May 12, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On May 5 at 14:42 (JST), there was a magnitude (M) 6.5 earthquake at a depth of approximately 10 km, offshore the Noto Peninsula (Note 1). 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 12 at 08 o'clock (JST) there occurred 96 earthquakes with seismic intensity 1 or over observed. The largest aftershock till 12th at 08 o'clock (JST) is an M5.9 earthquake on 5th at 21:58 (JST) with maximum seismic intensity 5 Upper observed in Ishikawa prefecture.
* The M6.5 earthquake on May 5, 2023 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 (Note 2) observation, 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 crustal deformation toward the satellite of maximum 20 cm or so was detected at the surrounding area of the epicenter of the M6.5 earthquake.
* 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.
* Accompanying the M6.5 earthquake on May 5, 2023, large accelerations were observed, such as 729 gal (synthesis of 3 components) at KiK-net Suzu observatory, Suzu City, Ishikawa prefecture.
* Although the seismic activity is decreasing, it continues to be still higher than that of the stage before the M6.5 earthquake. This situation will continue for the time being, and in the area of strong shaking, we should continue to pay attention. 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, 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. From December 1, 2020 till May 12, 2023 at 08 o'clock (JST), there occurred 409 earthquakes with seismic intensity 1 or over observed: of these 61 earthquakes with seismic intensity 3 or over observed.
In the series of the seismic activity, the M6.5 earthquake on May 5, 2023 is the largest earthquake.
* The series of the seismic activity was mainly observed in the four areas. Especially the activity was high in the northern and eastern areas. The M6.5 event occurred at the north of the eastern area and the M5.9 event occurred in the northern area.
The seismic activity hitherto showed several distributions of sources dipping generally southeast and moved from deeper to shallower region as time passed. The M6.5 earthquake occurred in the shallower side of the region, where the activity became high after the occurrence of M5.4 earthquake on June 19, 2022 which was the largest, and then the activity extends further to the northwestern shallow 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 seismic activity area of the M6.5 earthquake on May 5, 2023. Beside these, as earthquakes accompanied with 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.
* 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.
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.