High accelerations produced by the Western Nagano Prefecture, Japan, earthquake of 1984

Tectonophysics, Elsevier

335

141 (1987) 335-343

Science Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

High accelerations produced by the Western Nagano Prefecture, Japan, earthquake of 1984 Y. UMEDA ’ Abuyama Seismological

‘, A. KUROISO Observatory,

Faculty

2, K. IT0

2 and I. MURAMATU

ofScience,Kyoto

3

University, Takatsuki, Osaka 569 (Japan)

’ Regional Center for Earthquake Prediction, FacuIty of Science, Kyoto University, Takatsuki, Osaka, 569 (Japan) ’ Department

of Earth Science, Faculty of Education, Gifu University, (Received

June 30, 1986; revised version accepted

Yanagido, Gifu 501-I I (Japan)

January

13, 1987)

Abstract Umeda,

Y., Kuroiso,

Prefecture,

Japan,

Many boulders

A., Ito, K. and earthquake

were thrown

out of their former

1984 (MJMA = 6.8). The anomalous boulders,

assuming

Almost Average

ratios (mountain-top:

The high acceleration with the length characterized

boulders

area defined

(12 km) of the assumed

by extremely

low activity

by the Western

of 4-16

were found

accelerations

Nagano

g were estimated

range of 5-10

using five aftershocks

Prefecture,

by the distribution

ridges

recorded

of thrown-out

main fault. Many cracks

boulders

A moderate earthquake in the Western Nagano Prefecture (35 O49.3’N, 137 o 33.6’E, MJMA = 6.8) struck the central area of Japan on September 14, 1984. The earthquake caused severe damage such as landslides and rockfalls in Ohtaki village and its vicinity, at the southern foot of Mt. Ontake. Immediately after the main shock, aftershock activity spread about 16 km with a trend of N70 o E,

depth slippage deposits

of

and saddles.

The topographic and at the foot.

range concerned.

is very small (1 x3 in this limited

km) compared

small area, which is

was

very

to be 2 km by the Japan (JMA, 1985). Although shallow,

no

could be found because from Mt. Ontake.

systematic

Meteorothe focal surface

of thick volcanic

Ito et al. (1985) promptly reported that many boulders were thrown out of their sockets at the epicentral area in Ohtaki village. The fact that the ground acceleration caused by some shallow earthquakes exceeds the earth’s gravity has been reported by several field surveys (Kawasumi, 1950; Richter, 1958; Morrill, 1971). Instrumental rec-

InformaTsunami

Observations Division of the Japan Meteorological Agency (1985). The earthquake involved a right-lateral strike-slip motion with a small dip-slip component on a steeply northward-dipping fault extending 12 km in the direction of N70 o E (Take0 and Mikami, 1986). The depth of initial breakage 0 1987 Elsevier Science Publishers

earthquake

large dislocation,

was determined logical Agency

according to the Earthquake Prediction tion Division and Earthquake and

Nagano

of thrown-out

on the mountain-top

were also found

and relatively

Japan,

from the displacement

foot) of seismic waves are 2-7 in the frequency

of aftershocks

by the Western

Hz.

on the mountain-tops,

Introduction

0040-1951/87/%03.50

produced

141: 335-343

sockets

high accelerations

of seismic waves were estimated

amplitude

I., 1987. High

that the seismic waves had a frequency

ail of the thrown-out

amplifications

Muramatu,

of 1984. Tectonophysics,

ords in source regions have also shown the high acceleration exceeding that due to gravity (1 g) (Trifunac and Hudson, 1971; Hartzell, 1980; Archuleta, 1982). The Western Nagano Prefecture (or Naganoken-seibu) earthquake of 1984 produced field evidence of high accelerations far in B.V.

excess of any ever recorded this paper, we introduce earthquake this

vibration,

phenomenon

sidering

between

surveys.

of

We also

the area struck by

and the source process,

the topographical

In

effect of the

and show the distribution by detailed

discuss the relationship high accelerations

by accelerometers.

this dramatic

amplification

by con-

of seismic

waves. Some examples of boulders thrown out from the ground

More than 80 boulders thrown out from their sockets or overturned in situ were mainly found on the four ridge crests of the mountains near the epicenter of the main shock. Figures la and b show the biggest thrown-out rock found on the mountain-ridge F (see Fig. 6). This boulder. weighing about 5000 kg was moved by 1.2 m in the N105” W direction. As seen in Figs. la and b. bushes on the southwest side were flattened, and a small tree about 4 cm in diameter was broken at a height

of 15 cm

from

its roots.

This

evidence that the boulder was tossed than 15 cm from the ground.

provides up higher

The former position of the boulder can easily be found by comparing the shape of the boulder and of its former socket in the soil. In particular, the moss distribution on the boulder clearly indicates the former situation, as the mossless portion on the northeast

surface

of the boulder

coincides

with the 50 cm deep socket on the northeast side. The moss grown on the upper half of the boulder was not scraped except at the position of collisions with bushes. From this evidence, we conclude that the boulder surface, direction

did not roll on the ground

but was thrown up and moved in the of N105” W by 1.2 m. The cracks on the

surface soil ranged over the mountain ridge, with a trend of N20 o W near the largest boulder. We found 20 thrown-out boulders on this mountain rigde. On the mountain saddle I in Fig. 6, we also found 23 boulders thrown out and/or overturned. A boulder and socket set is shown in Figs. 2a and b. and this example corresponds to no. 6 in Fig. 4. The boulder, about 45 kg in weight, was lifted out of the ground without cutting the edge of its

Fig. 1. a. Schematic The upper right

diagrams

former

position

mossy portion. to Fig. Indicated

view of dislodged

left diagram

are vertical

views.

of the rock.

Shading

b. Photograph

la. This

rock (heavy solid curves).

is a horizontal

photograph

by the open arrow

view:

Dotted

the lower

and

show

the

on the rock shows

the

of dislodged was taken

curves

rock corresponding from

the direction

in Fig. la.

former 9 cm deep socket, and overturned at a distance of 43 cm from the socket. The boulder must have been thrown up at a high angle. The ground surface was covered with moss. No scratch trace or bruise mark could be found on this mossy surface between the dislodged boulder and the socket. The boulder with a thickness of 17 cm did not roll on the surface, but it must have been

+‘W.

+-----

0.43 m ---+

‘1

Boulder i,

Fig. 2. a. An example of a thrown-out boulder: vertical (upper) and horizontal (lower) views. This boulder is turned upside down and rotated about 60 ’ anticlockwise, as shown by solid thin arrows. Other notes are the same as for Fig. la. b. Photograph of the thrown-out boulder (right) and its former socket (left) corresponding to Fig. ‘la.

thrown up at least 17-18 cm in height. Another example shown in Figs. 3a and b was found on mountain-top C in Fig. 6. The boulder, weighing about 600 kg, was clearly tossed up out of the 20-70 cm deep socket and stood 3 m away from the socket. It did not turn upside down, and it rotated anticlockwise by 80“ within the horizontal plane. The bottom of the southwest side of the boulder was slightly thrust into the soil. The bark of a bush, of 5 cm thickness, standing near the socket was injured at a height of 60-70 cm from the root. Therefore, it is certain that the

Fig. 3. a. An example of a thrown-out boulder: vertical (upper) and horizontal (lower) views. Other notes are the same as for Fig. la. b. Thrown-out boulder (right lower, with scale) and its former socket (left upper), corresponding to Fig. 3a.

boulder had been thrown up at a high angle and moved by 3 m in the direction of SSOo W. The distribution of #mm-out

boulders

An example of the distribution of boulders from our preliminary report (Kuroiso et al., 1985) is shown in Fig. 4. This small area is part of mountain saddle I in Fig. 6. The arrowed boulders were thrown off from their former sockets, indicated by dotted lines. Boulders Nos. 10 and 13 were only overturned at their former sites. Boulders with no numbers were not moved, because they were almost buried in the ground. No. 9 indicates the socket only, since this boulder must have

I’ t

Fig. 4. Distribution saddle

(I)

the boulders boulders

of thrown-out

in Fig. 6. Heavy and

their former

are overturned.

tions (after Kuroiso

boulders

on the mountain

solid and broken sockets,

Arrows

imply

circles indicate

respectively.

Hatched

the throw-out

direc-

the solid and dashed curves in Fig. 6, boulders were shifted only in the range of several centimeters to several tens of centimeters. Although we could not cartify whether or not these boulders had been thrown up, they must have been struck by high accelerations over 1 g. The high-acceleration area defined by our surveys measures 3 km in the direction of N70” E and 1.3 km in the N-S direction. The smallness of this area strongly suggests that the largest rupture occurred just beneath it. Estimation of ground velocity and acceleration

et al.. 1985).

fallen down into the valley out of this saddle. The distribution of thrown-out boulders was surveyed as extensively as possible to cover the epicentral area, except for steep slopes over about 30 ‘. Our survey routes are shown in Fig. 5. The thrown-out boulders were mainly found on the flat tops, ridges and saddles of mountains. Within the shaded area in Fig. 6, we confirmed that the 80 boulders and four fallen trees were undoubtedly thrown out from their former sockets by the earthquake. In the valleys, it is difficult to certify whether or not boulders have been tossed up, because many boulders have rolled to the foot of the mountain. Only three thrown-out boulders were confirmed in the valleys. From the results of the detailed surveys, as shown in Fig. 6, we could define the area where the boulders were thrown out. In the area between

/”

Matuzawa (1944) discussed a method of estimating ground velocity and acceleration from the displacement distance of the ornamental stone dog “Komainu” associated with the shrine, near the epicenter of the Tottori earthquake of 1943. After Matsuzawa’s method, we now estimate the ground velocity and acceleration in the area of the Western Nagano Prefecture earthquake of 1984. We take a coordinate system fixed on the earth (6. n); the origin is at the former seat of boulder. 5 is taken along the horizontal displacement and q (6, q) is positive upward. Since the coordinate thereby located is moved by earthquake vibration we must adopt another coordinate system (X, Y 1

which is fixed in space. Then, the equations of motion of the boulder are: x+,$=0 Y+t=

(1) -g

(2)

KWX.%Va h

137’ 30’ E

Fig. 5. Routes

surveyed:

K, C, F and I correspond

to Fig. 6.

339

and the ground. We assume that a boulder had been lifted up slightly by vibration before it was thrown out from the socket and, just at that time, the ground collided with the boulders in the

I

boulder

135O34’E

course

of more

sumptions velocity

vibration.

be accepted.

because

the ground

with the boulders,

of the boulder

before

Secondly,

after the collision coefficient V,= Then,

and after

is negligibly

(V,) of the boulder

is as follows,

using a restitution

(e):

V;(l+e)

(9)

eqn. (8) is written

as follows:

A=
is assumed to be as

X= Y=A sin wt

(3)

where w is the angular frequency, let us assume that: &+0,

<=v=o,

at t=O

(4)

We obtain the solution by integrating the equations of motion (1) and (2) twice, under the above assumptions of the ground displacement (3) and the initial condition (4): X+.$=Awt

(5)

Y+q=

(6)

-gt2/2+Awt

If we substitute (3) into (5) and (6), the flight time (tl) and the ground displacement (A) are given

We can estimate smaller than that

(10)

the ground movement to be obtained from eqn. (8) by a

factor of l/(1 + e). However, it is difficult to estimate the real value of the restitution coefficient. The value varies with the coupling conditions owing to the shape of boulders, and the solidity or adhesion of the soil. As far as this problem is concerned, we shall describe the results of an experiment for some cases in the next section. In this section, the value is taken as 0.3, according to Ito et al. (1984). The angular unknown. The

frequency of the ground is also accelerograms recorded near the

epicenters of large earthquakes, for example, the San Fernand, California, earthquake of 1971 (Trifunac

and

by:

quake

of 1976 (Hartzell,

4

predominant frequency ground displacement

=

{a-

(7)

?)hy2

A = 5/( ot, - sin at,) Now,

(8)

we consider

the collision

between

a

Hudson,

acceleration quency

range

1971), and 1980),

the Gazli show

Accelerations calculated from the dislodged distances for two examples

(Am’)

Frequency A (m)

(Hz)

Aw (m SC’) Au* (m sC2)

5 0.044

8 0.026

1.4

1.3

43

66

of boulders

5

8

0.019

0.083

0.052

1.2

2.6

2.6

10

16

82

the

were calculated in the freof 5-10 Hz, for two examples of

Fig. 3 (E = 3.0, q = -0.9m)

Fig. 2 (6 = 0.43, 9 = 0.09 m)

earth-

that

is 5-10 Hz. Therefore, the (A), velocity (Au) and

TABLE 1

Sample

the

and the velocity

the collision

the velocity

two as-

the ground

mass is infinitively

large compared small.

Next, First,

(V,) does not vary before

collision,

Fig. 6. The high-acceleration area. In the shaded areas (C, F, I and K ), thrown-out boulders are distributed densely, as shown in Fig. 4. The areas enclosed by a heavy solid line and a dotted line correspond to the area struck by extremely high accelerations and by accelerations comparable to 1 g, respectively.

violent

should

130

10 0.041 2.6 160

dislodged boulders shown in Figs. 2a and b and 3a and b. As shown in Table 1, surprising high accel-

tion of accelerograms.

erations

and 2-4 m sP ‘, respectively.

of 43-160

m ss2 (4.4 - 16 g) were even-

tions and velocities

We found that the accelerawere as high as 8 g (7840 Gal)

Now, let us show how eqns. (7) and (8) predict

tually obtained.

these Vibrating table experiment

time

experimental

results.

We obtain

(t, = 0.62 s) by inserting

the flight

the displacement

(& = 1.9 m, n = 0 m) into eqn. (7) and the period In order boulders throw

to understand

and

effect

of

of 0.25 s (w = 8a) from the accelerograms

to

7. Then

The

vibrating

soil, we set up an experiment

up boulders

vibrating

the coupling

from

a vibrating

table was covered

table.

with different

was put

on it. Three

materi-

als and

a stone

kinds

of

material

were used to cover the table; foam rubber,

clayey soil, and paper clay (made by dehydrating a mixture of paste (rice gruel) and scraps of paper). Variously shaped boulders weighing 0.3-0.5 kg were placed on, or slightly buried in, the materials on the table. The experiment was carried out for various combinations of boulders and materials. The table was shaken manually. Vertical and horizontal accelerometers were fixed on the table, and their outputs and the boulder trajectories were monitored by a data recorder and a videorecorder, respectively.

above

the displacement table parameters

(A = 0.126

is obtained (t,.

by

5 and

in Fig.

m) of the

substituting w) into

the

eqn.

(8).

Thus,

we obtain

the acceleration

of the vibrating

table

as 02A/g

= 7.9 g, which

agrees

with

the

value obtained experimentally. When the displacement of a boulder exceeds several tens of centimeters, the maximum acceleration is not dependent on the kind of cover of the vibrating table, except in the case of adhesive material such as clayey soil. We then ascertained that the surprising high acceleration was actually produced on the mountain-tops, ridges and saddles by the earthquake. Geology

and topographical effects

Examples of experimental accelerations and velocities are shown in Fig. 7. In this case, we used

The geological basement in and around the high-acceleration region consists of Mesozoic sedi-

foam rubber

mentary rocks. Lava of Quaternary age, produced from the volcanic Mt. Ontake, covers the base

as the “soil”

material,

and a stone 9

cm in diameter and weighing 0.4 kg. The resultant horizontal displacement (L) of the stone is 190 cm, and the maximum velocity

waveforms

height (H)

were obtained

l_:lSOcm

is 60 cm. The by the integra-

H:GOcm

89

RCC-V

T

l?--Q-= VEL-V

2ms-I

T

rocks to a thickness of 800 m in regions C and K (Fig. 6) (Kisodani Subgroup, Matsumoto Basin Collaborative Research Group, 1985). The uppermost 30 m is composed of alternating strata of volcanic products such as pumices, scoria, tuffbreccias and ash. As described in the previous sections, the thrown-out boulders are mainly found on the mountain-tops, ridges and saddles. In order to understand the effect of the topography on seismic waves, we observed instrumentally the aftershocks at the mountain-top and foot in region C (Fig. 6).

VEL-H

4 ms-’

1 25ams

Fig. 7. An example accelerometers

table. The velocities integration

of the output

(ACC-V

of two-component

fixed

(VEL-V and VEL-H)

of accelerograms.

H is the-height

signals

and ACC-H)

on the vibrating

were obtained

L is the horizontal

of the thrown-out

boulder.

by the

distance

and

Fig. 8. Topography, C in Fig. 6.

geology

and observation

stations

of region

341

fl

UD

N20’

W

N70’

E

4.00E-03

cm/set

1.0

i

J

Sec.

Examples of three-component seismograms recorded at the mount~n-top and foot are shown in Fig. 9. It was found that the seismic waves on the mountain-top are considerably amplified. The spectra of these waves are shown in the lower part of Fig. 10, and the spectral ratios (top : foot) are also shown in the upper figure. For five microearthquakes recorded at short epicentral distances [S-P time G 1 s], the spectral ratios are shown in Fig. 11. The average ratios, shown by solid curves, show that the seismic waves on the mountain-top are amplified by a factor of 2-7 in the frequency range 5-10 Hz. Discussion and summary

N20’

N70’

W

E

Fig. 9. Three-component velocity seismograms recorded at the mount~n-top and foot, as shown in Fig. 8.

The cross-section of the observation site is shown in Fig. 8. A three-component velocity-type seismograph with a natural period of 0.5 s was put in the soil at the top and foot.

The Western Nagano Prefecture earthquake of 1984 is a multiple-sh~k earthquake. The solid circle in Fig. 12 represents the rupture starting point, which was determined by JMA (1985). The initial rupture propagated in the S70 OW direction from the starting point and the largest rupture occurred 1.5 s later (Ishikawa et al., 1985). Finally, the fault plane, 12 km long and 6 km wide, was dislocated as shown by a thin straight line in Fig. 12. Takeo and Mikami (1986) determined the dislocation distribution at the fault plane by an inverse method. The fault portion with the largest slip, as determined by them, coincides with the

TOP/FOOT

LOG

FREQUENCY

LOG

1 FREQUENCY

(Hz)

LOG

FREQUENCY

LOG

-2 1 FREQUENCY

(Hz)

_OG

FREQUENCY

LOG

1 FREQUENCY

(Hz)

I. (1’” 0

2 (Hz)

0

0 (Hz)

2 (HZ)

Fig. 10. Lower: spectral amplitudes on the mountain-top and foot. Upper:spectralratios (mountain-top : foot).

FREOUENCY

(Hz)

Fig. 11. Average

(Hz ) FREQlJLNCY : foot) for five micro-earthquakes.

FREQUENCY

amplitude

ratios (mountain-top

high-acceleration region obtained by our field surveys. We could not find any systematic slippage on the surface, but many cracks were found on the mountain-tops, ridges and saddles. Particularly in regions C and F in Fig. 6, relatively large cracks, 30-50 m long and 0.1-1.0 m wide, were running intermittently along the mountain ridge. Many boulders were also found rolling in the open cracks. These boulders must have been tossed up by severe vibration. Cracks were also found on the paved road in the valleys, as well as on the mountain-tops, ridges and saddles. These cracks were densely distributed within the high acceleration area indicated by a solid curve in Fig. 12. It should be noticed in Fig. 12 that the aftershock activity is very low in the high-acceleration area. This aftershock gap was promptly reported by the staff of Earthquake Prediction In-

Sep.l4-15.

ML3.0,

1

formation Division and Earthquake and Tsunami Observations Division Japan Meteorological Agency (1985). Our investigations in this paper are summarized as follows: (1) More than 80 boulders and four fallen trees thrown out from their sockets by the Western Nagano Prefecture earthquake of 1984 were found on the mountain-tops, ridges and saddles near the epicenter of the main shock. (2) The anomalous high accelerations of ground movement of 4-16 g at 5-10 Hz were estimated from the displacement of the boulders. (3) The experiments to throw up small boulders by use of a vibrating table suggest that the high accelerations actually affected the boulders on the mountain-tops. (4) The aftershock observations reveal that the ground’ motion is amplified 2-7 times at the

Depth=O-2Okm.

Ohtakl

(Hz

Nzl38

river 0

3km

/ Fig. 12. Aftershock

distribution

during

main shock. The straight

line indicates

tion area. It is noticeable

that aftershock

24 hours immediately the assumed activity

after the main shock. The solid circle shows the starting

fault of the main shock.

is very low in and around

Heavy solid and dotted the high-acceleration

point of the

curves show the high-accelera-

area.

343

ground

motion

is amplified

mountain-top,

2-7

as compared

times

to the

at

foot

the

of the

is relatively

with the total length (6) Surface

high acceler-

Japan

small (1 x 3 km) in comparison are densely

distributed

large dislocation (1981,

Meteorological

with the region

and the aftershock

large,

shallow

in

1985) suggested

that

of

gap.

Kawasumi,

there

Umeda

was a specific

rupture processes of large earthquakes must be obtained by further investigation of this specific

Earth-

(JMA),

1985. The Seismological

Meteorological

Agency

for September

description.

earthquake

of December

Bull. Earthquake

Res. Inst.,

28: 355-367. Kisodani

Subgroup,

Matsumoto

geology

of southern

89-104

(in Japanese).

Kuroiso,

Basin Collaborative

1985. The landslide

and mudflow earthquake,

for surface

area of the western tember

ruptures

Nagano

Univ., 28(B-1):

171-184

Earth

Y. and

Sci., 39:

Muramatu,

I.,

and a high acceleration

prefecture

earthquake

of Sep-

Prev. Res. Inst. Kyoto

(in Japanese).

T., 1944. On the movements earthquake.

caused

and the Quaternary

14, 1984. Ann. Rep. Disaster

Matuzawa,

Research

disasters

slope of Mt. Ontake.

A., Ito, K., Iio, Y., Umeda,

1985. Surveys

Tottori

region.

of “Komainu”

Bull. Earthquake

by the

Res. Inst., 22: 60-65

(in Japanese). Merrill,

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