In the series of Kumamoto earthquakes that occurred five years ago, the faults moved even at a distance of about 10 km from the active fault that caused the earthquake, and steps and deviations were confirmed on the ground surface in a total of about 230 places. It was.

It was found that this phenomenon, which is also called a "friendship fault" because it is triggered and moves, has been confirmed one after another in the subsequent inland earthquakes. It is necessary to understand that there are risks such as building damage. "

It is believed that part of the "Hinagu Fault Zone" and the "Futagawa Fault Zone" were displaced during the series of Kumamoto earthquakes, but Kumamoto City, which is about 5 km away from this fault zone, and about 15 km away. Steps and deviations on the ground surface were found one after another in the northwestern part of Mt. Aso.

When the Geospatial Information Authority of Japan analyzed the data of artificial satellites that can capture topographical changes in units of several centimeters, it was found that the total number of displaced faults, including those with a length of about several hundred meters, is about 230. I did.

These faults are also called "triggered surface seismic faults", commonly known as "Otsukai faults", because they move in a way that is triggered by the fault that caused the earthquake.

The "Otsukai Fault" has been confirmed in the northern Osaka prefecture earthquake and the eastern Hokkaido Eastern Iburi earthquake that occurred two years after the Kumamoto earthquake.

Geodetic Director Satoshi Fujiwara of the Geospatial Information Authority of Japan said, "It has been thought that each fault causes an earthquake, but it has become clear that this is not always the case. I need to. "

Professor Shinji Toda of Tohoku University, who is familiar with active faults, said that even if the displacement of the moving fault is not so large, if there is a building directly above it, it may cause serious damage. It's important to understand that there are risks around you. "

Even after the Kumamoto earthquake, "friendship faults" have been confirmed one after another in various places.

By utilizing the latest technology, the existence of the "Otsukai Fault" was confirmed in the subsequent inland earthquake.

According to the analysis by the Geospatial Information Authority of Japan, due to the earthquake in northern Osaka Prefecture in June 2018, a part of the "Arima-Takatsuki Fault Zone", which is about 3 km northwest of the epicenter, extends for about 5 km and is about several centimeters up and down. It means that traces of lateral displacement were confirmed.

The "Arima Takatsuki Fault Zone" is also certified as a major active fault zone.

In addition, in the earthquake in the eastern part of Hokkaido Eastern Iburi that occurred in September of the same year, a vertical shift of about 10 km was confirmed intermittently at a place about 10 km west of the epicenter.

The maximum deviation is about 10 cm, which means that it is a place that has not been confirmed as an active fault.

According to Geodetic Director Satoshi Fujiwara of the Geospatial Information Authority of Japan, the detailed mechanism by which the fault moves due to "friendship" is not yet known, but it moves triggered by the seismic motion from the active fault that caused the earthquake and the change in the force applied underground. It is thought that it was.

In addition, there are cases where known active faults are moved, such as the earthquake in northern Osaka, and cases where unknown faults are moved, such as the earthquake in Hokkaido, so further research is needed. I will.

The movement of 230 faults is revealed from space

It was an artificial satellite equipped with the latest technology that revealed the existence of a large number of faults that were triggered and shifted during the Kumamoto earthquake.

In particular, the "Synthetic Aperture Radar" mounted on the Earth observation satellite "DAICHI-2" can grasp the detailed topography by emitting microwaves from the satellite and receiving reflected waves bouncing off the earth.

Although the existence of faults that move by "friendship" has been confirmed so far, there are also a number of slight topographical changes that are often overlooked in the mountains and in the mountains where it is difficult to investigate on the ground by comparing the data before and after the earthquake. It means that you can now grasp it with an accuracy of centimeters.

There is also a risk of serious damage in urban areas

The swelling of the ground that appeared like a wave on the course of a golf course.

Countless cracks and cracks that appeared in the parking lot.

A river revetment that splits diagonally.

All of these are the damages caused by the "induced surface earthquake faults" that were displaced by the series of Kumamoto earthquakes.

Professor Shinji Toda of Tohoku University, who has been conducting field surveys immediately after the earthquake, points out that "it may lead to more serious damage" in urban areas.

The proof is the fault confirmed in the residential area of ​​Higashi-ku, Kumamoto City.

A crack was found across the road, triggered by two major earthquakes and slipping.

There is a consultant company building on the extension of the crack, and in the photo taken by Professor Toda immediately after the earthquake, multiple cracks can be seen on the wall.

According to the man who runs the company, the cracks extended to the floors and ceilings of the room, and the entire building was also slightly tilted.

The man said, "At that time, there wasn't much damage to the surrounding buildings, and I thought that the damage was severe because our building was old. However, I learned about the existence of the fault in a survey conducted after the earthquake. I was surprised and scared because the building was cracked along the cracks in the road. "

When rebuilding, the man changed the location of the building so that it was not directly above this fault.

Many of the active faults in each region and the "major active fault zones" that the national government conducts intensive surveys and evaluations pass through urban areas because they have a particularly large impact on society and the economy.

Professor Toda said, "When a large-scale inland earthquake occurs, not only shaking but also damage due to surface displacement may spread over a wide area. First of all, it is necessary to know the locations of major active faults and active faults. I think this is the first step. "