GEOPHYSICAL RESEARCH LETTERS, VOL. 32, XXXX, DOI:10.1029/2005GL023588,

Rupture processes of the 2004 Chuetsu (mid-Niigata
prefecture) earthquake, Japan: A series of events in acomplex fault system

Kazuhito Hikima
Earthquake Research Institute, University of Tokyo, Tokyo, Japan
Technical Center, OYO Corporation, Saitama, Japan
Kazuki Koketsu
Earthquake Research Institute, University of Tokyo, Tokyo, Japan

An edited version of this paper was published by AGU.
Copyright 2005 American Geophysical Union.
Further reproduction or electronic distribution is not permitted.

Kazuhito Hikima, Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku,
Tokyo, 113-0032, Japan. (hikima@eri.u-tokyo.ac.jp)

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HIKIMA AND KOKETSU:

THE 2004 CHUETSU, JAPAN, EARTHQUAKEWe relocated the hypocenters of the 2004 Chuetsu earthquake sequence,
Niigata, Japan, using the double-di erence method. The distribution of aftershocks reveals a complex fault system consisting of ve di erent fault planes.
Inversions of strong motion records for the ve major events within the sequence indicate that the mainshock ( W 6.6) and the largest aftershock ( W
6.3) occurred on parallel WNW-dipping fault planes. The zones of large slip
(asperities) of these two events are located near the hypocenters. In contrast,
the three other large earthquakes ( W 5.9, 5.7 and 5.9) occurred on eastdipping fault planes oriented perpendicular to the fault plane of the mainshock. We used slip distributions deduced from waveform inversions to estimate the Coulomb failure stress changes that occurred immediately prior
to each event. CFF values were positive at the hypocenters and within theasperities of the major aftershocks. These results suggest that stress changes
resulting from individual earthquakes led directly to the occurrence of subsequent earthquakes within the complex fault system.
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Introduction

An earthquake of JMA magnitude ( JMA) 6.8 occurred in the Chuetsu region of Niigata
prefecture, central Japan at 17:56 on October 23, 2004 (JST=UT+9 hours) [JMA: Japan
Meteorological Agency, 2005]. This mainshock was followed by vital aftershock activities,
and the number of large aftershocks was signi cantly greater than recent crustal earthquakes in Japan [JMA, 2005]. The four especially large aftershocks occurred at 18:03( JMA 6.3), 18:11 ( JMA 6.0) and 18:34 ( JMA 6.5) on October 23, and at 10:40 on October 27 ( JMA 6.1). The mainshock and four 6 aftershocks of the earthquake sequence
represent a series of ve events within a single fault system (Figure 1).
Despite the moderate magnitude of the earthquakes, the seismic sequence resulted in
46 deaths, 4,301 injuries, the destruction of 2,827 houses [FDMA: Fire and Disaster Management Agency, 2005] and thousands of landslides [Sidle et al., 2005]. Kato et al. [2005]
investigated the velocity structure of the source region of the earthquakes and the geometry of the fault system in order to understand the cause of the earthquake sequence. This
study also pursues the cause by determining the accurate geometry of the fault system.
We relocate the hypocenters of the major events and smaller aftershocks, and use thesedata to determine candidate fault planes. We will then perform the inversions of strong
motion records to determine the fault planes and the rupture processes of the major
events. Finally, we use fault geometry and slip distributions to calculate changes in static
stress, and we discuss the nature of interaction between the ve major events.
M

M

M

M

2.

Aftershock Distribution

M

M

The accurate location of aftershock hypocenters within the Chuetsu earthquake sequence is an important clue to the geometry of the fault plane of the mainshock [e.g.,
Page, 1968]. The Seismological and Volcanological Bulletin of Japan [JMA, 2005] provides a reliable catalog of hypocenter data for earthquakes in Japan, but the locations
described in this bulletin contain a bias toward the southeast because of the highly irregular velocity structure within the source region [Kato et al., 2005]. We have used the
double-di erence (DD) method [Waldhauser and Ellsworth, 2000] with arrival time data
published in the bulletin to relocate the hypocenters of the major events and smaller aftershocks of the Chuetsu seismic sequence. Because the DD method can determine not
only the relative positions but also the absolute locations of aftershocks, overcoming unmodeled heterogeneity of velocity structure [Menke and Scha , 2004], it was suitable forthe relocation of these events.
The depth pro le of the aftershock distribution suggests activity upon ve distinct
fault planes (Figure 2). We determined the orientations of the plane referring to the focal
solutions provided by NIED (National Research Institute for Earth Science and Disaster
Prevention) [2004]. The mainshock (event 1) and largest aftershock (event 4) are locatedon parallel westward-dipping planes (planes A and D respectively). Event 5 is located on
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plane E oriented perpendicular to planes A and D. The orientations of the fault planes
related to events 2 and 3 are less easily determined. The hypocenter of event 3 is close tothat of event 1, but appears to be located on a poorly de ned plane (plane C) oriented
perpendicular to plane A. A second poorly de ned plane (plane B) oriented perpendicular
to planes A and D contains event 2. Therefore, all events should have had di erent fault
planes, if rupture process inversions con rm that event 2 and 3 occurred on planes B andC, respectively.
3.

Rupture Process Inversions

We used three-component seismograms recorded by borehole instruments at eleven KiKnet stations [Aoi et al., 2000; Figure 1] to perform the rupture process inversion. Boreholedata were chosen to avoid site e ects related to shallow soil conditions. The observed
accelerograms were numerically integrated to obtain velocity waveforms. The resultant
velocities were ltered out with a pass band of 0.02 { 0.5 Hz and re-sampled with aninterval of 0.2 s. We used the re ectivity method of Kohketsu [1985], with an extension
to buried receivers [Koketsu et al., 2004], to calculate the Green's functions for borehole
seismograms.
In order to obtain accurate Green's functions, we used an inverse scheme similar tothat described by Ichinose et al. [2003] to determine a set of one-dimensionally strati ed
velocity models adapted to each station. The seismograms from the moderate magnitude aftershock ( W 5.0) at 18:57 on October 23 were inverted with xed point source
parameters and layer velocities. The initial model was constructed from the results ofseismic explorations in central Japan [e.g., Takeda et al., 2004]. The partial derivatives
were calculated numerically by taking the di erences between the synthetic seismogramsfor the initial model and those generated with 5% perturbations of a layer thickness. We
then determined the layer thicknesses by an iterative non-linear inversion using the focal
solution of the NIED [2004]. Figure S1 shows examples of the resultant velocity models
and comparisons of the observed and synthetic seismograms. The models were validatedby comparison with other aftershocks located near the mainshock.
The rupture process inversions followed the method of Yoshida et al. [1996], which isbased on the formulation of multiple time windows. Slip orientation data indicate reverse
faulting of these events [e.g., NIED, 2004]. Slip vectors are therefore represented by alinear combination of two components in the directions of 9045Æ. Each component isconstrained to a positive value using the non-negative least square algorithm of Lawson
and Hanson [1974] rather than the penalty functions of Yoshida et al. [1996]. The
inversion was subject to a smoothness constraint with a discrete Laplacian in space andtime. The weight of the constraint was determined using ABIC [Akaike, 1980]. Although
we constructed the set of adaptive 1-D velocity models ourselves, the models remain
incomplete. To minimize artifacts resulting from the incomplete nature of the models,M

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scalar time shifts were added to the Green's functions, and the amounts of the time shifts
were also determined by the inversion method of Graves and Wald [2001].
4.

Fault Models and Slip Distributions

We use the fault planes shown in Figure 2 as fault models for analyzing slip distribution.
Fault model orientations were derived from focal solutions provided by NIED [2004] andpreliminary inversions of far- eld body waves. We used the epicenter locations detailedin Figure 2 for determining the horizontal locations of initial rupture during the major
events. The depths of these events were shifted slightly to minimize the residuals between
the observed and synthetic waveforms. The fault models were divided into 2  2 km2
subfaults and the slip histories were represented by a combination of ramp functions witha rise time of 1 s. As described earlier, it is diÆcult to identify the fault planes of events
2 and 3 from the aftershock distribution. To overcome this limitation, we performed
rupture process inversions for planes B and C (Figure 2) and perpendicular planes, andchose the plane with the better t. We measured the t by both total residual and thepartial residual at distinct phases in the near-source waveforms.
The most suitable orientation for the fault planes related to events 2 and 3 is that ofplanes B and C, perpendicular to the fault planes of the mainshock and largest aftershock(Figure S2), dipping to the east. The rupture process inversions con rm that the ve
major events occurred on ve di erent fault planes. All ve fault planes strike in a NNESSW direction (Figure 3), but two of the planes dip westward while the other three dip
eastward. The parameters of these fault planes are summarized in Table 1.
All rupture process inversions produced sound results, as evident in waveform comparisons of the mainshock and largest aftershock (Figure S3). Figure 3 also shows slip
distributions projected onto the ground surface. Rupture associated with the mainshock
initiated from a deep part of the fault plane, and large slips occurred about the hypocenter. A maximum slip of about 1.7 m was recorded several kilometers to the north of
the hypocenter (Figure 3a), suggesting that the direction of rupture propagation was
dominantly northward. The asperity of the largest aftershock is also centered about the
hypocenter and recorded a maximum slip of about 1.0 m. The rupture propagated to thesouth and the slip distribution is more complex than that of the mainshock (Figure 3b).
The other events have relatively simple slip distributions consisting of a single asperity
around the hypocenter (Figure 3c).
5.

Discussion and Conclusions

To assess the nature of any interaction among the major events, we calculated static
Coulomb failure stress change (CFF) using the recovered slip distributions. We used
the formula of Okada [1992], assuming the fault system to be buried in a homogeneous
halfspace with an apparent frictional coeÆcient of 0.4. The receiver faults were set acD R A F T

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cording to the geometry described in Table 1 and assuming pure reverse faulting. Figure 4
shows the distribution of CFF immediately prior to the largest aftershock (event 4) andfollowing events 1, 2 and 3 on a horizontal plane at the depth of the hypocenter of event
4. The CFF is positive around the future site of the event 4 hypocenter and asperity
as imaged from the horizontal and vertical distributions of CFF in Figure 4. PositiveCFF values in the range 0.03 { 0.28 MPa were also determined for the hypocentersof the other major aftershocks for the time period immediately prior to rupture (FigureS4). Although the association of positive CFF and aftershock activity has been previously reported [e.g., Reasenberg and Simpson, 1992; Harris, 1998], the majority of suchstudies describe aftershocks far from the area of the mainshock. Figures 4 and S4 showthat interaction among moderate magnitude closely located aftershocks can be explainedby reference to CFF values calculated from recovered slip distributions. The positiveCFF values calculated about the hypocenters for the time immediately prior to rupturesuggests that the major aftershocks were triggered by stress changes caused by preceding
events.
The 2004 Chuetsu earthquake sequence is characterized by the large number of large aftershocks. 46 events of JMA magnitude 4.5 or greater occurred in the month following themainshock [JMA, 2005]. In contrast, the 1995 Kobe earthquake ( W 6.9) and the 2000
western Tottori earthquake ( W 6.7) generated only 13 and 8 aftershocks respectivelyof JMA4.5 or greater. Yamanaka and Shimazaki [1990] examined the linear scaling ofaftershock numbers with respect to the seismic moment of crustal earthquakes in Japan.
According to their scaling model, the Chuetsu earthquake should have produced only 9 aftershocks with the mainshock and largest aftershock moments of 1.21019Nm. Yamanaka
and Shimazaki [1990] concluded that aftershocks occur within areas of high strength on
the mainshock fault that failed to rupture during the mainshock event. The aftershocks of
the Chuetsu earthquake occurred on the mainshock fault and four other nearby faults. We
consider the disparity in predicted and actual aftershock numbers related to the Chuetsu
earthquake to be directly related to the occurrence of aftershocks on multiple fault planes.
We determined the fault geometry and rupture processes for the ve major events ofthe 2004 Chuetsu earthquake sequence using strong motion records, hypocenters relocatedusing the DD method, and calibrated one-dimensionally strati ed velocity models. We
conclude that the ve major events occurred on ve di erent fault planes. The CFF
due to prior events was positive around the major aftershocks.
The source region of the earthquakes contains complex geological structures and numerous faults and cracks [Kato et al., 2005] related to past tectonic activities [Sato, 1994].
High strain rates have been also recorded in this region [Sagiya et al., 2000]. These features of the tectonic setting controlled the occurrence of the mainshock. The mainshockcaused changes in the local stress distribution that triggered large aftershocks. Each afMMM

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tershock also had an e ect upon the stress distribution and in uenced the occurrence andlocation of subsequent aftershocks.
We thank NIED for the strong motion records from KiK-net andthe mechanism solutions from F-net. We are also grateful to JMA et al. for the hypocenter
list and arrival time data. Thoughtful comments from two anonymous reviewers improved
this manuscript. We thank Drs. Waldhauser, Wessel and Smith for the hypoDD and
GMT codes. Drs. S. Sakai and A. Kato provided valuable suggestions and discussion.
This study was supported by the DaiDaiToku Project I and the Grant-in-Aid for Scienti c
Research No. 15540403 from MEXT of Japan.
Acknowledgments.

References

Akaike, H. (1980), Likelihood and Bayes procedure, in Bayesian Statistics, edited by J.
M. Bernardo et al., pp. 143-166, Univ. Press, Valencia, Spain.
Aoi, S., K. Obara, S. Hori, K. Kasahara, and Y. Okada (2000), New strong-motion observation network: KiK-net, Eos Trans. AGU, 81 (48), Fall Meet. Suppl., Abstract
S71A-05.
FDMA (2005), The 2004 Chuetsu, Niigata, earthquake (No. 66),
http://www.fdma.go.jp/data/010502221022317478.pdf.
Graves, R. W., and D. J. Wald (2001), Resolution analysis of nite fault source inversion
using one- and three-dimensional Green's functions 1. Strong motions, J. Geophys. Res.,
106, 8745-8766.
Harris, R. A. (1998), Introduction to special section: Stress triggers, stress shadows, andimplications for seismic hazard, J. Geophys. Res., 103, 24347-24358.
Ichinose, G. A., H. K. Thio, P. G. Somerville, T. Sato, and T. Ishii (2003), Rupture
process of the 1944 Tonankai earthquake ( S 8.1) from the inversion of teleseismic andregional seismograms, J. Geophys. Res., 108, doi:10.1029/2003JB002393.
JMA (2005), The Seismological and Volcanological Bulletin of Japan for October 2004,
JMA, Tokyo.
Kato, A. et al. (2005), Imaging the source region of the 2004 mid-Niigata prefectureearthquake and the evolution of a seismogenic thrust-related fold, Geophys. Res. Lett.,
32, doi:10.1029/2005GL022366.
Kohketsu, K. (1985), The extended re ectivity method for synthetic near- eld seismograms, J. Phys. Earth, 33, 121{131.
Koketsu, K., K. Hikima, S. Miyazaki and S. Ide (2004), Joint inversion of strong motion
and geodetic data for the source process of the 2003 Tokachi-oki, Hokkaido, earthquake,
Earth Planets Space, 56, 329{334.
Lawson, C. L., and R. J. Hanson (1974), Solving Least Squares Problems, Prentice-Hall,Englewood Cli s., N. J.
M

D R A F T

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HIKIMA AND KOKETSU:

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Menke, W., and D. Scha (2004), Absolute earthquake locations with di erential data,
Bull. Seismol. Soc. Am., 94, 2254-2264.
NIED (2004), Earthquake Mechanism Information, http://www.fnet.bosai.go.jp/freesia/event/hypo/joho.ht
Okada, Y. (1992), Internal deformation due to shear and tensile faults in a half-space,
Bull. Seismol. Soc. Am., 82, 1018-1040.
Page, R. (1968), Aftershock and microaftershocks of the great Alaska earthquake of 1964,
Bull. Seismol. Soc. Am., 58, 1131{1168.
Reasenberg, P. A., and R. W. Simpson (1992), Response of regional seismicity to thestatic stress change produced by the Loma Prieta earthquake, Science, 255, 1687-1690.
Sagiya, T., S. Miyazaki, and T. Tada (2000), Continuous GPS array and present-day
crustal deformation of Japan, Pure Appl. Geophys., 38, 2303-2322.
Sato, H. (1994), The relationship between late Cenozoic tectonic events and stress eld
and basin development in northeast Japan, J. Geophys. Res., 99, 22261-22274.
Sidle, R. C., T. Kamai, and A. C. Tranda r (2005), Evaluating landslide damage during
the 2004 Chuetsu earthquake, Niigata, Japan, EOS Trans. AGU, 86(13), 133{136.
Takeda, T. et al. (2004), Crustal structure in the northern Fossa Magna region, central
Japan, from refraction/wide-angle re ection data, Earth Planets Space, 56, 1293{1299.
Yamanaka, Y., and K. Shimazaki (1990), Scaling relationship between the number ofaftershocks and the size of the main shock, J. Phys. Earth, 38, 305{324.
Yoshida, S. et al. (1996), Joint inversion of near- and far- eld waveforms and geodetic
data for the rupture process of the 1995 Kobe earthquake, J. Phys. Earth, 44, 437{454.
Waldhauser, F., and W. L. Ellsworth (2000), A double-di erence earthquake location
algorithm: method and application to the Northern Hayward fault, California, Bull.
Seismol. Soc. Am., 90, 1353-1368.

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38.0˚
an

f
ao

Jap

ef
ec
tu
r

e

Se

ean

FKSH01
NIGH07

Ni
ig
at
a

cif

Pa

Pr

c
ic O

37.5˚

NIGH01

4(18:34)
1(17:56)
3(18:11)2(18:03)
FKSH215(Oct.27)
NIGH12
FKSH06
NIGH15
FKSH0737.0˚

NIGH19GNMH07
GNMH0920 km
138.5˚

139.0˚

139.5˚

36.5˚140.0˚

Location of the 2004 Chuetsu earthquake sequence. The rectangle indicates
the target area of this study. Black stars represent the epicenters of the ve major events.
Chronological order and timing of rupture events is also indicated. Triangles represent
KiK-net stations that recorded strong motion data analyzed in this study.

Figure 1.

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139.0˚
37.5˚

(a)

A
2
4

1

5

3

A’

10 km

0.0

A

A’
(b)

Depth (km)

B
C
10.0

E
A
20.0

D

(a) Relocated epicenters of the major events (stars) and smaller aftershocks(circles). (b) Depth pro le of hypocenters. The pro le is oriented perpendicular to thestrike of the mainshock fault plane (A-A'). The candidates of fault planes (color segments)
were determined referring to NIED [2004].

Figure 2.

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139.0˚

139.0˚

139.0˚

37.5˚

(a)

(b)

(c)

event 1

event 2
event 4
event 3

event 5
(m)

(m)
2.0

10 km

1.0

10 km

0.0

10 km

(m)
1.0

1.0

0.5

0.5

0.0

10 km

0.0

Surface projections of the recovered slip distributions for (a) the mainshock(event 1), (b) the largest aftershock (event 4) and (c) the other major aftershocks (events
2, 3 and 5). Yellow stars indicate the hypocenter locations of the relevant slip event.
Brown dots represent the hypocenters of other events.

Figure 3.

Summary of Fault Parameters
No. Date Time Strike
Dip
Depth JMA WoÆÆ( ) ( ) (km)(Nm)
1 Oct.23 17:56 216 53 9 6.8 6.6 8 8  101817
2 Oct.23 18:03 20 34 7 6.3 5.9 8 5  1017
3 Oct.23 18:11 20 58 9 6.0 5.7 4 1  1018
4 Oct.23 18:34 216 55 12 6.5 6.3 3 2  1017
5 Oct.27 10:40 39 29 11.5 6.1 5.9 7 5  10
Depth of hypocenter
Table 1.

M

M

M

:

:

:

:

:

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139.0˚
37.5˚

Depth=12km

0.1

A - A’
-10.0

0.0

0-.
010..0 - 1.
0 0. 0
-11.
0

0.0

-0.1
-0
-.0010.
.0.00

-1.0

0

0

1.

-0
.1

10.0

-1
.0

.0

-1
.1

Depth(km)

0
1.

0.0
0.00.0

A

10.0

1

A’
1

MPa

0..0
0

-0.

0

-0.

-1.00 -0.10 -0.01 0.00 0.01 0.10 1.00

∆CFF
0.1

10 km

Distribution of CFF resulting from events 1 { 3 (white rectangles) on ahorizontal plane at the depth of the hypocenter of the largest aftershock (event 4). The
zones of positive CFF are shown in red. The hypocenter and fault plane are indicated by
the yellow star and yellow rectangle respectively. Crosses indicate the error bars. Contours
represent slip of 0.5 m within the asperity. The right-hand gure is a cross-section along
the line A-A'.

Figure 4.

>

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Auxiliary Material Submission for Paper 2005GL023588
Rupture processes of the 2004 Chuetsu (mid-Niigata prefecture) earthquake, Japan: A
series of events in a complex fault system
Kazuhito Hikima (Earthquake Research Institute, University of Tokyo / OYO
Corporation, Technical Center)
Kazuki Koketsu (Earthquake Research Institute, University of Tokyo)
Introduction
These gures are additional materials which we could not include in the paper. The
examples of the inversion for one-dimensionally strati ed velocity structures are shownin Figure S1. As the results of the rupture process inversions, the choise of fault plane is
explained in Figure S2a-b and the comparisons of resultant waveforms are given in
Figure S3a-b. Figure S4 shows the distributions of delta-CFF for the four aftershocks.

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HIKIMA AND KOKETSU:NIGH07(cm)
0.1

THE 2004 CHUETSU, JAPAN, EARTHQUAKE

Velocity (km/s)
0.0

0.0

- 0.1
0.1

2.0

4.0

6.0

FKSH21(cm)
0.1

Velocity (km/s)

8.0 10.0

0.0

0.0

0.0

0.0

2.0

- 0.1
0.1

2.0

4.0

2.0

4.0

6.0

8.0 10.0

4.0

Depth (km)

- 0.1
0.1

0.0

- 0.1
0.0

(cm)
0.2

20.0

40.0
(sec)

Depth (km)

0.0

0.0

6.0
8.0

- 0.1
0.1

10.0

0.0

12.0

- 0.1

14.0

8.0
10.0
12.0

0.0

20.0

40.0
(sec)

(cm)
0.1

NIGH196.0

14.0

FKSH07

0.0

0.0

- 0.1
0.1

- 0.2
0.2

0.0

0.0

- 0.2
0.2

- 0.1
0.1

0.0

0.0

- 0.1

- 0.2
0.0

20.0

Velocity (km/s)
2.0

4.0

6.0

0.0

20.0

Velocity (km/s)
0.0

8.0 10.0

0.0

0.0

2.0

2.0

4.0

4.0

Depth (km)Depth (km)

0.0

40.0
(sec)

6.0
8.0
10.0
12.0
14.0

Obs.
Syn.

2.0

4.0

6.0

40.0
(sec)

8.0 10.0

6.0
8.0
10.0
12.0

Vp
Vs

14.0

Figure S1 The one-dimensionally strati ed velocity models obtained by the structural
inversions for the four stations NIGH07, NIGH19, FKSH21, and FKSH07. The radial,transverse, and vertical components of the observed seismograms from an M5 aftershock(black traces) are compared with those of the synthetics for the nal velocity models(blue traces). The depth pro les of P-wave and S-wave velocities represent the velocity
models.

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(a)
0.30

Eastward
Westward

Relative Variance

0.25
0.20
0.15
0.10
0.05
0.00
0

5

10

15

Depth of Hypocenter

20

(km)

(b)
NIGH12
NS

EW

UD

1.549

1.659

.995

NS

EW

UD

1.549

1.659

.995

Dipping Eastward

Dipping Westward

OBS

SYN

0.0

10.0 20.0 30.0 40.0 50.0 60.0

Time

(s)

Figure S2a Figures S2a and S2b display the diagrams supporting the choices of fault
planes for the events 2 and 3, respectively. (a) The comparison of the relative variancesof the rupture process inversion results for the fault planes dipping eastward andwestward. (b) The synthetic seismograms at the station NIGH12 for the optimum
solutions for both the fault planes overlying the observed seismograms.

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(a)
0.30
Eastward
Westward

Relative Variance

0.25
0.20
0.15
0.10
0.05
0.00
0

5

10

15

Depth of Hypocenter

20

(km)

(b)
NIGH12
NS

EW

.219

UD
.435

.354

Dipping Eastward

NS

EW

.219

UD

.354

.435

0.0

10.0 20.0 30.0 40.0 50.0 60.0

Dipping Westward

OBS

SYN

Figure S2b

D R A F T

Time

(s)

HIKIMA AND KOKETSU:

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OBS

4.419

5.043

4.830

NIGH12NSNIGH12
EW

NIGH12
UD

15.21614.832

7.851

NIGH01NS

NIGH01EW

NIGH01
UD

2.222

1.591

2.532

NIGH15NSNIGH15EW

NIGH15
UD

1.733

2.223

1.609

FKSH21NSFKSH21EW

FKSH21
UD

1.176

1.010

1.160

NIGH07NS

NIGH07EW

NIGH07
UD

.734

.917

2.105

FKSH07NS

FKSH07EW

FKSH07
UD

.740

2.082

2.646

FKSH06NS

FKSH06EW

FKSH06
UD

.928

.947

1.110

NIGH19NS

NIGH19EW

NIGH19
UD

.766

.520

.707

FKSH01NS

FKSH01EW

FKSH01
UD

1.399

1.031

1.558

GNMH07NS

GNMH07EW

GNMH07
UD

1.469

1.430

1.073

GNMH09NS

GNMH09EW

GNMH09UD

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Time

(s)

SYN

0.0 10.0 20.0 30.0 40.0 50.0 60.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0

Time

(s)

Time

(s)

Figure S3a Comparisons between the observed (red) and synthetic (black) waveforms forthe mainshock (event 1) and the largest aftershock (event 4). The rupture was initiatedat 0 s. The gures above the stations codes represent the maximum velocities in cm/s.

D R A F T

X - 17

X - 18

HIKIMA AND KOKETSU:

THE 2004 CHUETSU, JAPAN, EARTHQUAKE

OBS
2.223

1.592

1.373

NIGH12NSNIGH12
EW

NIGH12
UD

5.884

2.395

2.202

NIGH01NS

NIGH01EW

NIGH01
UD

1.488

1.331

.953

NIGH15NSNIGH15EW

NIGH15
UD

.600

.654

.634

FKSH21NSFKSH21EW

FKSH21
UD

.103

.137

.150

NIGH07NS

NIGH07EW

NIGH07
UD

.386

.494

.482

FKSH07NS

FKSH07EW

FKSH07
UD

.337

.524

.456

FKSH06NS

FKSH06EW

FKSH06
UD

.494

.481

.717

NIGH19NS

NIGH19EW

NIGH19
UD

.398

.605

.798

GNMH07NS

GNMH07EW

GNMH07
UD

.616

.712

.618

GNMH09NS

GNMH09EW

GNMH09
UD

.096

.106

.130

FKSH01NS

FKSH01EW

FKSH01UD

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Time

(s)

0.0 10.0 20.0 30.0 40.0 50.0 60.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0

Time

(s)

Figure S3b

D R A F T

SYN

Time

(s)

HIKIMA AND KOKETSU:

THE 2004 CHUETSU, JAPAN, EARTHQUAKE

139.0˚

139.0˚

37.5˚

Depth= 7km

Depth= 9km

event 2

event 3
0.1

0.1

0.0

0.0

--00-.
.1001.0

1.0

0.1

-1
.0

1.0

-1.

-0
.0

-1.
0
0.0
-0.0

.1
-0

1.0
.0
0-0.0

0

0
0.

-0
.1

0.0

0.1

-1.0

1
-0.

0.1

01
0-0-0.0.1.0.

-0.1

0.0

1.0

10 km

10 km
139.0˚

139.0˚

37.5˚

Depth=11.5km

event 4

-0.1
0

-1.0

1.

0.1

1
.00.
0.0
-00.

-00..
010

1.
0.1

-1
1

0

-0.0
00.
.00

0

-0
-.0010.
.0.00

.0

-1.0

1.

0.

-0

.1

1

-0.

0
0-0.0.
-1.0-0

0.0
0.00.0event 5.10-.
010..0 - 1.
0 0. 0
-11.
0

0.1

0.1

Depth=12km

0..0
0

1

0.0
-00.

-0.

0.0

0

-0

-1.

-1.0
.1

0

-0.

0.1

10 km

10 km

MPa
-1.00 -0.10 -0.01 0.00 0.01 0.10 1.00
∆CFF

Figure S4 Distributions of delta-CFF on the horizontal planes at the depths of the
hypocenters of the major aftershocks. The white rectangles denote the fault planes ofthe preceding events. The hypocenter and fault plane of a target event are shown by ayellow star and rectangle, respectively.

D R A F T

X - 19