The Chūetsu earthquake (中越地震, Chūetsu jishin) occurred in Niigata Prefecture, Japan, at 17:56:00 JST (08:56:00 UTC) on 23 October 2004. The Japan Meteorological Agency (JMA) officially named it the "Heisei 16 Niigata Prefecture Chūetsu Earthquake" (平成16年新潟県中越地震, Heisei ju-roku-nen Niigata-ken Chuetsu Jishin). The reverse-faulting shock achieved a maximum JMA seismic intensity of Shindo 7 and Modified Mercalli intensity of XI–XII (Extreme). It caused widespread destruction and casualties in Niigata Prefecture, while also causing damage and injuries in Fukushima, Gunma, Saitama and Nagano Prefectures.
At least 68 people were killed, of which 52 were disaster-related deaths aggravated by illnesses and stress attributed to the earthquake and its effects. More than 131,600 houses were damaged in four prefectures, of which 17,700 partially or fully collapsed. It caused economic losses of US$40 billion, which made it the second costliest natural disaster in history at the time.
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. The earthquake is thought to have occurred due to reverse movement on one of these reactivated faults.
The United States Geological Survey (USGS) recorded a moment magnitude of 6.6, while the Japan Meteorological Agency put the magnitude at 6.8.
At 18:03:12 JST (09:03:12 UTC), another event, with a magnitude of Mw 6.1 or MJMA 6.3, struck near, with a maximum intensity of Shindo 5+ at Kawaguchi and Nagaoka. A third earthquake occurred at 18:11:57 JST (09:11:57 UTC), measuring Mw 5.8 or MJMA 6.0 and having a maximum intensity of Shindo 6+ at Ojiya. A fourth event, measuring Mw 6.3 or MJMA 6.5, struck at 18:34:04 JST (09:34:04 UTC) with a maximum intensity of Shindo 6+ in Kawaguchi, Nagaoka and Tōkamachi. 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.
The mainshock was caused by a rupture on the Muikamachi fault. 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.
GPS observations revealed that Ojiya experienced a horizontal movement of 8.1 cm (3.2 in) in a west-southwestern direction and an uplift of 26.5 cm (10.4 in), while Sumon experienced a horizontal movement of 21 cm (8.3 in) in a west-northwestern direction and subsidence of up to 6.3 cm (2.5 in). The earthquake also triggered at least 442 documented landslides in Niigata Prefecture, resulting in six fatalities. The earthquake struck several days after Typhoon Tokage made landfall in the country, with heavy rainfall associated with the typhoon contributing to at least 37 mudslides triggered by the earthquake. Observed modes of landslides and slope failure included translational soil slips, deepseated rotational slumps and debris flows.
The earthquake achieved 7 on the Japanese Shindo intensity scale at Kawaguchi, Niigata, making it the second earthquake to reach such intensity after the 1995 Kobe earthquake. It is also the time Shindo 7 was observed after the JMA scale's latest revision in 1996. It corresponded to a maximum Modified Mercalli intensity (MMI) of XI–XII (Extreme). 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). A peak ground acceleration of 2,515 gal (2.56 g) was recorded in Kawaguchi, Niigata, with an acceleration of 1,500 gal (1.53 g) recorded at Ojiya.
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. Most of these aftershocks were distributed within a 30 km (19 mi) 20 km (12 mi) area.
At least 68 fatalities and 4,805 injuries were related to the earthquake; all deaths and 4,795 injuries occurred in Niigata Prefecture, with 6 others injured in Gunma Prefecture, 3 in Nagano Prefecture and 1 in Saitama Prefecture. Of the 68 fatalities, 10 were caused by collapsing structures, 6 were killed by landslides and 52 died due to illnesses or stress related to the earthquake and its effects. At least 19 people died in Ojiya, 12 in Nagaoka, 8 in Tōkamachi, 6 in Kawaguchi, 5 in Yamakoshi, 3 each in Mitsuke and Koshiji, 2 in Yunotani, and 1 each in Yoshida, Yuzawa, Tsubame, Kawanishi, Yamato, Oguni, Tochio, Horinouchi, Koide and Hirokami. A landslide in Myoken, Nagaoka buried a car, killing a mother and one of her two children inside. One hospital patient also died when an artificial respirator detached during the earthquake, while five more died from complications due to sheltering in their cars, including blood clots from inactivity.
At least 3,175 homes were completely destroyed, 14,552 were partially destroyed and 113,912 others were damaged. At least 1,063 homes were damaged in Nagano, Fukushima and Gunma Prefectures, with the rest of the damage occurring in Niigata. The number of partially or fully collapsed houses by area include 5,977 in Nagaoka, 3,100 in Ojiya, 1,006 in Kawaguchi, 977 in Tōkamachi and 858 in Koshiji. Economic losses amounted to at least ¥6.28 trillion (US$40 billion), making it, at the time, the second costliest natural disaster in history behind the 1995 Kobe earthquake.
Housing damage, while heavy, was relatively light compared to the extreme ground motions from the earthquake. Most homes in the affected region were traditionally built with wood, bamboo and clay. Homes of this type performed very poorly in many Japanese earthquakes, particularly the 1995 Kobe and 2024 Noto earthquakes. In Niigata and nearby regions, traditional houses tend to have larger and more numerous interior columns and heavier roof beams to protect them from heavy snowfall, and many were built less than two decades prior to the earthquake, which in turn made them more resistant to earthquake shaking, although collapses were widespread in Ojiya and especially Kawaguchi.
Sewage and water mains ruptured, and gas and telephone services were cut off. Over 300,000 homes were affected by power outages, mainly in Ojiya and Nagaoka. Two tunnels along the Kan-etsu Expressway collapsed, and roads near the epicenter were severely damaged. In the Imo River basin of Yamakoshi, multiple landslide dams were formed by the earthquake, leaving officials concerned about potential flooding in the area, with 439 people living in 101 households temporarily evacuated.
For the first time in its 40-year history, a Shinkansen train derailed while in service, the train being too close to the epicenter to be halted by the automatic UrEDAS earthquake detection system. Eight out of ten cars of the Toki 325 service (a 200 Series Shinkansen train) derailed on the Joetsu Shinkansen line near Kawaguchi, between Nagaoka Station in Nagaoka and Urasa Station in Yamato; no injuries were reported among the 155 passengers. The railbed, bridges and tunnels were all affected.
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. A Souryu serpentine robot was used to inspect a collapsed house, marking the first time a snakebot was used at a natural disaster site.