A recent seabed study has confirmed that an active seismic fault more than 18 km long and running near a nuclear power plant was the cause of the magnitude-6.9 earthquake that jolted the Noto Peninsula and its vicinity in March, according to the National Institute of Advanced Industrial Science and Technology.

About 15 km of the fault, located near Hokuriku Electric Power Co.'s Shika nuclear plant, and some 5 km that extends onshore from it, are believed to have caused the quake, the institute said, countering the utility's view that three different faults were responsible for the temblor.

Prior to the Noto quake, the fault had been active once or twice in the last 20,000 years, the institute said.

In the seafloor tectonic survey conducted July 3 to 10 jointly with the Japan Coast Guard off the city of Wajima and the town of Shika in Ishikawa Prefecture, the institute found the fault near the quake's focus, using a high-resolution sonic profiling device it has developed.

Near the fault, an erosion surface formed about 20,000 years ago and sediment layers covering it were distorted about 3 meters up and down, indicating quakes had occurred once or twice before the March 25 temblor.

As a result of the quake, the seabed south of the fault was elevated about 50 cm, the institute said.

The institute plans to conduct a similar study on the magnitude-6.8 earthquake that hit Niigata Prefecture on July 16, research team leader Yukinobu Okamura said.