Noto Peninsula is a 100km long active fault zone… Possibility of earthquake due to displacement of some active faults
Tsunami reaches widely from Hokkaido to Kyushu… Due to the shallow terrain
The Yomiuri Shimbun reported that the Noto Peninsula earthquake in Ishikawa Prefecture, Japan, caused a tsunami over a wide area from Hokkaido to Kyushu and resulted in the collapse of buildings. The Yomiuri Shimbun reports that the multiple active faults and characteristic topography on the western coast of Japan appear to be the factors that caused the severe damage. This was reported on the 3rd.
The magnitude 7.6 earthquake that occurred on the 1st in Ishikawa Prefecture was the largest in the Noto region since 1885, when statistics from the Japan Meteorological Agency were compiled. Regarding the strong earthquake that occurred on the Noto Peninsula at 4:10 pm on the 1st, Tohoku University professor Shinji Toda (seismology) said, “It was a worst-case scenario earthquake.”
◆ Cluster earthquakes over a long period of time may have stimulated the active fault zone.
In the north of the Noto Peninsula, there is an active fault zone about 100 km long from northeast to southwest. According to a report released in 2014 by an expert meeting of Japan’s Ministry of Land, Infrastructure, Transport and Tourism, in the worst-case scenario where the entire region moves in unison, a magnitude 7.6 earthquake like this one was expected to occur.
In addition to this active fault zone, there are large active faults in the waters around the peninsula, and in 2007, a 6.9 magnitude earthquake occurred when part of a 20km long active fault on the west side of the peninsula shifted. An active fault is a weak fault on which seismic activity is likely to occur frequently.
After the magnitude 7.6 earthquake, seismic activity became active in a 150 km wide area, and there are concerns that it will become more widespread in the future. However, the Japanese government’s earthquake investigation committee presented the opinion that “there is a possibility that a fault about 150 km in length may have moved misaligned” and stated that its relationship with the active fault zone was unclear.
In the Noto Peninsula area, more than 700 earthquakes of magnitude 1 or higher have occurred since December 2020 until the 2nd of this month. One cause appears to be that fluids such as water existing underground make the surrounding rock easier to slip. Yomiuri analyzed that there is a possibility that the balance of forces applied to the underground bedrock changed due to the long-term effects of cluster earthquakes (small-scale earthquakes that occur several times locally) that occur concentrated in a specific area, thereby stimulating the active fault zone. .
◆Tsunami, from Hokkaido to Kyushu… Shallow water to reach a wider area
In this Noto Peninsula earthquake, a tsunami reached a wide area in the West Sea of Japan, from Hokkaido to Kyushu. According to Fumihiko Imamura, a professor at Tohoku University (tsunami engineering), the water depth in the West Sea of Japan is relatively shallow far from the coast. It is said that this terrain with shallow water over such a long distance is the reason why the tsunami reached such a wide area.
In shallow waters, the speed of tsunamis tends to slow down. Because of this, the waves around the coast are amplified as if waves coming from behind catch up and hit them. Moreover, the tsunami near the coast did not return to the sea, but repeatedly amplified and spread along the coast over time.
Professor Imamura pointed out, “In the West Sea of Japan, the first wave arrives very quickly and there are many cases where higher tsunamis follow, so caution is needed.”
In addition to the area around the Noto Peninsula, there is a fault that could cause a large-scale earthquake that could cause a tsunami in the waters near the western coast of Japan. The expert meeting of the Ministry of Land, Infrastructure, Transport and Tourism points out that there are at least 60 active faults, which could potentially cause a tsunami of several meters to 12 meters in size. However, Yomiuri reported that the government’s assessment of the probability of an earthquake occurring due to such a fault is delayed.
◆“Ishikawa Prefecture seismic waves comparable to the Great Hanshin Earthquake”
Some analyzes say that the seismic waves from the Noto Peninsula earthquake are comparable to those of the Great Hanshin Earthquake.
According to Yomiuri, the series of building collapses due to this earthquake may have been caused by the unique seismic waves that strongly shook wooden houses. In addition, it appears that not only the weak ground, but also the continued strong shaking led to increased damage to the wooden house.
Seismic waves contain waves of various periods, and it is believed that people can easily feel shaking when it is a slight shaking with a period of less than 1 second. On the other hand, what is likely to cause the collapse of wooden houses, etc. is shaking with a period of 1 to 2 seconds, and as a result of analyzing observation data released by the Japan Research Institute of Disaster Prevention Science and Technology, Yomiuri reported that this type of seismic wave was locally strong.
Kyoto University professor Yuki Sakai (earthquake disaster prevention engineering) analyzed that seismic waves of the type that destroyed buildings were observed in Anamizu-machi, Ishikawa Prefecture, with an intensity comparable to the Great Hanshin Earthquake (1995), which caused extensive damage to buildings. Professor Sakai told Yomiuri, “The magnitude of the earthquake was large and the soft ground may have contributed to the strong shaking.”
Source: Donga
Mark Jones is a world traveler and journalist for News Rebeat. With a curious mind and a love of adventure, Mark brings a unique perspective to the latest global events and provides in-depth and thought-provoking coverage of the world at large.