Seismology [S]
S53A
CC:227
Friday
1330h
The 23 October 2004 Chuetsu Earthquakes and Related Disasters, Niigata, Japan II
Presiding:
R Sidle, Disaster Prevention Research Institute, Geohazards Division, Kyoto University; Y Iio, Disaster Prevention Research Institute, Kyoto University
S53A-01
13:35h
Generation process of the 2004 Niigata-Chuetsu earthquake and its major aftershocks inferred from aftershock distributions
* Iio, Y
(iio@rcep.dpri.kyoto-u.ac.jp)
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Matsumoto, S
(iio@rcep.dpri.kyoto-u.ac.jp)
, Institute of Seismology and Volcanology
Faculty of Sciences, Kyushu University, Shimabara, Nagasaki, Japan, Shimabara, 855-0843 Japan
Katao, H
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Matsushima, T
, Institute of Seismology and Volcanology
Faculty of Sciences, Kyushu University, Shimabara, Nagasaki, Japan, Shimabara, 855-0843 Japan
Ohmi, S
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Shibutani, T
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Takeuchi, T
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Uehira, K
, Institute of Seismology and Volcanology
Faculty of Sciences, Kyushu University, Shimabara, Nagasaki, Japan, Shimabara, 855-0843 Japan
Nishigami, K
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Enescu, B
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Hirose, I
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Kano, Y
, Graduate School of Sciences, Kyoto University, Uji, Japan, Uji, 611-0011 Japan
Kono, Y
, Graduate School of Sciences, Kyushu University, Shimabara, Nagasaki, Japan, Shimabara, 855-0843 Japan
Korenaga, M
, Graduate School of Sciences, Kyushu University, Shimabara, Nagasaki, Japan, Shimabara, 855-0843 Japan
Mamada, Y
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Miyazawa, M
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Tatsumi, T
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Ueno, T
, Graduate School of Sciences, Kyoto University, Uji, Japan, Uji, 611-0011 Japan
Wada, H
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Japan, Uji, 611-0011 Japan
Yukutake, Y
, Graduate School of Sciences, Kyoto University, Uji, Japan, Uji, 611-0011 Japan
The 2004 Niigata-Chuetsu earthquake (Mjma 6.8) occurred in a concentrated deformation zone along the Japan Sea coast (NKTZ:
Niigata-Kobe Tectonic Zone), which was identified from strain patterns determined by the dense GPS network. The observed
difference in deformation rate between the NKTZ and the surrounding region, is almost one order of magnitude (Sagiya et al., 2000). It was, however, controversial whether or not this large deformation rate reflects stress concentration. This is
because larger inland earthquakes occurred outside of the NKTZ, while active folds are observed within the NKTZ. It is
inferred from deformation rate profiles and the estimated stress field in and around the NKTZ that a weak zone with low
viscosity exists in the lower crust beneath the NKTZ (Iio et al., 2002). The large deformation rate may result from aseismic
slip of many ductile fault zones in the lower crust that constitutes the weak zone. The complicated fault plane
distributions of the mainshock and large aftershock sequences are possibly produced by a number of the ductile fault zones in the lower crust. Further, rupture processes of major earthquakes, estimated from aftershock distributions, suggest that the
weak zone beneath the Niigata-Chuetsu region is weaker than in the surrounding areas.
S53A-02 INVITED
13:50h
Multi-Fault System of the 2004 Mid-Niigata Prefecture Earthquake and its Aftershocks
* Hirata, N
(hirata@eri.u-tokyo.ac.jp)
, The Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
Sato, H
(satow@eri.u-tokyo.ac.jp)
, The Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
Kato, A
(akato@eri.u-tokyo.ac.jp)
, The Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
Sakai, S
(coco@eri.u-tokyo.ac.jp)
, The Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
Kurashimo, E
(ekura@eri.u-tokyo.ac.jp)
, The Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
The 2004 Mid-Niigata Prefecture Earthquake (Japan Metrological Agency; JMA), central Japan, was a damaging event that
destroyed up to 10 000 dwellings, with 40 fatalities and leaving 3000 injured. The earthquake occurred at 17:56 (JST) on
October 23, 2004 with a local body wave magnitude (MJMA) of 6.8. The hypocenter was located at a relatively shallow depth of
13 km in an active fault-and-fold system overlain by thick sediments. Data recorded by a seismic network deployed the day
after the mainshock are used to determine the major source faults responsible for the mainshock and major aftershocks. Using
this high-resolution seismic data, three major source faults are identified: two parallel faults dipping steeply to the west
located 5 km apart, and the other dipping eastward and oriented perpendicular to the west-dipping faults. The analysis also
reveals that the lateral variation in seismic velocity observed at the surface extends to a depth of 15 km, encompassing the
source area of the mainshock. This strong heterogeneity of the crust, related to the complex geological and tectonic
evolution of the area, is considered to be responsible for the prominent aftershock activity following the 2004 Niigata event
http://www.eri.u-tokyo.ac.jp/hirata/chuetsu/0401104yotiren1.htm
S53A-03
14:05h
Aftershock distribution and seismic velocity structure in and around the focal area of the 2004 Niigata-Chuetsu earthquake using double-difference tomography, and its relationship with the earthquake generation process
* Okada, T
(okada@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Umino, N
(umino@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Matsuzawa, T
(matuzawa@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Nakajima, J
(nakajima@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Uchida, N
(uchida@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Nakayama, T
(nakayama@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Hirahara, S
(hirahara@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Sato, T
(sato@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Hori, S
(hori@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Kono, T
(kono@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Yabe, Y
(yabe@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Ariyoshi, K
(ariyoshi@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Gamage, S
(sng@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Shimizu, J
(shimizu@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Suganomata, J
(suga@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Kita, S
(kita@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Yui, S
(yui@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Arao, M
(kt.arao@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Hondo, S
(s.hondo@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Mizukami, T
(mizu@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Tsushima, H
(tsushima@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Yaginuma, T
(yagi@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Hasegawa, A
(hasegawa@aob.geophys.tohoku.ac.jp)
, Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku Univ., 6-6 Aramaki-Aza-Aoba, Aoba-ward, Sendai, 9810966 Japan
Asano, Y
(asano@bosai.go.jp)
, National Research Institute for Earth Science and Disaster Prevention, 3-1,Tennodai, Tsukuba, 3060066 Japan
Zhang, H
(hjzhang@ice.geology.wisc.edu)
, Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street,
Madison, WI 53706 United States
Thurber, C H
(clifft@geology.wisc.edu)
, Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street,
Madison, WI 53706 United States
A destructive large earthquake sequence (2004 Niigata-Chuetsu earthquake, magnitude 6.9) occurred in the central part
(Chuetsu district) of Niigata Prefecture, central Japan on Oct. 23, 2004. We deployed a dense temporary seismic network
composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month.
Using arrival time data from the temporary seismic network and surrounding permanent stations, we obtained the precise
aftershock distribution and 3D seismic velocity structure in and around the rupture zones of the main shock and its four
major (M=>6) aftershocks by double-difference tomography. The results show three major aftershock alignments. Two of them
are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond with the fault plane of the main shock and that of the largest aftershock, respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds with the fault plane of the M6 aftershock on Oct. 27. A significant feature of the
obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall
(eastern part of the focal area) has higher velocity. This observation suggests that the 2004 Niigata-Chuetsu earthquake
sequence occurred along faults that acted as normal faults in the Miocene and reactivated as reverse faults under the current compressional stress regime. The main velocity boundary seems to migrate to the west at a location near the central part of
the focal area, where the main shock rupture initiated. Some parts of the fault planes were imaged as low velocity zones.
This complex crustal structure may be one of possible causes of the complicated character of the 2004 Niigata-Chuetsu
earthquake sequence.
http://www.aob.geophys.tohoku.ac.jp/
S53A-04 INVITED
14:20h
Imaging S wave reflectors in and around hypocentral area of the 2004 Niigata-ken Chuetsu Earthquake (M6.8)
* Matsumoto, S
(matumoto@sevo.kyushu-u.ac.jp)
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin-yama 2-5643-29,
Shimabara, 855-0843 Japan
Iio, Y
(iio@rcep.dpri.kyoto-u.ac.jp)
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University
, Gokasho, Uji, 611-0011 Japan
Matsushima, T
(mat@sevo.kyushu-u.ac.jp)
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin-yama 2-5643-29,
Shimabara, 855-0843 Japan
Uehira, K
(uehira@sevo.kyushu-u.ac.jp)
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin-yama 2-5643-29,
Shimabara, 855-0843 Japan
Shibutani, T
(shibutan@rcep.dpri.kyoto-u.ac.jp)
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University
, Gokasho, Uji, 611-0011 Japan
We studied about a distribution of S wave reflectors in and around hypocentral area of the 2004 Niigata-ken Chuetsu
Earthquake (M6.9). The earthquake accompanied with several large after shocks greater than M6. Moreover, the aftershocks were not only located on the fault plane of the main shock, but also on a conjugate fault and that parallel to the main shock
one. In order to discuss the relationship between such a complex activity and the crustal heterogeneities, we analyzed
seismograms observed at seismic stations in this region. Normal moveout processing (NMO) was applied to the data of the
aftershock. From the NMO sections for every stations, many S wave reflector could be found. Especially, relatively strong
reflectors for S waves exist in the lower crust at about a depth range of 20 - 25km in the middle part of aftershock area. In addition, reflectors were found beneath the fault planes of the main shock and largest aftershock. This suggests a
possibility of the correlation of the crustal heterogeneities to the earthquake occurrence.
S53A-05
14:35h
Aftershock Distribution of the 2004 Niigata-Chuetsu Earthquake (Mj = 6.8) and Heterogeneous Structure in and Around the Source Region
* Shibutani, T
(shibutan@rcep.dpri.kyoto-u.ac.jp)
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Iio, Y
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Matsumoto, S
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin'yama, Shimabara,
Nagasaki, 855-0843 Japan
Katao, H
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Matsushima, T
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin'yama, Shimabara,
Nagasaki, 855-0843 Japan
Ohmi, S
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Takeuchi, F
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Uehira, K
, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin'yama, Shimabara,
Nagasaki, 855-0843 Japan
Nishigami, K
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Enescu, B
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Hirose, I
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Kano, Y
, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502 Japan
Kohno, Y
, Graduate School of Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukioka, 812-8581 Japan
Korenaga, M
, Graduate School of Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukioka, 812-8581 Japan
Mamada, Y
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Miyazawa, M
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Tatsumi, K
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Ueno, T
, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502 Japan
Wada, H
, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 611-0011 Japan
Yukutake, Y
, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502 Japan
The 2004 Niigata-Chuetsu Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe
Tectonic Zone where large strain rates (> 0.1 ppm/y contraction) were found from GPS data analyses (Sagiya et al., 2000).
The strong ground motion caused by the mainshock (1,722 gal was observed near the epicenter) severely destroyed ~5,000
houses and killed 40 people.
We deployed three temporary online seismic stations in the aftershock area, combined data from the temporary stations and
from permanent stations located around the aftershock area, and determined aftershock locations with a joint hypocenter
determination (JHD) technique. The resulting aftershock distribution showed that major events including the mainshock, the
maximum aftershock (Mj = 6.5), etc. formed at least three different fault planes parallel or perpendicular to each other. We
also found that large differences in the travel time residuals (~1 s for P and ~2 s for S) between Niigata Plain to the west and Echigo Mountains to the east from the source area significantly affected the aftershock locations.
We carried out a tomographic inversion of the P- and S- travel times for 3-D Vp and Vs structures. The station corrections
obtained in the JHD process were subtracted from the travel times beforehand. The result showed that the source area was
highly heterogeneous. Low velocity anomalies to the west and high velocity anomalies to the east were clearly found down to
~10 km deep. One of the boundaries of the anomalies sloped downward to the west and corresponded well to the mainshock
fault.
These features are probably a main cause that many larger aftershocks occurred in the sequence of the 2004 Niigata-Chuetsu
Earthquake.
S53A-06
14:50h
Triggering Sequence by Static Stress Changes of Large aftershocks of the Niigata-Chuetsu, Japan Earthquake
* Miyazawa, M
(linen@rcep.dpri.kyoto-u.ac.jp)
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Mori, J J
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Iio, Y
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Shibutani, T
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Matsumoto, S
, Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, Shin'yama, Shimabara,
Nagasaki, 855-0843 Japan
Katao, H
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Ohmi, S
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Nishigami, K
, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011 Japan
Following the 2004 Niigata-Chuetsu earthquake (M6.8), 4 large aftershocks (M6.3, 6.0, 6.5, 6.1) occurred: three within 40
minutes and one 4 days later. We examine the possibility for this triggering of these large aftershocks by static stress
changes. For the close spatial triggering, it is important to have information about the fault geometries, slip distribution, and focal mechanisms. We determine the fault planes orientations from the aftershock distributions. Slip distributions of
the main shock and largest aftershock are obtained by seismic waveform inversions of local strong-motion records. Mechanisms
for the events are taken from CMT solutions. The temporal variations of Coulomb failure function changes (ΔCFF) are
calculated on the fault planes of the large aftershocks before their rupture. Positive ΔCFF values (> 0.5 MPa) are
obtained on the fault planes, indicating the possibility that static triggering from the main event and large aftershocks can explain the occurrence of aftershocks.