Today is Disaster Prevention Day. Exactly 100 years ago, on September 1, 1923 (Taisho 12), at 11:58, a M7.9 earthquake occurred with its epicenter in western Kanagawa Prefecture. It is the “Taisho Kanto Earthquake” that caused the Great Kanto Earthquake.
It caused devastating shaking with a maximum seismic intensity of 6 in Otemachi, Tokyo, and a wide area of the metropolitan area, and more than 105,000 people died or went missing. A large number of fires occurred because it overlapped with lunch time, and it is said that 90% of the dead died due to the fire.
*Earthquake elements are from the Japan Meteorological Agency, and damage statistics are from Rika Chronology. At that time, there were seven levels of seismic intensity from 0 to 6.
“Earthquake” while being a “subduction zone earthquake”
Earthquake disasters can be divided into “subduction zone earthquakes (plate boundary earthquakes)” and “inland earthquakes (inland earthquakes).”
Trench-type earthquakes occur near plate subduction zones, and are a type of mega-earthquake that can reach magnitudes of 8-9. Because the epicenter is often far from the land, the tremor tends to attenuate and become smaller compared to the scale of the energy, but it is characterized by being prone to tsunami damage.
Inland earthquakes, on the other hand, occur on active faults within land plates, and most of them have a maximum scale of M7. However, even if the energy scale of the earthquake is smaller than that of a subduction-zone earthquake, it is characterized by the fact that the tremors are extremely large due to the proximity of the epicenter.
When asked which type the Taisho Kanto Earthquake, which caused the Great Kanto Earthquake, was, in fact, it can be said that it applies to both types.
The Taisho Kanto Earthquake is thought to have occurred at the plate boundary along the Sagami Trough, where the Philippine Sea plate subducts into the landward plate. As with other subduction-zone earthquakes, tsunami damage has also occurred. A 12m tsunami hit Atami in Shizuoka prefecture.
However, since the plate boundary of the Sagami Trough is close to the land, the “seismic focal region” spreads directly under the land. The “hypocenter” where the bedrock rupture started is said to be western Kanagawa Prefecture, and the entire “hypocenter area” where the bedrock rupture occurred is Sagami Bay along the Sagami Trough, Miura Peninsula, southern Tokyo Bay, and southern Boso Peninsula. believed to have been reached.
Along the Sagami Trough, huge earthquakes occur once every 180 to 590 years, and the Genroku Kanto Earthquake occurred in 1703 before the Taisho Kanto Earthquake.
This year marks the 100th anniversary of the Great Kanto Earthquake, so it cannot be said that a mega-earthquake is imminent in light of past cycles.
Two types of “Tokyo Inland Earthquake”
Great Kanto Earthquake Burnt ruins in front of Tokyo Station, toward Nihonbashi
In addition, the periodicity is not the only reason why we cannot be cautious about an earthquake directly below the Tokyo metropolitan area.The Cabinet Office’s Tokyo Inland Earthquake Model Study Group classifies Tokyo Inland Earthquakes into two major categories. One is “(1) an M8 class earthquake along the Sagami Trough” that caused the Great Kanto Earthquake. The other is “(2) an M7-class earthquake directly beneath the capital.” This occurs regardless of the period of the former.
Furthermore, (2) there is more than one possible type of M7-class earthquake directly beneath the capital. The Pacific plate, the Philippine Sea plate, and the landward plate overlap in a complex manner directly beneath the capital, and various types of earthquakes, including fault-type earthquakes, occur. Both types are likely to reach the scale of M7 class, and in that case, it is thought that the shaking will be comparable to (1) M8 class earthquakes along the Sagami Trough in the Tokyo metropolitan area.
We can’t let our guard down outside the metropolitan area
Including earthquakes with a seismic intensity of less than 1, it can be said that there is no area where earthquakes do not occur.
Because the Japanese archipelago is located along the plate boundary, large earthquakes occur in the subduction zone of the plate, and strain easily accumulates inside the plate on the land side, causing earthquakes everywhere.Starting with the Nankai Trough megathrust earthquake that has been feared in recent years, huge earthquakes along the Japan Trench and the Ryukyu Trench, large earthquakes caused by thousands of inland active faults, and earthquakes caused by unknown active faults. It can be said that it is not.
It is ideal to be fully prepared for earthquake disasters on a daily basis, but it is important to review disaster prevention and mitigation at such milestones.
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2023-08-31 21:50:00
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