Earthquake in Sea of Japan Possibly Triggered by Water from Pacific Ocean

By Keiichi Nakane and Yoshimune Numata / Yomiuri Shimbun Staff Writers

1:00 JST, May 7, 2023

The earthquake that registered a maximum intensity of upper 6 on the Japanese seismic scale of 7 in Ishikawa Prefecture on Friday afternoon was the largest in a series of earthquakes that have occurred in and around the Noto Peninsula since late 2020.

There is a growing belief that an accumulation of water deep underground had pushed up the bedrock and activated seismic activity. Experts said extreme caution is required.

Over 300 quakes felt since late 2020

“There is a possibility that large-scale earthquakes may occur in the future, so please make sure there are no dangerous conditions around you, such as nearby cliffs,” Toshihiro Shimoyama, a senior coordinator for seismological information at the Japan Meteorological Agency, said at a press conference Friday evening.

At the tip of the Noto Peninsula, many earthquakes occurred since December 2020, and the government’s Earthquake Research Committee issued an opinion just last month saying, “Seismic activity [in the area] is expected to continue for some time to come, and caution is required against strong tremors.”

According to the agency, there have been 313 noticeable earthquakes of intensity 1 or greater in the region up to the latest one. The areas where the earthquakes frequently occur have been moving and spreading clockwise from the south to the northeast.

The latest quake, with a magnitude of 6.5, was the largest in the series and occurred north of the tip of the peninsula. There have been a number of earthquakes in this area since around June 2021, and in June last year, a 5.4-magnitude earthquake with a maximum intensity of lower 6 on the Japanese scale was observed.

Since the beginning of this year, earthquakes have occurred around 10 times each month. Friday’s earthquake came amid a phase when the occurrence had been leveling off. The agency explains that “seismic activity continues in the area, making predictions difficult.”

Earth surface rose

A research team from Kanazawa University, Kyoto University, and other institutions believes that one reason for the frequent occurrence of earthquakes is the presence of fluids, such as water, that have been accumulating underground.

The Noto Peninsula extends into the Sea of Japan, but the underground water is thought to ultimately come from the Pacific Ocean, on the other side of the country, where the Pacific Plate is sinking beneath Japan, carrying water and other fluids with it at high temperatures and pressures. The rise of this water may have exerted a force that pushed the bedrock up, or the water and other fluids may have entered a fault in the Noto area, causing it to slide and thereby triggering the earthquake, the researchers said.

According to the Geospatial Information Authority of Japan, the ground surface in Suzu, Ishikawa Prefecture, where the earthquake measured a maximum intensity of upper 6, has risen about 4 centimeters since December 2020, and experts believe this is evidence that water is rising.

The research team has yet to confirm the water’s presence. However, measurements of weak electric currents flowing underground at the tip of the peninsula indicate that there is a high probability that water and other fluids are passing through.

Yoshihiro Hiramatsu, a professor of seismology at Kanazawa University, said: “This is considered an example of water-induced seismic activity. It is difficult to foresee when the quakes will die down, but there is a possibility that the earthquakes will continue for six months to a few years to come.”

New type of warning

In the wake of the upper 6 quake on Friday, the agency issued its first earthquake early warning against “long-period ground motion.”

Long-period ground motion is a long, slow shake with a period (the time for one complete shake back and forth) of 1.6 seconds or longer. Skyscrapers and long bridges in urban areas “resonate” and shake greatly. The agency has established four levels of shaking, ranging from “somewhat large shaking” (Class 1) to “extremely large shaking” (Class 4), depending on the nature of the quake.

And since February this year, the agency has begun operating a new system to issue an earthquake early warning to areas expected to be hit with shaking of Class 3 or greater. In the latest quake, a few seconds after the seismographs detected seismic waves, an announcement was issued for the Noto area, predicting Class 4 shaking from long-period ground motion. In reality, though, Class-3 shaking was observed.

Shimoyama of the meteorological agency said that the discrepancy between the level announced and the one actually observed was “within a margin of error.” He added: “We believe that when the early warning was issued, people could have taken action to protect themselves. We will continue our efforts to improve the accuracy.”