Numerous earthquakes have shaken the Noto Peninsula in Ishikawa Prefecture for a year while crustal deformations persist, and scientists are still unsure of the cause.

Naoshi Hirata, chairman of the government's Earthquake Research Committee, stated after a meeting of the committee in December that "we have no clear notion why the seismic activity and the crustal motions are occurring." "Something we've never seen before could be happening."

In Suzu at the point of the Noto Peninsula, according to the Geospatial Information Authority of Japan, the ground has been steadily rising since around December 2020. The total land uplift has increased by nearly 3 cm in one place.

69 earthquakes that registered at least 1 on the Japanese seismic intensity scale of 7, the lowest class that can be felt by people, struck the region between January and December 22, according to the Kanazawa Local Meteorological Office. On September 16, one earthquake had a weaker strength of 5.

For all temblors of magnitude 1 or more, whether or not they were felt, the count exceeds 3,000.

According to Takuya Nishimura, an associate professor of geodesy at the Research Center for Earthquake Prediction at Kyoto University's Disaster Prevention Research Institute, a major earthquake is typically followed by a series of aftershocks, which become less frequent over time.

But, he continued, "the Noto earthquakes have been going on for so long." "The only explanation for that is to suppose that some forces are still being exerted below the surface."

What then is going on underground?

The Earth's crust has expanded at depths of 10-15 kilometers beneath the seismic source locations since the beginning of the crustal deformation, according to joint GPS monitoring by Kyoto University and Kanazawa University. That suggests something has assembled in significant numbers there.

Nishimura asserted that he thinks the "something" to be water.

The oceanic tectonic plate, which is sliding from beneath the Pacific and subducting beneath the Japanese archipelago, is one potential source of the water.

When a tectonic plate is a part of the seabed, seawater seeps through fissures in the rock. The plate also has mineral crystallization water that is structurally incorporated.

The majority of the water content is transported deeper into the Earth's interior when the tectonic plate lowers.

According to Nishimura, some of the water may have split up for an unidentified reason at a depth of around 250 km and risen to a depth of 10 to 20 km below the surface.

An additional illustration of "something" gathering below the surface is a magma chamber beneath a volcano. However, Nishimura noted that there are no volcanoes close to the Noto Peninsula, making such a subterranean phenomena implausible.

The Matsushiro Earthquake Swarm, which began in August 1965 and continued for almost five years in Nagano Prefecture, was likewise supposed to have been caused by an inflow of water.

Over the course of the five years, more than 60,000 audible earthquakes were recorded, with the greatest one registering magnitude 5.4.

The bedrock was thought to have been weakened by water that rose from deeper levels and seeped in, causing the earthquakes.

A mountain rose 1 meters in height in addition to the swarm, and there were "water eruptions," or water spilling out of the earth.

However, there doesn't seem to be a straightforward example that applies here.

The earthquakes in the Noto swarm have deeper origins than the seismic sources of the Nagano swarm, which were less than 10 km deep.

It is also unclear if the Noto earthquakes can be linked to the same process that caused the Nagano quakes or to a water-induced reallocation of forces acting on the local bedrock.

Nishimura stated that there are currently "no signs" that the crustal deformation would stop. As long as the crustal deformation persists, there will probably be additional earthquakes, thus we should continue to be ready for them.

In Suzu, Ishikawa Prefecture, on September 16, an earthquake beneath the Noto Peninsula reported a weaker 5 intensity on the Japanese seismic scale of 7. The X indicates the epicenter. (From the website of the Japan Meteorological Agency)
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