June 9, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* In the crust of the Noto region, Ishikawa prefecture, the number of earthquakes has been increasing since about 2018, then the seismic activity became high since December 2020, and it has become higher since about July 2021. From December 1, 2020 till June 8, 2023 at 08 o'clock (JST), there occurred 437 earthquakes with seismic intensity 1 or over observed: of these 63 earthquakes with seismic intensity 3 or over observed.
In the series of the seismic activity, the largest earthquake is the magnitude (M) 6.5 earthquake on May 5, 2023 at 14:42 (JST), offshore the Noto Peninsula (Note 1). Maximum seismic intensity 6 Upper was observed in Ishikawa prefecture and some damage was caused.
In Ishikawa prefecture were observed long-period ground motion class 3, tsunami of height 10 cm at Wajima Port observatory (Ports and Harbours Bureau), and of height 4 cm at Nagahashi observatory (JMA), Suzu City, respectively. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust.
* The series of the seismic activity before the occurrence of the M6.5 earthquake, was mainly observed in the four seismic activity areas. Especially the activity was high in the northern and eastern sides of the Noto Peninsula. The seismic activity showed several distributions of sources dipping generally southeast and moved from deeper to shallower region as time passed. Note that the largest one before the M6.5 event was the M5.4 earthquake on June 19, 2022.
The M6.5 event occurred at the north part of the eastern side of activity areas and the seismic activity then extends further to the northern and eastern sea area of the activity area before the M6.5 event.
* After the M6.5 earthquake, till June 8 at 08 o'clock (JST) there occurred 124 earthquakes with seismic intensity 1 or over observed. The largest aftershock till June 8 at 08 o'clock (JST) is an M5.9 earthquake on May 5 at 21:58 (JST) with maximum seismic intensity 5 Upper observed in Ishikawa prefecture. Although the seismic activity is decreasing, it continues to be still higher than that of the stage before the M6.5 earthquake.
* According to the result of the GNSS (Note 2) observation, since about December 2020, the following crustal deformation had been observed: the cumulative displacement of 1 cm over in the south-southeast direction and the uplift of 4 cm or so at the Suzu observatory, Suzu City, Ishikawa prefecture and the cumulative displacement of 1 cm over in the south-southwest direction at the Noto observatory, Noto Town and so on.
* Accompanying the M6.5 earthquake on May 5, 2023, the following crustal deformations were observed: the displacement of 10 cm or so in the west-southwest direction at SZMT observatory (DPRI, Kyoto Univ.), Suzu City, and of 9 cm or so in the southwest direction at M Suzu Sasanami observatory (GSI), the uplift of 18 cm or so at SZHK observatory (Kanazawa Univ) and of 13 cm or so at M Suzu Noroshi observatory (GSI) and so on. The analysis of the Synthetic Aperture Radar Images by Advanced Land Observing Satellite "the Daichi-2" showed the uplift of maximum 20 cm or so was detected at the surrounding area of the epicenter of the M6.5 earthquake.
After the M6.5 event, slow deformations in both horizontal and vertical components are observed at several observatories.
* The source fault of the M6.5 earthquake on May 5, 2023, estimated from the focal mechanism, the distribution of the seismic activity, GNSS observation, the analysis of the data by Advanced Land Observing Satellite "the Daichi-2" and the result of the seismic waves analysis, is a reverse fault dipping southeast which extends in an NE-SW direction. The slip on the fault plane propagated from the focus to the shallower side.
* In the surrounding area of the Noto Peninsula there occurred large earthquakes with damage. On March 25, 2007, there was "the 2007 Noto-Hanto Earthquake"(M6.9) with maximum seismic intensity 6 Upper observed. Also in 1993, there was an M6.6 earthquake in the north of the seismic activity area of the M6.5 earthquake on May 5, 2023. Beside these, as earthquakes accompanying damage in the vicinity of the seismic activity area of the recent M6.5 earthquake, are known an M6.6~7.0 earthquake in 1729 and an M5.7 earthquake in 1896 and so on.
* In the ocean bottom offshore the north coast of the Noto Peninsula, active faults are known to exist. These active faults are estimated to be of reverse fault type dipping southeast with strikes generally in an NE-SW direction.
* Judging from the variation of the crustal deformation area, movement of the seismic activity to shallower depth, and the distribution of electrical conductivity, it is possible that some fluid movement is involved in the series of the activity. Based on the situation of the seismic activity and the crustal deformation, the series of the seismic activity will continue for the time being. We should continue to pay attention for strong shaking and tsunami.
Note1: [The Noto region, Ishikawa prefecture] was the regional name of the epicenter in JMA announcement.
Note2: GNSS is a general name of satellite positioning system such as GPS.