Photo/Illutration A road buckled by the Jan. 1 earthquake in Noto, Ishikawa Prefecture (Yoshinori Doi)

The fault responsible for the deadly magnitude-7.6 quake in Ishikawa Prefecture on New Year’s Day apparently ruptured over a far wider area than the local terrain at the center of an earthquake swarm over the past three years, experts said.

They said the fault may have shifted for around 150 kilometers from the westernmost point of the Noto Peninsula to an area in the Sea of Japan near Sado Island in Niigata Prefecture, east of Ishikawa Prefecture.

By contrast, Japan Meteorological Agency officials said the earthquake swarm that kicked off around December 2020 occurred within a 30-km-square area around Suzu city at the northern tip of the Noto Peninsula.

Experts said the earthquake was responsible for both the violent shaking and tsunami that followed as the fault is located close to the boundary between land and sea and the temblor occurred in a shallow underground area.

Hiroyuki Goto, a professor of earthquake engineering at Kyoto University, said a seismograph at the National Research Institute for Earth Science and Disaster Resilience in Wajima, Ishikawa Prefecture, apparently recorded a waveform of a “long-period seismic pulse,” indicating the fault rupture occurred directly under the city.

Seismic waves at intervals of 1 to 2 seconds, which can easily damage low- to mid-rise buildings such as wooden homes, were also recorded by seismographs in the Noto Peninsula.

The seismic waves are likely a key reason so many buildings collapsed, experts said.

Yukinobu Okamura, emeritus researcher at the National Institute of Advanced Industrial Science and Technology who has studied earthquakes for many years, said the Jan. 1 temblor could have been triggered by an active fault on the seafloor.

Central and local government officials have been paying close attention to undersea active faults north of the Noto Peninsula.

“We need to verify the accuracy of our assumptions through further detailed analysis,” Okamura said.

The direct relationship between the three-year earthquake swarm and the Jan. 1 temblor is not yet understood.

But the fault rupture started in the area where cluster earthquakes have occurred and spread over a wider area.

“In areas where small-scale seismic activities have frequently occurred, the probability of medium- to large-scale earthquakes increases,” said Shinji Toda, a professor of earthquake geology at Tohoku University. “The earthquake swarm made it easier for an active fault that had already accumulated strain to shift.”

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