November 10, 2004 |
Seismic Activity in the Chuetsu Region, Niigata Prefecture
On October 23, at 17:56 (JST) there was a M6.8 earthquake* with a maximum seismic intensity 7, at a depth of approximately 10km in the Chuetsu region, Niigata prefecture. Also within an hour, there were three earthquakes equal to or greater than M6.0. (see the 133rd Evaluation of the Earthquake Research Committee gNiigata Chuetsu Earthquake on October 23, 2004h) Judging from the sequence of occurrence, it is thought that the seismic activity was a main shock - aftershock sequence, with the M6.8 earthquake as the main shock. Immediately following the main shock, comparatively many large earthquakes occurred. Generally, the aftershock activity is declining, however, there was a M6.1 event on October 27, and a M5.9 event on November 8.
Most aftershocks were distributed in an area of about 30km in length and 20km in width oriented in a NNE-SSW direction. The following information was obtained from the rapid aftershock observations and the analysis results:
1) There is a distribution, including the mainshock, steeply dipping downward to the northwest.
2) There is a distribution parallel to 1), which includes the largest aftershock (M6.5 at 18:34 on October 23)
3) There is a distribution nearly perpendicular to 1) and 2), located on the east edge of the aftershock region, which includes the M6.1 event on October 27.
There are inferred fault planes corresponding to each of the above features. As described, it is inferred that there are several source faults, and it is thought that there is a complicated fault configuration under the surface. In addition, the focal mechanism of the main shock corresponds to aftershock distribution 1), so it is thought that the main shock was caused by a NNE-SSW striking reverse fault, with the northwest side up.
According to the GPS data, crustal movements associated with this event were observed centered on Niigata prefecture, with an uplift of approximately 27cm at the Ojiya observation station (Ojiya city, Niigata prefecture) located near the southern edge of the aftershock area. Also there was a displacement of approximately 21cm in a NW direction and a subsidence of approximately 6cm at the Sumon observation station (Sumon, Uonuma city / old Sumon-mura, Minamiuonuma-gun, Niigata prefecture) located on the eastern side of the aftershock area. Crustal movements associated with the event were observed in the data from the Synthetic Aperture Radar. The source model determined from these observation data corresponds to fault movements for the mainshock with uplift on the northwestern side. In addition, it was observed at the Sumon observation station near the epicenter, that the M6.1 event on October 27 and the M5.9 on November 8 caused displacements less than a few centimeters.
From the analysis of the focal process of the main shock, it is estimated that the rupture initiated on the deeper part of the fault plane, and propagated upward towards shallower depths along the fault plane.
It is estimated that there is an approximately 20 percent probability of occurrence of M5.5 or greater aftershocks (seismic intensity 5 Upper in some areas), and an approximately 30 percent probability of occurrence of M5.0 or greater aftershocks (seismic intensity 5 Lower in some areas) within the 3 days from 18:00 on November 10.
Around this active region, there are many active faults. According to the aftershock distribution, there is a possibility of activity on the northern part of the fault zone located in the western margin of the Muikamachi Basin. Research so far shows that there are small surface displacements along this fault zone.
*: The Japan Meteorological Agency named the event the "Mid Niigata Prefecture Earthquake in 2004".