1984 Otaki earthquake

1984 Otaki earthquake

Date	September 14, 1984 local time (September 13, 1984 UTC)
Magnitude	M_s 6.3 (M_j 6.8)
Depth	2 km (1 mi)^[1]
Epicenter	35°47′N 137°29′E / 35.79°N 137.49°E / 35.79; 137.49Coordinates: 35°47′N 137°29′E / 35.79°N 137.49°E / 35.79; 137.49
Type	Intraplate
Areas affected	Japan
Max. intensity	Shindo 6 (estimate)
Peak acceleration	0.3~ g
Tsunami	No
Landslides	Yes
Casualties	14 dead 10 injured 15 missing^[2]

The 1984 Otaki earthquake hit Nagano Prefecture, Japan on September 14, 1984 at 08:48 local time (September 13, 1984 at 23:48 UTC). Registering a magnitude of M_s 6.3, the earthquake destroyed Otaki (Japanese: 王滝(おうたき)), and elicited landslides.^[3]^[4] The earthquake left at least 29 people dead or missing, making it the deadliest earthquake in 1984.

Geology

Although the epicenter was only 2 km (1 mi) deep, no visible fault appeared. The Japan Meteorological Agency estimated that two faults, one 15 km (9 mi) long and one 5 km (3 mi) long, had ruptured simultaneously.^[1]

Relation to other earthquakes

Seismologists including Akeo Yoshida state that the 1948 Fukui earthquake, a 7.0 M_j earthquake in Gifu Prefecture in 1961, a 6.6 M_j earthquake in Gifu Prefecture in 1969, and this earthquake were earthquakes that occurred in a cycle for a little while in the same area.

Main Shock

Since there was no seismometer in the area, the Japan Meteorological Agency made an estimate intensity, Shindo 6. Some unofficial estimates put it to Shindo 7. There are reports that rocks and pieces of wood flew in the air in areas near the epicenter, due to ground accelerations faster than gravitational acceleration in the 5 Hz~10 Hz shaking range.^[5] A seismometer at Makio Dam, 4 km (2 mi) away from the epicenter, observed very strong shaking but couldn't record more than 0.3 g which was the limit.

Damage

This lake was formed after a part of the Otaki river was blocked by a landslide

Due to torrential rains in the area before the earthquake, many landslides occurred.

Total Damage
Dead	14
Missing	15
Injured	10
Collapsed buildings	14
Half collapsed buildings	73
Damaged Buildings	517
Economic cost	US$28 million

Incidents

A massive landslide occurred on the south side of Mount Ontake, sending 3.45 million cubic meters of earth at estimated speeds of 80 to 100 km/h down the side of the mountain and buried over 3 km (2 mi) of land, including a hot spring facility, under 50 m (164 ft) of earth. A nearby valley was completely buried under 30 m (98 ft) of earth. The 4 people in the facility's owner's family and 5 people who were looking for mushrooms in the area were also buried and became missing.

A large landslide in the Matsugoe District caused part of an inn and a road to collapse and crashed into a concrete factory on the bottom of the slope, killing 13 people working in the forest and the factory. There were no people staying at the inn, and the owner miraculously survived since she was able to stay on her tatami mat that stayed over the landslide as she was swept down. Then she was able to climb up the cliff shaped by the landslide, even though she was badly injured. She was then found and hospitalized for 2 weeks.^[6]

A house in the Takigoe District collapsed after a landslide crashed into it, killing 1 person.

5 people that were driving in the Kōrigase District were buried in a landslide and became missing. 2 people were able to survive by running away from the landslide.^[7]

1 person became missing in the Yanagigase District.

A man driving a truck was killed after being swept away by a landslide.

A lake was formed after a landslide blocked a river.

Precursors

According to Nagoya University, hot spring observatories near the epicenter recorded a big difference in the ratio of Methane and Argon, and the amount of Hydrogen before the earthquake. The change in the amount of Hydrogen is thought to be from small rupturing before the main earthquake.^[8]

A change in the percentage of Radon was observed at 141 observatories over 3 faults, including the Japan Median Tectonic Line.

Response

9 check dams were built on Mount Ontake in 4 years after the earthquake.

References

← Earthquakes in 1984 →

Morgan Hill (6.2, April 24) Dominican Republic (6.7, June 24) Llŷn Peninsula (5.4, July 19) Otaki (6.3, September 13) Northern Sumatra (7.2, November 17)

Earthquakes in Japan

Historical	869 Sanriku 1293 Kamakura 1498 Nankai 1605 Nankai 1611 Sanriku 1703 Genroku 1707 Hōei 1771 Great Yaeyama 1792 Unzen 1854 Nankai 1854 Tōkai 1855 Edo 1858 Hietsu 1889 Kumamoto 1891 Mino–Owari 1894 Tokyo 1896 Sanriku

20th century	1911 Kikai Island 1923 Great Kantō 1927 Kita Tango 1930 North Izu 1933 Sanriku 1936 Miyagi 1943 Tottori 1944 Tōnankai 1945 Mikawa 1946 Nankai 1948 Fukui 1952 Hokkaido 1963 Kuril Islands 1964 Niigata 1968 Hyūga-nada 1968 Tokachi 1974 Izu Peninsula 1978 Miyagi 1983 Sea of Japan 1984 Otaki 1993 Hokkaidō 1994 offshore Sanriku 1995 Hanshin 1998 Ryukyu Islands 2000 Tottori

21st century	2001 Geiyo 2003 Hokkaidō 2004 Chūetsu 2005 Fukuoka 2005 Miyagi 2006 Kuril Islands 2007 Chūetsu 2007 Kuril Islands 2007 Noto 2008 Iwate–Miyagi 2009 Izu Islands 2009 Shizuoka 2010 Bonin Islands 2011 Tōhoku April 2011 Fukushima April 2011 Miyagi 2012 Chiba 2012 Kamaishi

Related articles	Earthquakes in Japan Tōkai earthquakes Nankai megathrust earthquakes Kantō earthquakes

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