Journal of Natural Disaster Science, Volume 25, Number 1, 2003, pp7-15

Near Field Effects of Hidden Seismic Faulting on a Concrete Dam
Tatsuo OHMACHI
Professor, Department of Built Environment, Tokyo Institute of Technology, Yokohama, Japan
Naoyuki KOJIMA
Civil Engineer, Metropolitan Expressway Public Corporation, Tokyo, Japan(Formerly, Graduate Student, Ditto)
Atsushi MURAKAMI
Engineer, Sumisho Electronics Co. Ltd, Tokyo, Japan(Formerly, Graduate Student, Ditto)
Nobuhiko KOMABA
Graduate Student, Department of Environmental Science and Technology,
Tokyo Institute of Technology, Yokohama, Japan(Received 21. Oct., 2002 and in revised form 24 Jun., 2003)

ABSTRACTThe 2000 Western Tottori earthquake (MJ 7.3), Japan, was caused by a hidden seismic fault underlying the
Kasho Dam, a 46 m-high concrete gravity dam. Strong-motion accelerometers registered peak accelerations of2051 at the top of the dam and 531 gal in the lower inspection gallery. Integration of the acceleration records inthe gallery showed a permanent displacement of 28 cm to the north, 7 cm to the west, and an uplift of 5 cm. The
dam survived the earthquake without serious damage, but the reservoir water level dropped suddenly by 6 cm followed by damped free vibration that continued for several hours. Based on numerical simulation and field observations, the water level change is attributed to ground displacement in the near field and subsequent seiching of
the reservoir. The vibration period of the dam in the upstream-downstream direction changed noticeably during
the main shock, probably due to hydrodynamic pressure variation. The earthquake caused cracking of concrete
floor beams in a sub-gate control room, which was repaired by post-tensioning with steel bars. Micro-tremor measurements were used to evaluate the effectiveness of the repair work.

1. INTRODUCTION
The 2000 Western Tottori earthquake (M J 7.3), Japan,
occurred at 13:30 (local time) on October 6. Kasho Dam, in thenear field of this earthquake, is a concrete gravity dam constructed
in 1989 with a height of 46.4 m and crest length of 174 m. The
seismic design of the dam was basically conducted by the seismic
coefficient method using a horizontal coefficient of 0.12. The fullwater level of the reservoir is EL 118 m, and at the time of themain shock the water level was EL 112 m.
Strong motion accelerometers at the dam registered a peakacceleration of 2051 gal at the top of the dam and 531 gal in thelower inspection gallery (Japan Commission on Large Dams,
2002). Despite such high acceleration, the dam body escaped serious damage, only suffering cracking of the concrete floor in its
sub-gate control room. Although no increase in water leakage wasrecorded, the water level of the reservoir suddenly dropped 6 cm
immediately after the main shock. This was followed by damped
free vibration that continued for several hours. The drop in reservoir’s water level was mysterious and raised concern among dam
engineers.
The location of the dam is shown in Fig. 1 together with the
hypocenter locations of the main and aftershocks. A seismic fault

appears to run through the Kasho Dam site. No clear trace of fault
rupturing was found on the ground surface in the epicentral area
(Inoue et al., 2002). The earthquake therefore appears to have beencaused by seismic rupturing of a hidden fault underlying the damsite (Ohmachi et al., 2002).
2. NEAR-FIELD GROUND DISPLACEMENT
2.1. Ground displacement inferred from strong-motion records
Several monitoring systems are installed at the dam. Threecomponent strong-motion accelerometers (seismometers) are bolted firmly to the concrete floor of the upper elevator room at EL
124.4 m and on the concrete floor of the lower inspection gallery at
EL 87.0 m (Fig. 2). The horizontal components of both accelerometers are set N-S and E-W (the dam axis is oriented N110º E). A
reservoir water-level meter is installed in a concrete well on the
crest 0.8 m in diameter. Figure 3 presents a downstream view ofthe dam, showing the elevator tower and small concrete well structure on the downstream side next to the dam’s crest.
Acceleration records of the main shock obtained at the two
seismometer installations are shown in Fig. 4. Earthquake acceleration was sampled at 100 Hz with 24-bit resolution and reliablefrom DC to 41 Hz. Seismic ground displacement was estimated by

KEY WORDS: 2000 Western Tottori earthquake, near field, Kasho Dam, strong motion, ground displacement, seiche.

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T. OHMACHI, N. KOJIMA, A. MURAKAMI AND N. KOMABA

Fig. 1 Map showing locations of the main shock, aftershocks, and Kasho Dam

Fig. 2

Locations of strong-motion accelerometers and water level
meter at Kasho Dam

twice integrating the acceleration histories at the lower inspection
gallery with respect to time.
Figure 5 shows that the dam underwent different modes ofdisplacement in three directions. In the N-S direction, it was displaced linearly to the north with a final permanent displacement of
27.6 cm. In the E-W direction, displacement varied sinusoidallywith a final permanent displacement of 6.5 cm to the west. In thevertical direction, displacement showed a sharp upward peak witha final permanent uplift of 4.7 cm.
2.2. Ground displacement from the numerical simulation
Using the fault parameters shown in Table 1 (GeographicalFig. 3

Downstream views of Kasho Dam from the right bank.

Table 1 Fault parameters used in the simulation

NEAR FIELD EFFECTS OF HIDDEN SEISMIC FAULTING ON A CONCRETE DAM

Fig. 5 Displacement histories obtained by integration of acceleration
records

Fig.4

Strong-motion acceleration of the main shock recorded at
Kasho Dam.

Fig. 7

Fig. 6

Seismic fault model for the simulation

Vertical ground displacement evaluated from the simulation by (a) BEM and (b) its interpolation (b)910T. OHMACHI, N. KOJIMA, A. MURAKAMI AND N. KOMABA

Fig. 8Fig. 9Ground displacement at Kasho Dam simulated by BEM

Fig. 10Observation points along the reservoir for the ground survey

Fig. 11Comparison of vertical displacements evaluated from thesimulation and ground survey

Horizontal displacement motion trajectories estimated fromstrong-motion records and the simulation

Survey Institute, 2000), the ground displacement associated withthe seismic faulting event was simulated by the 3D boundary element method (BEM). The seismic fault of the main shock was aleft lateral strike slip fault with a strike of N152º E. Besides thestatic parameters shown in Table 1, dynamic parameters, e.g., therupture velocity Vr and rise time τr, were tentatively assumed (Fig.
6).
Simulation was conducted on a rectangular area 23 km long
(N-S) by 17 km wide (E-W) that encompassed Kasho Dam and its
reservoir. The ground was simplified as a half-space of homogenous elasticity with a shear wave velocity of 4 km/s. In the firststep of the simulation, seismic ground displacement was calculated
at every node of a 500 m×500 m mesh. Results are shown in Fig.
7(a), in which thick broken lines indicate the projection of the fault
plane, thick solid lines the reservoir, and thin solid ones lines of
equal displacement. The iso-displacement lines run almost parallelto the fault projection in the vicinity of the reservoir, indicating
uplift of the reservoir to the northeast and settlement to the southwest.
The displacement in the area indicated by the thick solid lines
in Fig. 7(a) was interpolated (Fig. 7(b)). The profile of the reservoir is drawn along the altitude of the design’s flood water level(EL123.2 m). Kasho Dam is located at the north end of the reservoir.
Histories of ground displacement around the dam, obtained
from interpolation of the simulated displacement, are plotted inFig. 8. They appear very similar to the recorded displacement histories in Fig. 5, but the amplitudes are somewhat smaller. The sim-

ulated permanent ground displacement at the dam is 10.9 cm to thenorth and 0.1 cm to the west, with an uplift of 2.5 cm. The horizontal displacements shown in Figs. 5 and 8 are plotted in Fig. 9 interms of motion trajectories. Interestingly, both trajectories in Fig.
9 have features in common; the direction of initial motion, elliptic
motion in the final stage, and a large transient shift to the NE or
NNE.
2.3. Comparison of vertical displacements found by simulation
and a ground survey
After the main shock, a ground survey was conducted at 65
points along the reservoir. Figure 10 shows that the straight lines A
to D parallel the fault projection in Fig. 7. With the distance fromline A as the reference, the vertical displacements from the simulation and survey are plotted in Fig. 11. Displacements obtained
from the survey are almost three times those obtained by simulation, particularly between lines B and C. The differences are attributable to several factors: the inaccuracy of parameters used in theNEAR FIELD EFFECTS OF HIDDEN SEISMIC FAULTING ON A CONCRETE DAM

Fig. 12

11

Relative displacement evaluated from plumb line readings

Fig. 14
Fig. 13(a) Reproduced free-vibration history of the water level and(b) the corresponding spectrum

Records of the reservoir water level at Kasho Dam

simulation and effects of local site conditions, as well as the effectsof the many aftershocks, not considered in the simulation.
2.4. Comparison with plumb line readings
To detect the relative displacement between the top and bottom of the dam, a plumb line was installed in a vertical shaft 30 cm
in diameter. The upper end of the steel plumb line is fixed to thefloor of the upper elevator room next to the strong motion
accelerometer, and the weight on the lower end housed in a measurement device located at EL 89.5 m in the lower inspection
gallery. Automatic measurement usually is conducted a few times
a day. Readings from the measurements from October 3 to October
13 are plotted in Fig. 12. The main shock induced a relative displacement of –2.8 mm in the x (right bank) direction and 0.7 mm
in the y (upstream) direction, with a resultant permanent displacement of 2.9 mm.
If the relative displacement indicated by the plumb line readings is attributable to the inclination of the dam body produced byearthquake-induced ground displacement of its foundation, thenthe inclination is quantitatively given by the resultant displacement
divided by the length of the plumb line; 2.9/34900 = 8.3×10−5. In
the vicinity of the dam, the inclination of the earthquake-induced
ground displacement, as estimated from the simulation, is 6×10−5(Fig. 11). Although this value differs slightly from the inclination
determined from plumb line readings in terms of magnitude anddirection of inclination, it is reasonable to assume that the dam site
was displaced by the seismic faulting of the main shock, as
inferred from integration of the strong-motion acceleration.
3. CHANGE IN THE RESERVOIR WATER LEVEL
3.1. Reservoir water level records
The reservoir water level is recorded continuously. The histo-

ΔH1 (-1.5cm)

ΔH2 (+2.5cm)

Fig. 15

Schematic explanation of the water level change caused byground displacementry at the time of the earthquake is shown in Fig. 13. The upper andlower histories in the figure respectively are records with low andhigh resolutions. They indicate that the water level dropped suddenly by about 6 cm at the time of the main shock, followed bydamped free vibration that continued for several hours.
The water level record was reproduced roughly from readingsof successive peaks and troughs in the original record. The reproduced history and Fourier spectrum are shown in Fig. 14(a) and(b). The free-vibration characteristics estimated from these findings give a period of 6.5 min and a damping ratio of 2%.
3.2. Cause of the sudden water level drop
Seismic faulting caused uplifting of the dam body and change
in the ground slope of the base of the reservoir as a result of nearfield ground displacement. These factors are regarded to constitutethe main cause of the sudden drop in the reservoir’s water level. A
schematic explanation is given in Fig. 15, in which the dam andground slope before and after the main shock respectively are indicated by thin and thick lines. In the figure, ΔH1 is the water levelchange due to the increased reservoir capacity estimated from theaverage settlement of the reservoir area, and ΔH2 is the water levelchange due to the uplifting of the dam. The simulated displace-

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T. OHMACHI, N. KOJIMA, A. MURAKAMI AND N. KOMABA

ments (Fig. 7) give ΔH1 of –1.5 cm and ΔH2 of 2.5 cm, with acorresponding water level change of −4.0 cm (ΔH =ΔH1+ΔH2),
which is in reasonable agreement with the observed −6.0 cm.
3.3. Seiching of the reservoir
Free vibration of a water body, such as a reservoir, is called
seiching (Lamb, 1932). The fundamental period T for a rectangularreservoir is approximated by
2a
（1）
T＝
gh
√
where a is the length and h the depth of the reservoir, and g theacceleration due to gravity. Introduction of a = 2 km and h = 10 m
into Eq. (1) for Kasho Dam gives a period of T = 7.0 min which isfairly close to the observed period of free vibration (6.5 min).
Accordingly, the free vibration is thought mainly to be due to theFig. 16

fundamental mode of the seiche. Seiching therefore was simulated
by a numerical technique recently developed for tsunami simulation (Ohmachi et al., 2001). This technique essentially involves
solving the Navier-Stokes equation for a system by the 3D finite
difference method (FDM).
In our simulation, for simplicity the dam and ground wereassumed to be rigid. The initial water surface was considered tohave the same slope as that obtained from the simulated grounddisplacement, which was released under the force of gravity,
inducing free vibration of the reservoir. The simulated water surface over the entire reservoir for intervals of 50 s from t = 0 to t =550 s, are shown in Fig. 16. The water level history is shown in
Fig. 17, for which the simulated seiche period is 7.5 min (450 s).
In Fig. 16, the reservoir’s water surface has a uniform slope at
t = 0, the highest water level being at the dam and the lowest at theWater surface of the simulated seiche (time unit : s)

NEAR FIELD EFFECTS OF HIDDEN SEISMIC FAULTING ON A CONCRETE DAM

Fig. 17

Fig. 18

13

History of the simulated seiche at Kasho Dam

Horizontal strong-motion records rotated with the azimuth of
Kasho Dam

upstream end. After 200 s, about half of the seiche period, thehighest point is at the upstream end and lowest at the dam. After
450 s, the water surface again resembles the initial shape. The
water surface pivots on an axis in the middle of the reservoir,
indicative that its free vibration motion primarily is due to the fundamental mode of the seiche.
4. STRUCTURAL RESPONSE TO THE EARTHQUAKEAND ITS AFTER EFFECTS
4.1. Change in the vibration period of the dam
Response characteristics of the dam were analyzed by transforming the two horizontal components of the strong-motion
records in the N-S and E-W directions, shown in Fig. 4, into the
upstream-downstream and dam-axis directions. The transformed
acceleration histories are presented in Fig. 18. After transformation, peak accelerations at the top of the dam were 2100 and 1684

Fig. 19

Amplification factors between the dam top and lower inspection gallery

gal, respectively in the upstream-downstream and dam-axis directions, and in the lower inspection gallery 570 and 503 gal.
Acceleration histories were divided into four 5.12-s segments(Fig. 18). At the top of the dam, the peak accelerations in the
upstream-downstream direction in these segments were 491, 2100,
531 and 227 gal. The amplification factors between the top andlower gallery, calculated from the Fourier spectral ratios between
pairs of segments, are shown in Fig. 19. In the upstream-downstream direction, the main peak is about 0.1 s, and appears to be14

Table 2

T. OHMACHI, N. KOJIMA, A. MURAKAMI AND N. KOMABA

Main and aftershocks recorded at Kasho Dam in the 2000
Western Tottori earthquake

Fig. 22

Fig. 20Peak periods of the dam in the upstream-downstream direction

Fig. 21Sub-gate control room and its dimensions

associated with the fundamental mode of dam vibration. The peak
periods of these amplification factors are 0.084, 0.096, 0.092, and
0.087 s for segments 1 through 4. During the main shock, the peak
period therefore initially lengthened then shortened, the change

Schematic explanation showing the post-tensioning of thefloor beams and micro-tremor measurement locations

exceeding 10%.
Lengthening of the vibration period during strong earthquake
shaking often is due to the non-linearity of material properties. In
this case, however, because lengthening is seen only in the
upstream-downstream direction, and thereafter the period soon
shortened, factors other than the non-linear properties appear to be
responsible for the change in the period. One probable factor is thehydrodynamic pressure acting on the upstream face of the dam
which would change in proportion to the intensity of earthquake
acceleration (Westergaard, 1933).
At Kasho Dam, strong-motion acceleration was well recorded
not only during the main shock but the aftershocks (Table 2)(Japan Commission on Large Dams, 2002), which shows peakaccelerations in the upstream-downstream direction. Note that inTable 2 the reservoir water at the times of the shocks was almost
the same levels, between EL 112.20 and EL 110.93 m. Peak periods in the upstream-downstream direction were similarly obtained
from the aftershock acceleration records and plotted against thepeak accelerations at the top of the dam (Fig. 20). The peak period
tends to increase with the peak acceleration, as shown by the broken line in the figure, whereas the plotted periods are somewhat
scattered. Further study is necessary to identify with certainty
those factors responsible for the period change.
4.2. Damage and repairs to the sub-gate control room
In the sub-gate control room, concrete was cracked on thefloor and sidewalls after the main shock. As shown in Fig. 21, theroom is supported by cantilever beams at the center of the dam
crest.
The cracks in the cantilever floor-beams were repaired bypost-tensioning with steel bars, shown schematically in Fig. 22.
Before and after the repairs, micro-tremor measurements were conducted at points on the dam’s crest and inside the room. Fourier
spectra of the velocity components recorded at Point B, before
repair work began, are plotted in Fig. 23(a). The vertical component had the largest amplitude and a peak period at 0.1 s. Spectral
ratios of the vertical components between Points A and B are plotted in Fig. 23(b). The peak period apparently was shortened from
0.1 to 0.07 s after the repairs, indicative of a remarkable increase in
the rigidity of the cantilever beams as a result of post-tensioning.
In the design of the dam, seismic loading was considered onlyin the horizontal direction. If the dynamic response characteristics

NEAR FIELD EFFECTS OF HIDDEN SEISMIC FAULTING ON A CONCRETE DAM

15gave a permanent displacement of 28 cm to the north, 7 cm to thewest, and about a 5 cm uplift.
The sudden 6-cm drop in the reservoir water level recorded
immediately after the main shock is attributable to permanent displacement of the ground in the near field of the earthquake. The
free vibration of the water level after the sudden drop is interpreted
as the seiching of reservoir water, characterized by a period of 6.5
min and a damping ratio of 2%.
The plumb line installed at the dam indicated a relative displacement of 2.9 mm between its top and the lower inspection
gallery. This displacement is attributable to incremental inclinationof the dam’s foundation induced by the near-field ground displacement.
During the strong shaking produced by the main shock, thevibration period of the dam body showed a noticeable transient
increase in the upstream-downstream direction. There is high probability that the change in period resulted from changes in thehydrodynamic pressure acting on the upstream face of the dam.
Further study is required to examine this effect in detail.
The sub-gate control room suffered cracking of its concrete
floor beams due to intense shaking during the main shock. The
cracks were repaired by post-tensioning with steel bars, resulting
in increased beam rigidity. The effectiveness of the repair work
was confirmed from micro-tremor measurements. Based on theexperiences at this site, it is strongly recommended that the earthquake response not only of the dam body but of appurtenant structures such as sub-gate control rooms be considered in the structuraldesigns of dams.
ACKNOWLEDGMENTS

Fig. 23

Results of micro-tremor measurements.

(or vertical loading) of the sub-gate control room had been taken
into account, measures to avoid concrete cracking could have beentaken in advance. This cracking in the sub-gate control room during the 2000 earthquake indicates that the earthquake response
and/or seismic loading should be considered in seismic designs of
not only of dam bodies but of appurtenant structures.
5. CONCLUSIONS
Kasho Dam, a concrete gravity dam, was in the near field of
the 2000 Western Tottori earthquake (MJ 7.3) caused by a hidden
seismic fault. Strong-motion acceleration at the dam during themain shock exceeded 2000 gal at the top and 500 gal in the lower
inspection gallery. Despite these large accelerations, the dam survived the earthquake without serious damage. The reservoir waterlevel, however, dropped markedly immediately after the main
shock, and the vibration period of the dam varied by more than
10% during the main shock. The causes and effects of these phenomena were investigated in this study, with the following conclusions:
Seismic rupturing of a hidden fault caused not only dynamic
but permanent displacement at the dam site. Integration of thestrong-motion acceleration recorded in the lower inspection gallery

We thank the Department of Civil Engineering, Tottori
Prefecture for providing valuable information regarding the effectsof the 2000 earthquake on Kasho Dam. This study was supported
in part by the Ministry of Education, Science, Sports and Cultureof Japan through Grants-in-Aid for Scientific Research Nos.
13480119 (T. Ohmachi) and 12555134 (K. Kawashima).
REFERENCES
Geographical Survey Institute: http://www.gsi.go.jp/wnew/pressrelease/2000/1007-2.htm.
Inoue, D., K. Miyakoshi, K. Ueta, A. Miyawaki and K. Matsuura, 2002.
Active fault study in the 2000 Tottori-ken seibu earthquake area,
Zisin, (Journal of the Seismological Society of Japan), Vol. 5, No.4,
557-573 (in Japanese).
Japan Commission on Large Dams, 2002. Acceleration records on dams
and foundations No. 2 (CD-ROM version).
Lamb, H. 1932. Hydrodynamics, Cambridge Univ. Press, Cambridge, U.
K. 190-191.
Ohmachi, T., N. Kojima and A. Murakami, 2002. Ground displacement
and change in reservoir water level due to seismic rupturing of a hidden fault underlying a dam site, Jour. Str. Mech. Earthquake Eng.,
JSCE, No.710/I-60, 337-346 (in Japanese).
Ohmachi, T., H. Tsukiyama and H. Matsumoto, 2001. Simulation of tsunami induced by dynamic displacement of seabed due to seismic faulting, Bull. Seism. Soc. Am., Vol. 91, No.6, 1898-1909.
Westergaard, H. M., 1933. Water pressure on dams during earthquakes,
Trans. ASCE 95, 418-433.