January 15, 2025
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
After a year has passed since the M7.6 earthquake occurred on January 1, 2024, the Earthquake Research Committee comprehensively discussed today (January 15, 2025) on the seismic activity and crustal deformation of the 2024 Noto Peninsula Earthquakes (*1), which have been active for more than four years since December 2020, collecting and analyzing the results of the surveys, observations and researches to date by relevant administrative agencies, universities, and other organizations.
Seismic activity in the Noto region, Ishikawa Prefecture has been active since December 2020. While relatively small earthquakes continued at the beginning of the activity, larger earthquakes such as an M5.4 earthquake (maximum seismic intensity 6 Lower) in June 2022 and an M6.5 earthquake (maximum seismic intensity 6 Upper) in May 2023 occurred. In January 2024, the largest earthquake in the series of activities, an M7.6 earthquake (maximum seismic intensity 7) occurred. The seismic activity up to December 2023 was limited to an area of about 30 km square in the northeastern Noto Peninsula. Immediately after the M7.6 earthquake, seismic activity became very active and spread to about 150 km, extending from northeast to southwest. Thereafter, activity in the focal region of the M7.6 earthquake has gradually decreased over time. However, large earthquakes continue to occur in the area, including an M6.0 earthquake (maximum seismic intensity 5 Upper) in June 2024 and an M6.6 earthquake (maximum seismic intensity 5 Lower) in November 2024. In Japan, we have never seen such seismic activity in the land and coastal areas as this one, which lasts for several years and is accompanied by a series of M6.6 and M6.5 magnitude earthquakes in addition to the M7.6 earthquake.
Faced with an event that has never been experienced, it is difficult to foresee future activity, including how long seismic activity will continue. Since large earthquakes are still possible, as suggested by many offshore active faults around the Noto Peninsula, the Earthquake Research Committee needs to strengthen its information dissemination. In addition to regularly evaluating seismic activity, we have decided to disseminate related information as the "View of the Earthquake Research Committee Chairman."
In general, seismic activity can be divided into two types: one in which a major earthquake is followed by smaller earthquakes (mainshock-aftershocks type), and the other in which earthquakes of the same scale continue over a relatively small area during a certain period (earthquake swarm type). In the former type, seismic activity increases due to the direct impact of the first major earthquake (mainshock) and then gradually decreases over time, and in some cases, such as "the 2011 off the Pacific coast of Tohoku Earthquake," seismic activity continues even after more than 10 years. On the other hand, in the latter case, seismic activity becomes higher possibly due to some external force acting on the seismically active area, such as mass transfer of magma or other fluids materials.
The causes of the ongoing seismic activity can be roughly classified as either the direct effects of the January 2024 M7.6 earthquake (the former) or the continuing effects of some external force (the latter) in addition to the former. Statistical analysis of seismic activity (*2) can be used to estimate the degrees of both effects (the one caused by the latter is called "background seismicity." According to this analysis, the background seismicity for the series of seismic activities since December 2020 has remained high since the latter half of 2021. In this context, an M7.6 earthquake occurred on January 1, 2024. This earthquake makes it difficult to assess the effect of external forces at this moment.
Previous studies have pointed out the effect of fluids as a cause of long-lasting seismic activity with similar magnitudes. It is possible that the series of seismic activity since December 2020, at least before the M6.5 earthquake in May 2023, has involved the mass transfer of fluids such as water. For details, please refer to the "View of the Earthquake Research Committee Chairman on the Seismic Activity in the Noto Region, Ishikawa Prefecture" published in July 2022. Since December 2020, expansion-like crustal deformation has been observed in the seismically active northeastern Noto Peninsula. Assuming that this crustal deformation is caused by fluid transfer, we can evaluate the degree of fluid effect from this deformation. However, due to the large post-seismic deformation of the M7.6 earthquake, it is not possible to determine whether the crustal deformation before M7.6 is continuing, and we are currently unable to evaluate the fluid effect.
Given the above observations and analysis of seismic activity and crustal deformation, it has become difficult to foresee future activity, such as how long seismic activity will continue.
In August 2024, the Earthquake Research Committee published a long-term evaluation of offshore active faults from off the northern coast of Hyogo Prefecture to off the Joetsu region of Niigata Prefecture. According to the distribution of seismic activity after the earthquake and analysis of seismic waves, the source faults of the M7.6 earthquake are estimated to be about 150 km in length, extending northeast-southwest from the eastern part of the Monzen-oki section of the Monzen Fault Zone to the North Coast Fault Zone of the Noto Peninsula and the southwestern part of the Western Margin Fault of the Toyama Trough among these evaluated offshore faults. It should be noted that there are surrounding active faults whose probability of an earthquake is increased by the current seismic activity; the M6.5 earthquake in May 2023 and the M6.6 earthquake in November 2024 are thought to have been activated on faults that differ from the source faults of the M7.6 earthquake. In addition to the offshore and inland active faults already evaluated, there seem to be faults that cause earthquakes around the focal region of the M7.6 earthquake, although their traces in the shallow sub-seafloor or at the surface are unclear.
Seismic activity began in December 2020 and has been declining since the M7.6 earthquake in January 2024 over a longer period, from several months to a yearly basis. However, given that seismic activity is still active and crustal deformation is continuing, seismic activity is expected to remain at the current level on a monthly basis. In addition, occasional large earthquakes may occur and become even more active.
In a series of activities, large earthquakes have already occurred, including an M7.6 earthquake in January 2024 and an M6.6 earthquake in November 2024. However, on the Sea of Japan side, there have been cases such as the 1993 Earthquake off the Southwest Coast of Hokkaido (M7.8), where an M6-class earthquake occurred even several years after the largest earthquake occurred. There have also been cases of M6-class earthquakes occurring with damage in and around the Noto Peninsula, such as "the Noto Hanto Earthquake in 2007," before the current series of earthquakes that began in December 2020.
In light of these facts, in and around the active area after the M7.6 earthquake, attention should continue to be paid to strong shaking for the time being, depending on the size of the earthquake and the area of residence. In addition, if a large earthquake occurs on the seafloor, it is necessary to be aware of tsunami. Once again, it is important to ensure earthquake preparedness daily.
*1: Name established by the Japan Meteorological Agency for the M7.6 earthquake on January 1, 2024 in the Noto region, Ishikawa Prefecture, and a series of seismic activities since December 2020.
*2: A statistical model (non-stationary ETAS model) is used to quantify seismic activity. This model assumes that parameters describing the background seismicity and aftershock productivity vary with time and can evaluate long-lasting seismic activity.