October 24, 2004
Niigata Chuetsu Earthquake* on October 23, 2004
On October 23, at 17:56 (JST) there was a M6.8 (preliminary) earthquake at a depth of approximately 10km under the Chuetsu region, Niigata prefecture, and a maximum seismic intensity 6 Upper was observed. Also on the same day, there was a M6.0 (preliminary) earthquake at 18:12, and a M6.5 (preliminary) earthquake at 18:34, with both events having a maximum seismic intensity 6 Upper. 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. These events were distributed in a NNE-SSW direction over a length of 30km. The focal mechanism of the main shock shows a reverse fault with a compression axis in a NW-SE direction. The direction of the estimated fault generally corresponds to the aftershock distribution. There was strong aftershock activity with three events over M6.0 within an hour after the main shock, but the activity is declining. In addition, before this activity, there was a M2.5 earthquake (preliminary) under the Chuetsu region, Niigata prefecture at 6:07 on the same day. This event had a seismic intensity 1 in Ojiya city, Niigata prefecture.
According to the GPS data, crustal movements associated with this event were observed centered on Niigata prefecture, with a displacement of approximately 10cm in a NW direction at the Niigata Yamato observation station (Yamato-cho, Minamiuonuma-gun, Niigata prefecture) located to the southeast of the event, and a displacement of approximately 6cm in a SE direction at the Kashiwazaki 1 observation station (Kashiwazaki city, Niigata prefecture) located to the northwest of the event. These are consistent with the focal mechanism of the main shock.
Around this active region, there are many active faults located parallel to the aftershock distribution. It is unclear if the recent activity is related to these active faults. The Nagaoka Plain Western Margin Fault Zone, that is located approximately 10km west of the main shock, is thought to be a reverse fault dipping to the west. The recent events were distributed to the east of the fault zone. So it is thought that this fault zone was not active at this time.
It is estimated that there is a 10 percent probability of occurrence of M6.0 or greater aftershocks (seismic intensity 6 Lower - 6 Upper in some areas), and a 20 percent probability of occurrence of M5.5 or greater aftershocks (seismic intensity 5 Upper in some areas), and a 40 percent probability of occurrence of M5.0 or greater aftershocks (seismic intensity 5 Lower in some areas) within the 3 days from 16:00 on October 24.
*: The Japan Meteorological Agency named the event the “Mid Niigata Prefecture Earthquake in 2004”.