LETTER

Earth Planets Space, 57, 545–549, 2005

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake
derived from a combined analysis of temporary online observations
and permanent observations
Takuo Shibutani1 , Yoshihisa Iio1 , Satoshi Matsumoto3 , Hiroshi Katao1 , Takeshi Matsushima3 , Shiro Ohmi1 , Fumiaki Takeuchi1 ,Kenji Uehira3 , Kin’ya Nishigami1 , Bogdan Enescu1 , Issei Hirose1 , Yasuyuki Kano2 , Yuhki Kohno4 , Masahiro Korenaga4 ,Yutaka Mamada1 , Masatoshi Miyazawa1 , Ken’ichi Tatsumi1 , Tomotake Ueno2 , Hiroo Wada1 , and Yohei Yukutake2
1 Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
2 Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
3 Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara 855-0843, Japan
4 Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan(Received February 16, 2005; Revised May 6, 2005; Accepted May 15, 2005)

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern partof the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followedby an anomalously intense aftershock activity that included nine Mj ≥ 5.5 aftershocks. We deployed three
temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary
stations with those from permanent stations located around the aftershock area, and determined the hypocenters ofthe mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock
distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on
27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular toeach other. This might be due to heterogeneous structure in the source region. The strain energy was considered
to have been enough accumulated on the individual fault planes. These features are probably a cause of theanomalous intensity of the aftershock activity.
Key words: The 2004 Mid Niigata Prefecture Earthquake, aftershock distribution, complexity of earthquake
faults, temporary online aftershock observations.

1.

Introduction

The 2004 Mid Niigata Prefecture Earthquake occurred
on 23 October 2004 with the Japan Meteorological Agency(JMA) magnitude (Mj) of 6.8 (Fig. 1). The maximum intensity of VII in JMA scale and the maximum acceleration of 1,722 gal were observed at Kawaguchi Town which
is located in the vicinity of the epicenter of the mainshock (Japan Meteorological agency, 2004). This earthquake killed 40 people and injured more than 4,600 people.
It severely destroyed ∼5,000 houses and partially ∼100,000
houses.
The mainshock was followed by four aftershocks with
Mj ≥ 6.0 and four with 5.5 ≤ Mj < 6.0. The remarkablefeature of this earthquake was that much more larger aftershocks (Mj ≥ 5.5) occurred than those in the 2000 Western
Tottori Earthquake (Mj = 7.3) and in the 1995 Hyogo-ken
Nanbu (Kobe) Earthquake (Mj = 7.3). In the case of the2000 Western Tottori Earthquake, there was only one aftershock with Mj ≥ 5.5 and one with 5.0 ≤ Mj < 5.5. In
the case of the 1995 Kobe Earthquake, there were six aftershocks with 5.0 ≤ Mj < 5.5. The feature that manylarger aftershocks occurred affected the lives of refugees,c The Society of Geomagnetism and Earth, Planetary and Space SciCopy right
ences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

who could not go home because of fears that their houses
would collapse due to strong shaking.
The 2004 Mid Niigata Prefecture Earthquake occurredin the Niigata-Kobe Tectonic Zone in which large strain
rates (>0.1 ppm/y contraction) were found from GPS data
analyses (Sagiya et al., 2000). Many large earthquakes
occurred along this tectonic zone such as the 1964 Niigata
Earthquake (Mj = 7.5), the 1847 Zenkoji Earthquake (M =7.4), the 1828 Echigo-Sanjo Earthquake (M = 6.9) in theneighboring areas of the 2004 earthquake (Sagiya et al.,2000).
We deployed three temporary online seismic stations justabove the aftershock area in order to constrain focal depths
and mechanisms of aftershocks. We combined data fromthe temporary stations and from permanent stations aroundthe aftershock area, and relocated aftershocks with a joint
hypocenter determination (JHD) technique. The purposesof this study are to discuss the source process of the earthquake with the improved aftershock distribution and to clarify the relation between the feature that many larger aftershocks occurred and the fact that the earthquake occurred in
the Niigata-Kobe Tectonic Zone.

2.

Data and Method

The distribution of 60 stations used in this study is shownin Fig. 1. DP.TDOM, DP.OJKW and DP.YMKS are the

545

546

T. SHIBUTANI et al.: AFTERSHOCK DISTRIBUTION OF THE 2004 MID NIIGATA PREFECTURE EARTHQUAKE

20 km
H.AHIH
H.ARKH

H.SKWH

J.SADO

38°

E.AKD

Pr
efe
ctu
re

H.SROH
E.FKA

40°

Ni
iga
ta
J.NAKAM A

J.IZUMOZ

H.STDH H.KMKH
DP.TDOM
H.TDMHDP.YMK S
J.HIROKA
H.YNTH

H.KWNH

H.ATD HJ.YANAI Z

H.SMGH

3.

H.INAH
H.MUIH

H.SZWH
E.TNN

H.MKOH

H.HMTH

H.NZWH

H.KYWH

H.MKGH
H.NKNH

H.YZWHH.MNKHJ .K UNI E.KU J

H.FJ2H
H.KYEH

37°
J.SHIOBA
H.YITH

J.KATASH

H.TGKH138°

H.ATKH

H.NGOH

H.MAKHJ .MATSU S

H.TWAH

H.MRMH

H.TSTHDP.OJKW

E.NUJ2
H.IGWH

H.KMOH

E.KZK

140°

H.NAZHJ.SASAKA

E.YHJ

H.TNEH

H.IICHH.TAYH

139°√
time was 1/1.73 (≈0.76) of that of the P time.
Then we applied a single-event location procedure withthe station corrections and the 1-D V p and V s models anddetermined 7,257 events which were selected with the following criteria: (1) the number of the P arrival times was
greater than or equal to 10, (2) the magnitude was greaterthan or equal to 0.0, and (3) the period was from 23 October
to 31 December in 2004.

140°

Fig. 1. Map showing the location of the stations (cross with square:
temporary online, cross: permanent online) used in this study. Gray
dots denote the aftershocks to be relocated in this study. The dashedlines indicate the borders of prefectures. In the inset map the target area
is shown by the dark gray rectangle with respect to greater parts of Japan
Islands.

Results

3.1 JHD relocation
After the JHD procedure the root mean square (RMS) of
the travel time residuals decreased to 0.024 s. The resulting
1-D velocity models are shown in Fig. 2. V p and V s arewell resolved at the depths of down to 17 km, and thestandard errors are smaller than 0.05 km/s for both the V p
and V s at most of the depths. At the depths shallower
than 3 km the obtained V s is significantly lower than the
JMA2001 velocity model (Ueno et al., 2002) shown by thebroken line, and the V p/V s shows relatively high values(∼2.0). On the other hand, at the depths of 5–17 km both of
the V p and V s are slightly higher than the JMA models, andthe V p/V s takes values 1.69–1.73 which agree well with
the JMA model.
The resulting station corrections for the P- and S-travel
times are illustrated in Fig. 3. Large positive travel time
residuals are found at stations in Niigata Basin to the west
from the source area, whereas negative residuals are foundin Echigo Mountains to the east. The large differencesin the travel time residuals (∼1 s for P and ∼2 s for S)
between the two areas significantly affected the aftershock
locations. As a result of the JHD relocation the hypocenters
moved horizontally ∼3 km in the direction of ∼N65◦ W andupwards ∼3 km in the direction of ∼45◦ from the zenith
with respect to the corresponding JMA hypocenters.
3.2 Single-event relocation
We examined the distribution of the standard errors of
the hypocenters and found that the errors were smaller than
0.31 km in EW, 0.27 km in NS and 0.83 km in Z for morethan 90% of the 7,257 relocated events. Figure 4 shows
the distribution of the selected aftershocks with the condition that the errors in all the EW, NS and Z directions are
smaller than 1.0 km. ∼93% of the relocated events were

DEPTH, km

temporary stations at which three components of short period (1 s) velocity-type seismometers were installed and the
waveform data digitized at the rate of 100 Hz were sent
through satellite telemetry systems (Urabe et al., 1998). We
started the operations at DP.TDOM and DP.OJKW on 27
October. The dynamic range of the satellite telemetry system used in both the stations was 120 dB. We could use AC
power at DP.TDOM, however, we had to use a portable-type
generator at DP.OJKW. The observation at DP.YMKS wasstarted on 29 October. The dynamic range of the satellite
telemetry system used in this station was 142 dB. We used
more than 15 car butteries to operate the system because no
AC power was available and all people had already evacuated their home town. The others are the permanent online
stations operated by National Research Institute for Earth
Science and Disaster Prevention (NIED), JMA and University of Tokyo. The waveform data from the permanent stations were also sent to our institute. Both of the data werecombined on our monitoring system. We picked P- andS-arrival times, P polarities and the maximum amplitudes
on the three component waveforms. The accuracies of thearrival times were typically estimated at 0.01–0.04 s for P
2
3
4
5
6
7 1.6 1.8 2.0 2.2
and 0.02–0.08 s for S. We also utilized P- and S-arrival
0
0
times from the JMA preliminary catalog.
We selected 1,579 events with the following criteria: (1)55
the numbers of both P- and S-arrival times were greater
10
10than or equal to 10, (2) the magnitude was greater than or15
15
equal to 1.7, and (3) the period was from 29 October to 31
Vs
Vp
Vp/Vs
December in 2004 during which data from the three tempo20
20
rary stations were available. We determined simultaneously
25
25
the hypocentral parameters, 1-D velocity models of P- and
2
3
4
5
6
7 1.6 1.8 2.0 2.2
S-waves and the travel time corrections of P- and S-waves
VELOCITY, km/s
Vp/Vs
for the 60 stations using a JHD technique (Kissling et al.,1994). In this technique, a positive (negative) station cor- Fig. 2. 1-D velocity models of the P- and S-waves from −1 to 25 km
rection means a positive (negative) travel time residual indepth. The V p/V s is also shown in the right side. The dotted lines
indicate the JMA2001 velocity models. The solid lines denote thedicating that an observed travel time is larger (smaller) thanvelocity models obtained from the JHD relocation.
a theoretical one in an average sense. The weight of the S

T. SHIBUTANI et al.: AFTERSHOCK DISTRIBUTION OF THE 2004 MID NIIGATA PREFECTURE EARTHQUAKE
138°

138°

O - C (P)O - C (S)- 0.5s- 1.0s

0.5 s

1.0 s

Height, m
2000
38°

ou

go

37°

M

a
at

(a)

139°

1000

ns

ai

nt

ou
oM
ig
h
Ec

hi
Ec

138°

1500

Ba
sin

in

a
nt

Ni
ig

a
at

20 km

Ni
ig

20 km

140°

139°

Ba
sin

38°

140°

139°

500

37°

(b)

140°

138°

547

0

140°

139°

Fig. 3. Travel time corrections for the P- and S-waves derived from the JHD relocation. (a) for the P wave and (b) for the S wave. The red circles
indicate the positive residuals, while the blue crosses denote the negative residuals. Reference sizes are shown in the insets. The small open circles
indicate major events in the 2004 Mid Niigata Prefecture Earthquake sequence. The altitude is shown by the color scale.
Table 1. Hypocenters and mechanisms of the major events.

Origin time (JST)

Latitude (◦ )Longitude (◦ )Depth (km)Mj

(S, D, R) (◦ )#1

2004/10/23
17:56:00.206

37.3046(0.104)138.8362(0.096)11.079(0.371)

6.8(212, 47, 93)(27, 43, 87)#2

2004/10/23
18:03:12.704

37.3613(0.426)138.9629(0.253)4.920(1.531)

6.3(218, 47, 107)
(14, 45, 73)#3

2004/10/23
18:07:30.934

37.3583(0.320)138.8336(0.244)6.627(1.096)

5.7(182, 36, 61)(37, 59, 110)#4

2004/10/23
18:11:56.388

37.2728(0.307)138.8029(0.310)4.330(1.955)

6.0(234, 37, 118)
(20, 58, 70)#5

2004/10/23
18:34:05.682

37.3174(0.150)138.8990(0.142)

12.594(0.438)6.5(221, 59, 94)(33, 31, 84)#6

2004/10/23
19:45:56.787

37.3122(0.139)138.8510(0.136)7.642(0.750)5.7(217, 40, 107)
(16, 52, 76)#7

2004/10/25
06:04:57.283

37.3419(0.134)138.9178(0.128)13.545(0.418)5.8(215, 53, 94)(29, 37, 85)#8

2004/10/27
10:40:50.221

37.2980(0.124)139.0060(0.117)12.281(0.341)

6.1(218, 60, 100)
(18, 32, 73)#92004/11/04
08:57:29.604

37.4379(0.170)138.8787(0.163)14.848(0.316)

5.2(190, 48, 77)(29, 43, 104)#10

2004/11/08
11:15:58.694

37.4016(0.182)138.9975(0.166)2.831(0.251)

5.9(209, 38, 103)
(13, 53, 80)

Standard errors in km are shown in the parentheses in the latitude, longitude and depth columns. (S, D, R) means the angles
of strike, dip and rake of the nodal planes. These values are from NIED MT solutions (NIED, 2004).

selected. The aftershocks distribute beneath Higashiyama
and Uonuma Hill Zones, and extend for ∼40 km in the direction of N30◦ E–S30◦ W. Major events are listed in Table 1 and shown by the open circles with the numbers. The
aftershock distribution is deeper (down to ∼15 km) at thecentral part, while it is shallower at the margins. The mainshock (#1) and the largest aftershock (#5) started near the
deepest points.
Figure 5 shows the X-Z cross-sections at every 4 km
from Y = 16 km to Y = −8 km. From the aftershock

distributions we can recognize the fault planes for someof the major events. The translucent red thick (broken)lines indicate the estimated fault plane of the mainshock(#1), which slopes downward to N60◦ W with the dip of∼50◦ and extends from Y =∼ −4 km to Y =∼ 12 km.
The estimated fault plane of the largest aftershock (#5) isindicated by the translucent blue thick (broken) lines. It is
inclined at ∼60◦ downward to N60◦ W and stretches fromY =∼ −4 km to Y =∼ 8 km. The fault plane of theaftershock occurred on 27 October (#8) is indicated by the

548

T. SHIBUTANI et al.: AFTERSHOCK DISTRIBUTION OF THE 2004 MID NIIGATA PREFECTURE EARTHQUAKE
#10

X, km
0
10

#9

Z, km
0 #10 10 #2 20

20

-10

#3

10

#6

Y, km

O

0

#2

20

#7
#3
#8

#7

10
0

S M

#5

#1

#6
#1
on

#4

(c)

(a)

S

0

#2

M

0 #4 10
20
Z, km

Z, km

#3
#5

#1

0 #7 10 #8 20
X, km

Z, km

#9

(a)

0

10

10

#2

(b)

20

0

0

10

10

#8

(c)

#7

20
0

#3

10

10
#5

(d)

20
S

0
Z, km

M

20
0

#6

10
20
0

Z, km

10
(e)
#1

20
0

#4

10

10
(f)

20
Z, km

Fig. 4. Distribution of the relocated hypocenters from 23 October to 31
December 2004. (a) Epicenter distribution. Note that the map is rotatedanticlockwise by 30◦ so that the Y axis becomes parallel to the aftershock distribution. The background color shows the altitude. The scaleis the same as shown in Fig. 3. The red lines indicate active faults, especially the one with the label M is the Muikamachi Fault, the one with thelabel O is the Obiro Fault and the one with the label S is the Suwatoge
Flexure. The blue crosses denote stations. (b) Depth distribution on the
X-Z cross-section. The symbols S and M at the top indicate the locations of the Suwatoge Flexure and the Muikamachi Faults, respectively.
(c) Depth distribution on the Y-Z cross-section. The sizes of the circles
are proportional to the magnitude of the corresponding aftershocks. The
open circles with the numbers are the major events listed in Table 1. The
size of the major events are a little bit emphasized.

20

0

0

(b)

20

10

#9

20

10
#6

10

20
Z, km

#10

#4

Z, km

#8
#5

0
#10

20

Y, km

20

20

0
Z, km

H ga yama
H s

#9

X, km
0
10

20

0

0

10

10
(g)

20
-10

0
10
X, km

20

20

translucent green thick (broken) lines. In contrast to theabove two events (#1 and #5), the fault plane of #8 slopes
downward to S60◦ W with the dip of ∼30◦ . It extends from Fig. 5. Depth distribution of the relocated hypocenters on the X-Zcross-sections at (a) Y = 16 km, (b) Y = 12 km, (c) Y = 8 km,
Y =∼ 0 km to Y =∼ 12 km. The estimated strike and(d) Y = 4 km, (e) Y = 0 km, (f) Y = −4 km and (g) Y = −8 km. In
dip angles are in good agreement with those of the MT
these cross-sections, events located in the range ±2 km from the planes
solutions (NIED, 2004) shown in Table 1. The translucent
were plotted. The translucent red, blue and green thick lines indicate
the estimated fault planes. The symbols S and M at the top of (e) indiyellow thick broken lines might indicate the fault plane
cate the locations of the Suwatoge Flexure and the Muikamachi Faults,of the aftershock (#4) occurred at 18:11 on 23 Octoberrespectively. The open circles with the numbers are the major events
although the estimated dip angle does not agree with that
listed in Table 1.
of the MT solution (NIED, 2004).

4.

Discussions

The station corrections obtained from the JHD relocationshows large positive travel time residuals in the Niigata
Basin and negative residuals in the Echigo Mountains. The
Niigata Basin and the Higashiyama and Uonuma Hill Zones
are located in the northern Fossa Magna, which was formedby the subsidence under the extension tectonics during theJapan Sea opening (16–13.5 Ma). Thick marine sediments
up to 7 km were deposited in this sequence (Takano, 2002).
These low velocity materials can cause the large positive
travel time residuals.
Our JHD relocation in which the large differences inthe travel time residuals between the Niigata Basin andthe Echigo Mountains were taken into account shifted the
hypocenters ∼3 km to the west-northwest from the corresponding JMA hypocenters. This feature was consistent
with the source models derived from crustal movement datafrom leveling and GPS observations (Geographical Survey
Institute, 2004) and with those derived from crustal deformation detected using SAR interferometry (Ozawa et al.,2005). This supports that the JHD relocation could improve
the absolute locations of the hypocenters.
The mainshock fault plane estimated by the aftershock
distribution (the translucent red thick line in Fig. 5(e)) intersects the surface near the Suwatoge Flexure, while thefault plane of the largest aftershock (#5) (the translucent
blue thick line in Fig. 5(d)) cuts the surface near the Muikamachi Fault (Fig. 4(a)). The Suwatoge Flexure and the
Muikamachi Fault have moved as reverse type faults dipping northwestward (Kim, 2004). Ground deformations
such as horizontal shortenings, landslides and fissures wereconvergently observed near the faults (Kim and Okada,2005). Therefore, the mainshock (#1) and the largest aftershock (#5) were probably caused by dislocations of parts

T. SHIBUTANI et al.: AFTERSHOCK DISTRIBUTION OF THE 2004 MID NIIGATA PREFECTURE EARTHQUAKEof the Suwatoge Flexure and the Muikamachi Fault, respectively.
Hirata et al. (2005) determined hypocenters of ∼600 aftershocks by using data from their rapidly deployed array of
14 stations. Kato et al. (2005) applied a double-difference
tomography to a combined dataset of the same data as Hirata et al. (2005) used and data from 28 permanent stations,and obtained hypocenters of ∼700 aftershocks and 3-D velocity models for the P- and S-waves. They identified threemajor source faults. The source faults of the mainshockand the largest aftershock were parallel, steep west-dipping
faults located ∼5 km apart. The source fault of the major
aftershock on 27 October was perpendicular to the westdipping faults. These features are consistent with our results
mentioned in Section 3.2.
A tectonic inversion from a tensional to a compressional
stress field took place at 7 or 6 Ma (Takeuchi, 1977).
Then the compressional stress became intense from 1 Ma
(Takano, 2002), which formed the Higashiyama and Uonuma Hill Zones by folding the relatively soft marine sediments. It also formed complicated reverse faults in thisregion, some of which were activated and caused the sequence of the 2004 Mid Niigata Prefecture Earthquake asshown in Fig. 5. In addition, the source area is located in
the Niigata-Kobe Tectonic Zone with large contraction rates
and had accumulated enough strain energy. These features
are probably a main cause that many large aftershocks occurred in the sequence.

5.

Conclusions

In order to obtain the detailed aftershock distribution of
the 2004 Mid Niigata Prefecture Earthquake, we relocatedthe hypocenters with the JHD technique using the P- andS-arrival times from the three temporary online stations andthe 57 permanent stations in and around the source area.
We found the large differences in the travel time residuals between the Niigata Basin (positive) and the Echigo
Mountains (negative), which caused biases (∼3 km in theeast-southeast and ∼3 km downwards) in the original JMA
hypocenters. Our relocated hypocenters were considered to
be improved in absolute accuracy.
The obtained aftershock distribution showed that at least
the mainshock, the largest aftershock and the aftershockon 27 October were generated by the different fault planes
which were nearly parallel or perpendicular to each other.
This must be one of the causes that the aftershock activity
contained many larger events.
Acknowledgments. We are very grateful to people in Tochio
City, Ojiya City, Yamakoshi Village and Niigata Prefecture who
kindly helped us in the aftershock observation. We thank Yuta
Asaka, Ling Bai, Mio Hori, Takeshi Katagi, Shigemitsu Matsuo,
Koichi Miura, Masahiro Morita, Sun Cheon Park and Atsushi

549

Watanabe for their help in the observation, and Aiko Nakao forher help in picking up the arrival times. We benefited from thethoughtful reviews of Haruhisa Nakamichi and an anonymous reviewer. We utilized waveform data from permanent stations operated by NIED, JMA and University of Tokyo and P- and S-arrival
times from the JMA preliminary catalog. The Generic Mapping
Tools (Wessel and Smith, 1991) was used for drawing figures.
This work was partially supported by Special Coordination Funds
for Promoting Science and Technology, from Ministry of Education, Sport, Culture, Science and Technology.

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T. Shibutani (e-mail: shibutan@rcep.dpri.kyoto-u.ac.jp), Y. Iio, S.
Matsumoto, H. Katao, T. Matsushima, S. Ohmi, F. Takeuchi, K. Uehira,K. Nishigami, B. Enescu, I. Hirose, Y. Kano, Y. Kohno, M. Korenaga, Y.
Mamada, M. Miyazawa, K. Tatsumi, T. Ueno, H. Wada, and Y. Yukutake