January 11 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* The Niigataken Chuetsu-oki Earthquake in 2007 (hereafter called the Niigataken Chuetsu-oki Earthquake) generally occurred by the movement of a reverse fault that dips downwards to the southeast (increasing depth from the ocean toward the land). In the northeastern part of the focal region, it is thought that there was movement of the fault that dips downwards to the northwest (increasing depth from the land toward the sea).
Associated with the earthquake, no displacements of the sea floor were observed. However, judging from the aftershock distribution, the fault dipping downwards to the southeast has a shallow extension that possibly connects to a known active fault.
The following is a summary of analysis results about the fault plane evaluation of the Niigataken Chuetsu-oki Earthquake.
* According to the detailed aftershock distribution determined from the temporary ocean-bottom and onshore seismic observations, aftershocks generally occurred on a fault that dips downwards to the southeast. In the northeastern part of the focal region, aftershocks also occurred on the fault that dips downwards to the northwest.
* For the crustal movement analysis referenced to the aftershock distribution, the data can also be explained well with a fault that dips downwards to the northwest in the northeastern part of the focal region, taking into account the fault that dips downwards to the southeast.
* Judging from the analysis of the strong-motion waveforms, it is estimated that the fault dipping downwards to the southeast is the source fault plane. This conclusion does not contradict the existence of the fault dipping downwards to the northwest in the northeastern part of the focal region which is suggested from the aftershock distribution and the crustal movement analysis.
* Judging from the tsunami analysis, it is estimated that there is subsidence on the northwestern side of the northeastern part of the focal region, and there is an uplift on the northwestern side of the southwestern part of the focal region. It is difficult to determine whether the fault plane dips to the northwest or to the southeast from only the tsunami analysis.
* According to the structural survey in the ocean area, to the northwest of the focal region, there is an active fault and an active fold structure that extend in the same direction as the source fault. Those active structures are estimated to have been mainly formed by the movement of a reverse fault that dips downwards to the southeast.
Associated with this earthquake, there were no displacements observed of the sea floor. However, estimated from the aftershock distribution, there is a possibility that the shallow extension of the fault plane dipping downwards to the southeast connects to the active fault, as mentioned above.
January 11 2008
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On July 16, at 10:13 (JST) there was a M6.8 earthquake at a depth of approximately 10km off-shore of Jo-Chuetsu, Niigata prefecture. This event had maximum seismic intensity 6 Upper in Niigata and Nagano prefectures. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred at shallow depth within the crust.
* Seismic activity was a main shock-aftershock sequence. The aftershock activity has mostly subsided. The aftershock activity is not so strong compared to other recent damaging earthquakes. The aftershock locations are distributed along a trend about 30km long in a NE-SW direction. The largest aftershock was a M5.8 earthquake at 15:37 on July 16, which occurred at a location in the deeper portion of the aftershock distribution dipping downwards to the southeast.
* According to the GPS data, there were crustal movements associated with the main shock, with the largest displacement of approximately 17cm to the northwest on the coast in Kashiwazaki city. From the geological field surveys and leveling results, a maximum uplift of approximately 25cm was observed in the area centered on Cape Kannon, Kashiwazaki city, and a subsidence of approximately 4cm at the Kashiwazaki tidal station. In addition, using Synthetic Aperture Radar data (SAR) from the advanced land observing satellite, "Daichi", crustal movements associated with this earthquake are observed to be consistent with a planar source, and centered in the area on the coast of the Chuetsu region, Niigata prefecture. The various geodetic observations of crustal movements are consistent with each other. In addition, according to the GPS analysis, postseismic movements were observed in the area near the focal region. (GPS analysis as of September 10, 2007)
* Using Synthetic Aperture Radar data (SAR) from the advanced land observing satellite, "Daichi", in the area near the Oginojo anticline on the western slope of the Nishiyama Hills located to the east of the focal region of the Niigataken Chuetsu-oki Earthquake in 2007, an uplift zone of approximately 15km in length and 1.5km in width was observed, which is associated with the earthquake. An approximately 10cm or more of uplift was seen, which is consistent with leveling results.
* Associated with this event, small tsunamis were observed centered on the coast in Niigata prefecture, such as 0.3m high waves in Kashiwazaki and Ogi. In addition, a 1m tsunami was observed in Kashiwazaki (Niigata prefecture jurisdiction).
* The earthquake caused large ground accelerations of over 1,000 gal observed at the Ikeura station in Nishiyama-cho, Kashiwazaki city.
* From the focal process analysis of the main shock and the aftershock distribution, it is thought that the rupture propagated from the northeast to the southwest.
* There is an active structure along the eastern margin of the Japan Sea, called a strain concentration zone. It is thought that this event is related to a part of that active zone.
* Although the 2004 Niigata Chuetsu Earthquake occurred to the east of the recent event, it is thought that the 2004 earthquake did not trigger the recent event.