moments

slightly

an 0.3

and the

for the earthquakes

the network

with than

network,

will be estimated

depend

by

larger

is constructed

tens of kilometers,

than

the calculated

steps

are properly

may

and

including

associated

from at least 2 stations

strain

caused

mechanism,

strainmeter

of the observed

mogeneity of the rock around the rather than by the errors of sensitivities

Reliubility

the fault deformation

has a

between the observed strain steps of No. 1 and No. 2 are smaller than

between

aseismic

of the sensitivity

steps ranges between 100 and 0.3 nanostrain. The unit of analogue/digital conversion is 50 picostrain for a dilatation change and it is difficult to detect the steps smaller than 0.3 nanostrain. The differences instruments

any

within

high reliability. The maximum

ble to determine

seismic

(3) These of the

moment results

of the earthquakes demonstrate

three-component

investi-

the high strainmeter.

measurement of static deformation by strainmeter will make it possible to evaluate

reliaThe this the

“ total fault mechanism” including aseismic deformation, whereas the focal mechanism determined by seismic waves cannot detect any aseismic motion associated with seismic faulting. Acknowledgements The authors wish to express our gratitude to Dr. Yoshimitsu Okada for his permission to use the computer program to calculate static deformations by faulting. We also like to thank Dr. Masakazu Ohtake for his critical reading of the manuscript.

214

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