Rupture process and strong ground motions of the 2007 Niigataken Chuetsu-Oki earthquake -Directivity pulses
striking the Kashiwazaki-Kariwa Nuclear Power Plant Summary

NS dip ① ( str,dip,rake) = (223,44,84)
北西傾斜 ① (STR,DIP,RAKE)=(223,44,84)
北西傾斜 ① (STR,DIP,RAKE)=(223,44,84)② (STR,DIP,RAKE)=(218,42,110) ② ( str,dip,rake ) = (218,42,110)② (STR,DIP,RAKE)=(218,42,110)

1000

KKZ1G1: Unit 1 (surface)
KKZ5G1:Unit 5 (surface)
KKZ1R2: Unit 1 (under ground)
KKZ5R2: Unit 5 (under ground)
1000

KKZ1G1 NS

0203040KKZ1R2 NS

2030403040304030400-800101300203040

KKZ5G1 NS-80010130020

KKZ5G1 EW00-130010500

Acc (cm/s/s)

-1000
10800

KKZ1R2 EW0Fig.1. The focal mechanism (determined by NIED) anddistribution of seismic intensity from the 2007 Niigata-ken
Chuetsu-Oki earthquake.

KKZ1G1 EW0-1000
10800

203040 -130010
500

00-50010-500102020KKZ5R2 EWKKZ5R2 NSTime(sec)30-1000
108002030

NIG018 NS

40r=24km0-100010800203040NIG018 EW

203040r=35km

NIG019 NS0102
1011000-800
10500-800
10500

203010-140203040

110102100

10-11000

Distance (km)Unit 1 (under ground)
Unit 2 (under ground)101100

K-NET
K-net
司・
翠川and Midrikawa
Shi

NIG019 EW

K-NET
K-net
Fukushima
and Tanaka(Rock)福島・田中(ROCK)
Fukushima
and Tanaka(Hard-Medium)
福島・田中(HARD-MEDIUM)

1

10100

Distance (km)1000

SE dip ① ( str,dip,rake) = ( 55,34,98)
南東傾斜 ①(STR,DIP,RAKE)=(55,34,98)② ( str,dip,rake) = (33,49,91)②(STR,DIP,RAKE)=(33,49,91)

0-500
10300

Acc. Amp (cm/s/s)

00

Unit 1 (surface)
Unit 5 (surface)

南東傾斜 ①(STR,DIP,RAKE)=(55,34,98)②(STR,DIP,RAKE)=(33,49,91)

-500
10300

203040103

103

r=51km

Acc. Amp (cm/s/s)

0NIG021 EW

0Acc. Amp (cm/s/s)

Fig.4. Relationship of observed peakhorizontal ground accelerations versus
-300
-300
1020304010203040
1010
shortest distances to the source fault.
300300
NIG025 EW
r=57km
NIG025 NS
101000
Left: Red solid and dotted curves
K-net
Shi and Midrikawa-300-300
show the empirical PGA attenuation101020304010203040101101001000
1101001000
Time(sec)
Time(sec)
distance relationship for surface data(no site classification) by Si and Midorikawa (1999) and its standard deviation.
Fig.3. Examples of horizontal acceleration waveforms
Right: Green and blue curves show that for the average of medium and hard soilin near-source region.
surface data and that for rock surface data.
NIG021 NS

102
10102
10K-NET
司・翠川

K-NET
K-net
福島・田中(ROCK)
Fukushimaand Tanaka(Rock)福島・Fukushima
田中(HARD-MEDIUM)
and Tanaka(Hard-Medium)-1-1

Distance (km)Distance (km)Event factors of observed ground motions are studied for two different dip models (NW dip and SE dip). The event factors are
estimated as average spectral ratio of observed response spectra to empirical spectra proposed by Abrahamson and Silva(1997) shown inFig.5. The results are shown in Fig.6. The top and bottom panels indicate the event terms due to assumed NW-dip (model A) and SE-dip(model B) faults. The data whose shortest distances are within 30 km are used for the estimation. The result suggests that the earthquake
is not a particular but almost a normal one.

lnSa ( f ) =High seismic intensity of 6-upper in the JMA scale were measured intensively in an area along the coast line south of the epicenter ofthis earthquake (Fig.1). The KKNPP is located inside the high intensity area. Strong ground motions on surface observed at K-NETstations are shown in Fig.2 and those on surface and in underground at the KKNPP in Fig.3. The records at NIG018 and NIG019 andKKNPP have two or three distinct pulses. Especially, at the surface station near Unit 5 of KKNPP, maximum acceleration over 1200gal
was recorded. The relationship between observed peak horizontal accelerations at K-NET and shortest distances to the source fault isshown in Fig. 4. The source faults are here assumed, determined by the Geographical Survey Institute using the GPS data. The PGA
attenuation-distance relationships generally follow the empirical relations in Japan, obtained by Fukushima and Tanaka (1990) and Siand Midorikawa (1999), even if either fault plane, SE dip or NW dip, is assumed. However, the strong ground motions in the site of theKKNPP had markedly large accelerations more than those expected from the empirical relations.

Unit 5KKZ1G1 EW

103

Attenuation relationship

1. How strong are ground motions from this earthquake ?
-Comparison between observation and empirical attenuation
relationships of PGA and response spectra-

Unit 1

r=20km

Acc. (cm/s/s)

The Niigataken Chuetsu-Oki earthquake occurred on July 16, 2007, northwest-off Kashiwazaki in Niigata Prefecture, Japan, causing
severe damages of ten people dead, about 1300 injured, about 1000 collapsed houses and major lifelines suspended. In particular,
strong ground motions from the earthquake struck the Kashiwazaki-Kariwa nuclear power plant (hereafter KKNPP), triggering a fire at
an electric transformer and other problems such as leakage of water containing radioactive materials into air and the sea, although theradioactivity levels of the releases are as low as those of the radiation which normal citizens would receive from the naturalenvironment in a year.
The source mechanism of this earthquake is a reverse fault, but whether it is the NE-SW strike and NW dip or the SW-NE strike andSE dip are still controversial from the aftershock distribution and geological surveys near the source. Results of the rupture processes
inverted by using the GPS and SAR data, tsunami data and teleseismic data so far did not succeed in determining which fault planes
moved.
Strong ground motions were recorded at about 390 stations by the K-NET of NIED including the stations very close to the sourcearea. There was the KKNPP which is probably one of buildings and facilities closest to the source area. They have their own strong
motion network with 22 three-components’ accelerographs locating at ground-surface, underground, buildings and basements ofreactors.
The PGA attenuation-distance relationships made setting the fault plane estimated from the GPS data generally follow the empirical
relations in Japan, for example, Fukushima and Tanaka (1990) and Si and Midorikawa (1999), even if either fault plane, SE dip or NW
dip, is assumed. However, the strong ground motions in the site of the KKNPP had very large accelerations and velocities more thanthose expected from the empirical relations. The surface motions there had the PGA of more than 1200 gals and even undergroundmotions at the basements of the reactors locating five stories below the ground had the PGA of 680 gals.
We simulated ground motions using the characterized source model (Kamae and Irikura, 1998) with three asperities and theempirical Green’s function method (Irikura, 1986). Then, we found that the source model should be a reverse fault with the NE-SW
strike and NW dip to explain the strong motion records obtained near the source area. In particular, strong ground motions in the site ofthe KKNPP had three significant pulses which are generated as directivity pulses in forward direction of rupture propagation. This is
the reason why the strong ground motions in the site of the KKNPP had very large accelerations and velocities. The source model isalso verified comparing the observed records at the KKNPP with the numerical simulations by the discrete wavenumber method(Bouchon, 1981).

KKZ1G1 NS1000

Acc. Amp (cm/s/s)

1031000

4020Time(sec)Fig.2. Example of horizontal acceleration waveforms at Units 1
and 5 of the KSWAPP.

f1(M, rrup)+ F ⋅ f3(M)Effect of hanging wall+ HW⋅ f4(M, rrup)+ S ⋅ f5( pgarock)
Ground response (non-linear)

Fig.5. Spectral attenuation relationship by Abrahamson
and Silva (1997).

Fig.6. Map showing two fault planes with the SE and NW dips,
respectively, by NIED (left) and estimated event terms due to thefault planes (right).

2. Source model estimated using the empirical Green’s function method
We estimate the source model of the Niigata-ken Chuetsu-Oki earthquake using the empirical Green’s function method. The strike anddip of the source model are estimated from the aftershock distribution determined by ERI of University of Tokyo as shown in Fig.7.
Referring the slip distribution estimated with the waveform inversion using the empirical Green’s functions by Nozu (2007) as shown inFig. 8, we set an initial model consisting of three asperities and obtained the best-fit model by forward modeling to minimize the residuals
between the observed and synthesized. Strong motion data at 16 stations in Fig 9 are used in this analysis.
The best-fit source model is shown in Fig10 and the source
parameters are summarized in Table 3. The first asperity (ASP 1) islocated near the hypocenter of the mainshock, the second (Asp 2) inthe deeper part of south-west direction of the hypocenter; and thethird (Asp 3) in the shallow part of south-east direction of Asp 2.
Asp 3 is largest among the three asperities. As examples, thesynthesized motions at NIG018, NIG019, NIG021, KKZ1R2 andKKZ5R2 are compared with the observed ones in Fig 11. Thesynthesized waveforms agree with the observed ones fairly well. In
particular, three pulses in synthesized velocity and displacement
Fig.7. Aftershock distributions in the horizanntal plane and crosswaveforms are well produced at KKZ1R2 and KKZ5R2 located at
section perpendicular to the strike within 8 hours after theB5F in underground of Unit 1 and Unit 5, respectively. The rupture
occurrence of the mainshock determined by ERI of University ofon Asperity 3 propagates from sea side to land side, making
Tokyo. We determined the strike and dip of the fault plane of thissignificant directivity pulse as the third pulse of the waveforms ofearthquake from this aftershock distribution.
KKZ1R2 and KKZ5R2.

Kojiro Irikura(1), Takao Kagawa(2), Ken Miyakoshi(2) and Susumu Kurahashi(1)(1) Aichi Institute of Technology, Japan, (2) Geo-Research Institute
3. Improvement of the source model by comparing observation with theoreticalsimulation
Fig.9. Map showing the locationsof K-NET stations and KKNPP(circle) and the epicenters of themainshock (red star) and anaftershock (blue star).

Fig.8. Slip model of this earthquake estimated with the wave
inversion using the empirical Green’s functions by Nozu (2007)(right). The epicenters of aftershocks and locations of K-NETstations used in the analysis are shown in the left.

Referring to the initial source model (Model 1) estimated by empirical Green’s function method, next we revised source model(Model 2) using theoretical Green’s function method. The discrete wavenumber method (Bouchon, 1981) associated with the reflection
transmission propagator matrix method (Kennett and Kerry, 1979) was used to calculate theoretical Green’s functions. Fig.12 shows Swave velocity structure at KKZ1R2 and KKZ5R2. Theoretical velocity waveforms, which were calculated from Model 1, areoverestimated at NIG018 (K-NET). Fig. 13 shows Model 2. Fig.14 shows theoretical velocity waveforms calculated from Model 2 at
KKZ1R2 and KKZ5R2. The calculated waveforms agree well with the observed waveforms. Fig.15 shows the comparison of theoreticalwaveforms at KKZ1R2 calculated from the rupture process having multi-hypocenters (1) and circular front (2). The directivity pulses atKKZ1R2 are recognized in theoretical waveforms calculated from the rupture process having multi-hypocenters. Strong ground motionsin the site of the KKNPP plant, which had three large pulses, are generated as directivity pulses in forward direction of rupture
propagation from three asperities. To explain such directivity pulses, the source model should be a reverse fault with the NE-SW strikeand NW dip.
S-wave velocity[m/s]
01000

2000

3000

4000

KKZ5R2KKZ1R20KKZ5R21000

Table 1. The information of the mainshock and an aftershock
used as the empirical Green’s function.
After shock

Origin time

07/07/16

Hypocentral

37.557，138.609

37.509，138.630

Depth

12km

20.4kmMw

6.6

4.4Mo（F-net）

8.37E+18Nm

5.21E+15Nm（str,dip,rake）（F-net）

25/223

10：13

51/42

Depth[m]Main shock2000

72/11107/07/16

187/39KKZ1R2

3000
4000

21:08

Fig.10. Source model consisting of three asperities
estimated by forward modeling.

5000
6000

Table 3. The source parameters for each asperity.

54/41 70/115

Table 2. The fault plane of the mainshock. We determined the strikeand dip using the aftershock distribution.
StraikeDiprakeLatitude

Longitude

depth

218

4590

37.5045138.7301

0km

Rupture start point

depth

Mo(Nm)

Asp1（2, 4 ）

12.0km

8.7×1017

Asp2（1, 3 ）13.4km9.0×1017

Asp3（3, 5 ）

9.2km

1.5×1018

L(km)×W(km)⊿σ(MPa)Risetime (second)

Asp1

5.2×5.2（N：4×4）

15.1

0.5

Asp2

5.2×5.2（N：4×4）

15.6

0.6

Asp3

6.5×6.5（N：5×5）13.3

0.6

Fig.12. S-wave velocity structure at KKZ1R2and KKZ5R2 (Kamae and Kawabe, 2007).

Fig.13. Initial source model (Model 1) and revised
source model (Model 2).
(1) multi-hypocentersFig.14. Theoretical velocity waveforms (red) calculated from
Model 2, comparing with the observed waveforms (black) atKKZ1R2 (left) and KKZ5R2 (right).

(2) circular front
asp1
asp2
asp3

asp1
asp2
asp3

(1)
(2)

(1)
(2)

Fig.15. Comparison of theoretical waveforms at KKZ1R2 calculated from the rupture process having multi-hypocenters (1) andcircular front (2).

Fig.11. Comparison between the observed records (black) and synthesized motions (red).
Acceleration (top), velocity (middle), and displacement (bottom) are shown at
NIG018,NIG019,NIG021,KKZ1R2 and KKZ5R2.
Left figure shows comparison of velocity waveform between observed and synthesized
motions of NS and EW components at 16 stations.

Acknowledgements
We appreciate National Research Institute for Earth Science and Disaster Prevention (NIED) for providing the K-NET, KiK-net andF-net data. We also thank to Tokyo Electric Power Company (TEPCO) for providing strong ground motion data at the KashiwazakiKariwa nuclear power plant.