The magnitude 7.6 quake that struck on New Year’s Day was caused by a movement of active faults in the sea off Ishikawa Prefecture's Noto Peninsula that had been dormant for 3,000 to 4,000 years, Japanese experts have said.

Researchers told a symposium hosted by Tohoku University on Tuesday that the slipping of a belt of active faults on the Noto Peninsula and the Sea of Japan explained the big jolt — which registered a 7 on the Japanese seismic intensity scale, the maximum level — as well as a tsunami that hit the shores soon afterward.

“While each active fault is short, a group of them have formed a band stretching over 100 kilometers,” said Shinji Toda, a seismologist at the university. “The active faults are right beneath the peninsula, and also in the sea, thus causing the strong jolt on the ground and the big tsunami.”

The Geospatial Information Authority of Japan has said that the quake caused the ground to be uplifted by as much as 4 meters in Wajima, Ishikawa Prefecture, based on images captured by the Advanced Land Observing Satellite-2, also called Daichi-2.

Toda estimated that, considering the peninsula’s geological features, where long-term uplifting of the ground had formed its terraces, seismic energy capable of causing ground uplift of 4 meters had built up over 3,000 to 4,000 years.

“The active faults that had been dormant for 3,000 or 4,000 years moved, causing the uplift this time,” he said.

Through such ground uplift, the total land area of the Noto Peninsula has expanded by 4.4 square kilometers, according to an estimate released Monday by the Association of Japanese Geographers.

The association’s researchers, led by Hideaki Goto of Hiroshima University, analyzed 300 km of the peninsula’s coast using aerial photography and satellite images taken between Jan. 2 and Friday. They found that the shoreline had advanced toward the sea, with the largest expansion seen in Wajima, where the coastline moved by 240 meters.

The Noto Peninsula had experienced a series of small earthquakes preceding the Jan. 1 jolt. Called seismic swarms, such small quakes do not fit the pattern of a main shock-aftershock sequence. Their number had grown dramatically over the past three years and had happened as often as 8,000 times per year, a major increase from an annual average of 20 before that, Toda said.

“It is common knowledge in the field of seismology that when the frequency of small quakes rises by 400 times, so too does the risk of big quakes by the same amount,” Toda said.

While Toda said more analysis was needed to ascertain all of the seismic forces that contributed to the quake, some researchers have argued that the swarms were caused by active faults being made slippery by crustal fluids seeping from the tectonic plates deep underground and into the faults.

Another researcher said that the Jan. 1 quake — whose death toll in Ishikawa stood at 203 as of 9 a.m. Wednesday — probably damaged far more structures than the government had then grasped.

Hiroyuki Fujiwara of the National Research Institute for Earth Science and Disaster Resilience told the symposium that, based on the institute’s real-time damage estimate using data from seismic intensity meters set up across Japan, the number of houses destroyed or partially damaged by the quake could run up to several tens of thousands, far bigger than the official tally, which stood at 1,787 as of Wednesday morning.

Tohoku University researchers, who conducted a field survey of buildings in the city of Nanao and the town of Anamizu in Ishikawa Prefecture on Thursday and Friday, also said that structures built before earthquake resistance requirements were strengthened in 1981 and 2000 had proved vulnerable.

The fact that the region has many houses with traditional roof tiles means they are also less resistant to quakes, as the heavy tiles put extra pressure on structures, researcher Ryuta Enokida said, noting that recently built structures he saw in Anamizu were unscathed.