What happened underground during the Noto Peninsula earthquake?

Seismic intensity of the earthquake that occurred around 4:10 pm on January 1 (from data released by the Japan Meteorological Agency). Credit: Kobe University

The Noto Peninsula earthquake on January 1 has put the challenges faced by earthquake-prone Japan once again in the spotlight. With a magnitude of 7.6, exceeding the 1995 Kobe earthquake (the Great Hanshin-Awaji Earthquake), it caused major disturbances in the landscape and significant ground upwelling. Also noteworthy is the large number of aftershocks. What exactly happened under the Noto Peninsula?

We asked Professor Yoshioka Shoichi of Kobe University's Urban Safety and Security Research Center, an expert on earthquake generating mechanisms, about the characteristics of this earthquake and future countermeasures.

How did you feel when you first heard about the Noto Peninsula earthquake?

I was at home in Kobe when the earthquake happened. I received an earthquake emergency warning on my smartphone, but I didn't feel any earthquakes, so at first I thought it was a false alarm. However, when I turned on the TV, I realized it was a big earthquake. I also received information from foreign researchers I know.

When a large earthquake occurs somewhere in the world, the United States Geological Survey (USGS) analyzes it and releases the information.

I started analyzing this earthquake while looking at such scientific information. The extent of the fault that moved during the Noto Peninsula earthquake was about 150 km. Given that the Kobe earthquake fault extended over 50 km, this was clearly a large earthquake. In terms of energy, it was nearly eight times larger than the Kobe earthquake.

What do you think happened underground on the Noto Peninsula?

The earthquake was of the reverse fault type, where the active fault moved directly down the northern part of the Noto Peninsula vertically. Analysis of seismic waves revealed significant displacement directly beneath the northern tip of the peninsula, as well as significant movements in active offshore faults between the peninsula and Sado Island (Nigata Prefecture).

Distribution of earthquake epicenters around the Noto Peninsula (from JMA data). Credit: Kobe University

Due to the vertical movement, the landward side was uplifted, with the coast around Wajima City rising by up to 4 metres.

Such rises are said to occur about once every 5,000 years on the Noto Peninsula, and there are stair-like floor features that indicate this. These changes in the landscape are permanent displacements that do not return to their original state, which means that the map of Japan has changed.

It is believed that severe damage to infrastructure, such as roads and water pipes, was caused by this permanent displacement and strong shaking. The maximum acceleration, which is a rough measure of the strength of the shaking, was recorded at 2,828 gpm at Shika (the Kobe earthquake recorded a magnitude of 891 gpm at the Kobe Marine Meteorological Station). It is said that rocks buried near the surface can be ejected when the acceleration exceeds 1,000 gallons, so it was a very strong earthquake.

There have been a series of earthquakes on the Noto Peninsula since the end of 2020, and in May 2023, a 6.5 magnitude earthquake (seismic intensity 6+ in Suzhou City) also occurred. Is there any connection to this earthquake?

The earthquake swarm could have settled on the Noto Peninsula, but on the other hand, some researchers suggested that it could be a precursor to a large earthquake. In general, earthquake swarms are more likely to be a series of small earthquakes rather than resulting in a large earthquake. However, this time the worst outcome happened.

As noted by the Earthquake Research Committee (a research organization established at the government's Earthquake Research Promotion Headquarters), the series of earthquakes on the Noto Peninsula are believed to be linked to the movement of groundwater rising from the Pacific Plate.

Water in rocks can be released under certain temperature and pressure conditions. It is speculated that this water was rising and accumulating directly under the Noto Peninsula, triggering a series of earthquakes over a period of about three years. Fluids act like lubricants, making it easier for the crack surface to slide. It is very likely that the water has gradually moved and now stimulated the main fault in this earthquake.

There have been previous examples of water being involved in earthquake swarms. The series of Matsushiro earthquakes in Nagano Prefecture, which began in 1965 (with a maximum seismic magnitude of 5), lasted for about five and a half years, and after a large amount of water came out from under the ground, it began to subside. Some researchers have speculated that a similar phenomenon may have occurred on the Noto Peninsula before this earthquake.

Illustration of the hypothetical mechanism that caused a swarm of earthquakes on the Noto Peninsula. Credit: Kobe University

One of the features of this earthquake appears to be the large number of aftershocks.

Since large earthquakes involve large areas of fault rupture, aftershocks occur at different places on the fault surface. When summed up, the number of aftershocks per unit time becomes large. This time, a very large fault was ruptured, about 150 kilometers long, so we think that's why there were so many aftershocks.

In addition, the fact that the fault moved at a shallow depth of about ten kilometers may also have contributed to the large number of aftershocks. Up to a depth of about 15 km, the rocks break with a “snap”, and if there are places that did not slide during the main shock within this depth range, cracks can occur later, and aftershocks are likely to occur. On the other hand, at greater depths, faults are hotter and more flexible, and move with a “mud” motion, so aftershocks do not occur.

Another feature of this earthquake is the early arrival of the tsunami.

It is believed that the tsunami arrived early because the active fault near the coast moved significantly. It appears to have made landfall within 1-2 minutes after the earthquake in some areas. Some researchers suggest that not only active fault movement, but also landslides on the sea floor, were a factor in generating the tsunami.

After the Kobe earthquake, the Earthquake Research Committee began investigating active faults in Japan and completed investigations on more than 100 major active faults. However, most of them are active faults on land, and investigations into active offshore faults have not yet begun, except for some parts of western Japan.

Investigations can reveal past fault activity, but investigating marine faults is very difficult and time-consuming. At the time of the Noto Peninsula earthquake, marine faults in the surrounding areas had not yet been investigated.

Aftershocks of the Noto Peninsula earthquake (from JMA data). Credit: Kobe University

Can this earthquake affect the occurrence of earthquakes in other regions?

It is not clear whether it will affect earthquakes in other areas, but seismic activity in inland areas is known to intensify as the expected massive Nankai earthquake approaches. Therefore, there is still the possibility of a major earthquake occurring inside.

The massive Nankai earthquake occurs on a 90-150 year cycle. The last earthquakes were the Showa Tonankai Earthquake in 1944 and the Nankai Earthquake in 1946, so it has already been 80 years.

In my classes, I tell my students that “the massive Nankai-True earthquake will happen at least in your children’s generation,” so it is absolutely essential that we be prepared. The Noto Peninsula earthquake caused all the damage possible from an earthquake at once, including house collapses, fires, tsunamis and soil liquefaction. We must realize that such damage would occur over a wide area of ​​western Japan in the event of a huge earthquake in the Nankai region.

How should we prepare?

Considering the damage caused by the Noto Peninsula earthquake, I feel it is necessary to consider not only the call for tsunami evacuation, but also the response to tsunami waves and the rescue of people trapped under buildings through centralization of information. As part of the measures taken against the massive Nankai True earthquake, it is necessary to establish a system capable of gathering information, quickly dispatching self-defense forces and rescue teams and accepting support from abroad.

The most important action that can be taken at both the individual and community levels is, of course, seismic retrofitting of buildings. If you live in a building built before 1981, when the Building Standards Code was revised, and have not done seismic retrofits, you should consider it urgently. Also, since we sleep about a third of the day, it is important to avoid placing furniture in the bedroom, sleep away from slopes such as mountains, and if you live in a two-story house, sleep on the second floor.

It is also important to collect information about earthquake risks in your area. I would like people to take advantage of the online National Earthquake Hazard Maps provided by the National Research Institute for Geosciences and Disaster Resilience, where one can see the vulnerability and risk level of tremors at their home location on the map.

Provided by Kobe University

Quote: Q&A: What happened underground during the Noto Peninsula earthquake? (2024, March 21) Retrieved March 21, 2024 from https://phys.org/news/2024-03-qa-underground-noto-peninsula-earthquake.html

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