TOKYO -- The large earthquake that shook a wide area of Japan on New Year's Day is the latest in a cluster of temblors that have been hitting the Noto Peninsula region on the Sea of Japan since late 2020, with experts suspecting deep groundwater as a factor and warning that more quakes could follow.
Monday's earthquake registered a preliminary magnitude of 7.6, larger than the 7.3 quake that struck Kobe in 1995 and killed over 6,000 people. The New Year's quake prompted a major tsunami warning in the prefecture of Ishikawa, in addition to toppling buildings and sparking fires.
According to the Japan Meteorological Agency, 19 tremors registering at least 1 on Japan's seismic intensity scale -- which gauges the degree of shaking at a given point rather than the release of energy measured by magnitude -- had occurred as of 5:30 p.m. on Monday. One had an intensity of 7, the biggest on the scale, while three were logged at upper 5, the fourth-highest reading. Aftershocks remain a concern.
Since 2020, the Noto region has experienced five earthquakes with intensities of upper 5 or higher that have caused human casualties. In May 2023, it saw a series of tremors clocking in at upper 5 and upper 6 on Japan's scale, resulting in one death, 49 injuries and the destruction of approximately 200 residential buildings.
More than 500 earthquakes of intensity 1 or higher have been confirmed in the region since December 2020. Government researchers warned in May that this activity was "expected to continue for some time," and called for strengthening disaster prevention measures.
Although the mechanism that triggered Monday's big earthquake remains uncertain, government researchers have suspected that movement of underground fluid is contributing to the seismic instability in the region, due to the way the ground lifts and the epicenter shifts.
Junichi Nakajima, professor in the Department of Earth and Planetary Sciences at the Tokyo Institute of Technology, said rising underground water can enter faults, making them slippery. Researchers' analysis of seismic waves indicates that water is accumulating at a depth of 20 to 30 kilometers, and that if it rises to around 10 to 15 km, it is likely to cause an earthquake.
"There are almost no observed cases of earthquake swarms exceeding magnitude 7," said Nakajima, cautioning that a series of relatively large earthquakes is a risk.
Aitaro Kato at the University of Tokyo's Earthquake Research Institute said that "seismic activity has been decreasing, but it is still at a high level." In Monday's quake, he said, "the fluid may have made it easy for the fault to slip, and caused a larger rupture. We need to continue to pay close attention to the activity."
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