May 6, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On May 5 at 14:42 (JST), there was a magnitude (M) 6.5 (provisional) earthquake at a depth of approximately 10 km in the Noto region, Ishikawa prefecture and maximum seismic intensity 6 Upper was observed in Ishikawa prefecture and some damage was caused. Also, in Ishikawa prefecture long-period ground motion class 3 was observed. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred within the crust.
* After the recent event, the high seismic activity continues in the area including the focus of the recent event, extending about 15 km in an E-W direction and about 15 km in an N-S direction. Till May 6 at 16 o'clock (JST) there occurred 52 earthquakes with seismic intensity 1 or over observed. The largest aftershock till 6th at 16 o'clock (JST) is an M5.9 (provisional) earthquake on 5th at 21:58 (JST) with maximum seismic intensity 5 Upper observed in Ishikawa prefecture.
* The recent event caused tsunami of height 0.1 m (preliminary) observed at Nagahashi observatory (JMA), Suzu City, and at Wajima Port observatory (Ports and Harbours Bureau) in Ishikawa prefecture.
* According to the GNSS observation, accompanying the recent event the following crustal deformations were observed: the displacement of 9 cm or so in the west-southwest direction at SZMT observatory (DPRI, Kyoto Univ.), Suzu City, the displacement of 8 cm or so in the southwest direction at M Suzu Sasanami observatory (GSI), and the uplift of 11 cm or so at M Suzu Noroshi observatory (GSI). The analysis of the Synthetic Aperture Radar Images by Advanced Land Observing Satellite "the Daichi-2" showed the crustal deformation toward the satellite of maximum 20 cm or so was found at the surrounding area of the epicenter of the M6.5 earthquake.
* Accompanying the M6.5 earthquake on May 5, 2023, a large acceleration was observed, such as 729 gal (synthesis of 3 components) at KiK-net Suzu observatory, Suzu City, Ishikawa prefecture.
* In the area of strong shaking, we should pay attention for a week or so, to the possibility of an earthquake which causes maximum seismic intensity 6 Upper or so. Especially for 2 or 3 days after the earthquake, there is a possibility that a large earthquake occurs. Also, in the case of a large earthquake at the ocean bottom, we should pay attention for tsunami.
* In the crust of the Noto region, Ishikawa prefecture, where the recent earthquake occurred, the number of earthquakes is increasing since about 2018, then the seismic activity became high since December 2020, and it has become higher since about July 2021. In the series of the seismic activity, from December 1, 2020 till May 5, 2023 at 16 o'clock (JST) there occurred 365 earthquakes with seismic intensity 1 or over observed: of these 57 earthquakes with seismic intensity 3 or over observed.
In the series of the seismic activity, the recent M6.5 event is the largest earthquake. In addition, on June 19, 2022 there occurred an M5.4 earthquake.
* The series of the seismic activity was mainly observed in the four parts. Especially the activity was high in the northern and eastern parts. The recent M6.5 event occurred at the north of the eastern part and the M5.9 event occurred in the northern part. The seismic activity then extends further to the northern ocean area.
* According to the result of the GNSS observation, since about December 2020, the following crustal deformation had continued: 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 were observed.
* 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 recent seismic activity area. Beside these, as earthquakes accompanied with damage in the vicinity of the recent seismic activity area, are known an M6.6~7.0 earthquake in 1729 and an M5.7 earthquake in 1896.
*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 in an NE-SW direction.
* Judging from the variation of the crustal deformation area, movement of the seismic activity to shallower ground, 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.
Note: GNSS is a general name of satellite positioning system such as GPS.