2004 Chūetsu earthquake

The northwestern side of Honshu lies on the southeastern margin of the Sea of Japan, an area of oceanic crust created by back-arc spreading from the late Oligocene to middle Miocene. The extensional tectonics associated with the spreading formed a series of N–S trending extensional faults and associated basins. Currently the area is being deformed by contractional tectonics, causing inversion of these earlier basins, forming anticlinal structures.^[2] The earthquake is thought to have occurred due to reverse movement on one of these reactivated faults.^[3]

The United States Geological Survey (USGS) recorded a moment magnitude of 6.6,^[4] while the Japan Meteorological Agency put the magnitude at 6.8.^[5]

At 18:03:12 JST (09:03:12 UTC), another event, with a magnitude of M_w  6.1^[6] or M_JMA  6.3, struck near, with a maximum intensity of Shindo 5+ at Kawaguchi and Nagaoka.^[7] A third earthquake occurred at 18:11:57 JST (09:11:57 UTC), measuring M_w  5.8^[8] or M_JMA  6.0 and having a maximum intensity of Shindo 6+ at Ojiya.^[9] A fourth event, measuring M_w  6.3^[10] or M_JMA  6.5, struck at 18:34:04 JST (09:34:04 UTC) with a maximum intensity of Shindo 6+ in Kawaguchi, Nagaoka and Tōkamachi.^[11] Intervening and subsequent earthquakes of lesser intensity also shook the region. Within 116 hours of the mainshock, 15 earthquakes with intensities of Shindo 5− or higher struck the Chūetsu region.^[12]

The mainshock was caused by a rupture on the Muikamachi fault.^[13] The focal mechanism of the earthquake suggests a rupture on a reverse fault along a northeast-trending plane dipping northwest. The rupture lasted up to 15 seconds, extending up to 20–30 km (12–19 mi) by 14–25 km (8.7–15.5 mi), with the greatest slip being concentrated within 10 km (6.2 mi) of the hypocenter.^[14]

Intensity

The earthquake achieved 7 on the Japanese Shindo intensity scale at Kawaguchi, Niigata,^[5] making it the second earthquake to reach such intensity after the 1995 Kobe earthquake.^[18] It is also the time Shindo 7 was observed after the JMA scale's latest revision in 1996.^[19] It corresponded to a maximum Modified Mercalli intensity (MMI) of XI–XII (Extreme).^[14] According to the USGS, around 206,000 people were exposed to shaking corresponding to MMI IX (Violent), 261,500 were exposed to MMI VIII (Severe) shaking and 1.38 million others were subjected to shaking levels of MMI VI-VII (Strong-Very strong).^[20] A peak ground acceleration of 2,515 gal (2.56 g) was recorded in Kawaguchi, Niigata,^[21] with an acceleration of 1,500 gal (1.53 g) recorded at Ojiya.^[18]

More information Intensity, Prefecture ...

Locations with a seismic intensity of Shindo 5− and higher^[5]

Intensity	Prefecture	Locations
7	Niigata	Kawaguchi
6+	Niigata	Ojiya, Yamakoshi, Oguni
6−	Niigata	Nagaoka, Tokamachi, Tochio, Koshiji, Mishima, Horinouchi, Hirokami, Sumon, Irihirose, Kawanishi, Nakasato, Kariwa
5+	Niigata	Minamiuonuma, Tsunan, Jōetsu, Izumozaki, Mitsuke
5−	Niigata	Kashiwazaki, Sanjō, Tsubame, Nishikan-ku, Kamo, Niigata City
	Fukushima	Tadami, Yanaizu, Nishiaizu
	Nagano	Iizuna
	Gunma	Katashina, Shibukawa, Takasaki
	Saitama	Kuki

Close

Throughout 23 October, 164 aftershocks were recorded in Niigata Prefecture, with 110 additional aftershocks occurring on 24 October. Aftershocks continued, with a total of 600 perceptible events recorded by 31 October and 825 overall by 30 November 2004.^[22] Most of these aftershocks were distrbuted within a 30 km (19 mi) 20 km (12 mi) area.^[14]

By 24 October, 98,000 households, most of them in Nagaoka and Ojiya, were still left without power; that number dropped to 34,000 on 26 October and 2,300 on 3 November. Damage assessments became difficult due to landslides and the disruption of communications in rural areas. There were reports of locals rescuing people from collapsed homes. By the morning of 24 October, the Japan Self-Defense Forces (JSDF) opened a disaster headquarters in Ojiya to coordinate relief efforts, and dispatched 300 personnel, 21 helicopters and 65 vehicles to transport food and water to evacuation sites. They also evacuated tens of thousands of residents to emergency shelters, and used helicopters to airlift stranded villagers from the hamlet of Shiotani.^[14] A Souryu serpentine robot was used to inspect a collapsed house, marking the first time a snakebot was used at a natural disaster site.^[56]

Rescuers found difficulties reaching the towns of Kawaguchi and Yamakoshi, whose residents had to depend on airlifted food and water until they could be rescued the following day. In Kawaguchi, about 300 people took refuge in an elementary school and awaited assistance. By the evening of 25 October, relief supplies were coming in over the roads, but still with not enough food for the population. In Yamakoshi, 250 residents spent two nights in a gymnasium without power or water before they were evacuated. All but six of Yamakoshi's 2,200 residents were evacuated and taken to three sites in Nagaoka. The national government applied the disaster relief law to 29 localities, which meant that government would cover the cost of relief and shelter. Over 82,000 people were evacuated to emergency shelters in total, including 34,000 in Nagaoka, with 14,500 people in Ojiya seeking refuge across 93 evacuation centers. On 25 October, thousands of people were airlifted from about 60 mountainous communities, which remained closed to road access. Thirty police and JSDF helicopters evacuated residents in seven municipalities. By 26 October, about 320 people were still stranded in five hamlets.^[14]

Aftershocks and landslide dams posed continuing problems. A M_JMA  5.2 aftershock of 4 November halted the Jōetsu Shinkansen line between Niigata and Nagaoka. A M_JMA  5.9 aftershock on 8 November again halted operations and closed a portion of the Kan-etsu Expressway. Continued rain during the ten days following the earthquake raised concerns about water behind landslide dams in Ojiya and Yamakoshi. The JMA set up alarm units and surveillance cameras at five landslide dams, Niigata Prefecture prepared pumps to lower water levels, and the prefecture asked 439 residents in 101 households to evacuate.^[14]

By 28 November, authorities attributed 40 deaths to the earthquake,^[14] with the death toll rising to 68 by October 2009.^[16] As a result of post-earthquake fatalities from stress-related illnesses, Niigata Prefecture encouraged people to move into JSDF tents or to stay in accommodations farther away. Elderly residents were especially susceptible to exposure and stress-related illnesses.^[14] Agricultural activities in Yamakoshi were disrupted by the earthquake, and residents feared permanent consequences. The village's main industry is the raising of carp, which it provides to ponds and lakes throughout Japan. Residents of the village also had to leave behind 1,000 cattle. In order to save his business, a cattle owner in Yamakoshi began to airlift 700 cattle out of the village by helicopter on 2 November, at considerable expense.^[14]

The Tokyo Stock Exchange fell on 25 October, with the largest losses to the East Japan Railway Company, companies with production facilities in Niigata, and insurers. However, The Japan Times reported on 1 November that shares of construction-related companies had increased sharply on the Tokyo Exchange, since investors anticipated a supplementary national budget to pay for reconstruction following the earthquake and recent typhoons. However, reconstruction became challenging to homeowners, since only 11.2% of households in Niigata Prefecture had earthquake insurance, compared to 17.2% nationally at the time.^[14]

• List of earthquakes in 2004
• List of earthquakes in Japan