December 11, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
There was no remarkable activity.
* On November 6, there was a magnitude (M) 5.0 earthquake at a depth of approximately 55 km, offshore of Fukushima Prefecture. The focal mechanism showed a reverse fault type with a compression axis in an NNW-SSE direction. This event occurred within the Pacific plate.
* On November 20, there was an M5.9 earthquake at a depth of approximately 50 km, offshore east of Aomori Prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
* 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 December 8, 2023 at 08 o'clock (JST), there occurred 501 earthquakes with seismic intensity 1 or over observed: the state of high seismic activity continues.
In the series of the seismic activity, the largest earthquake is the M6.5 earthquake on May 5, 2023 at 14:42 (JST) offshore of the Noto Peninsula (#1). The series of the seismic activity before the occurrence of the M6.5 earthquake, was mainly observed in the land area and the coastal area of the northeastern Noto Peninsula. After the occurrence of the M6.5 earthquake, the seismic activity then extends further to the northern and eastern sea area. Since November 1 till December 8 at 08 o'clock (JST), there occurred 8 earthquakes with seismic intensity 1 or over observed: the largest in November is an M3.1 earthquake. The seismic activity is decreasing as time passes, and as a whole the number of earthquakes has returned in general to that of the stage before the occurrence of the M6.5 earthquake.
According to the result of the GNSS observation, since about December 2020 till the occurrence of the M6.5 earthquake, the following crustal deformation had been observed: the cumulative displacement of 1 cm over in horizontal direction and the uplift of 4 cm or so at the Suzu observatory, Suzu City, Ishikawa Prefecture and so on. The GNSS observation and the analysis of the synthetic aperture radar images by Advanced Land Observing Satellite "the Daichi-2" showed that accompanying the M6.5 earthquake, the crustal deformations of the order maximum 20 cm was observed at the surrounding area of the epicenter. The deformation in different tendency from the pre-earthquake stage observed at several observatories after the M6.5 event has recently decayed, and it seems that the tendency in the pre-earthquake stage has returned.
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.
* On November 1, there was an M4.9 earthquake at a depth of approximately 45 km under the Kiisuido Strait. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.
* In the ocean area near the Tokara Islands (near Kuchinoshima and Nakanoshima Islands), from about April 1, there was a small cluster of earthquakes, and since May 11 the seismic activity became higher. From about middle June, magnitudes of earthquakes have become smaller, and the number of earthquakes was decreasing. Since November 7 the seismic activity has become rather active.
In November there were 21 earthquakes with seismic intensity 1 or over observed: of these 4 earthquakes with seismic intensity 3 or over. The earthquake of the largest magnitude in November is the M4.3 earthquake on November 11.
* On November 11, there was an M5.0 earthquake at a depth of approximately 100 km under Kagoshima Bay (#2). The focal mechanism had a tension axis in the subducting direction of the Philippine Sea plate. This event occurred within the Philippine Sea plate.
* There has been observed no unusual change which indicates that the possibility of occurrence of a great earthquake along the Nankai Trough has relatively raised compared to the normal time.
* On December 2, there was a moment magnitude (Mw) 7.5 earthquake in Mindanao, the Philippine Islands. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Philippine Sea and the Eurasia plates. This event caused tsunamis observed in the Pacific coasts from Chiba to Okinawa Prefectures, the Izu-Ogasawara Islands, and the Amami-Tokara Islands such as tsunami of 0.4 m (preliminary value) at Yaene, Hachijojima Island, the Izu Islands.
#1: [the Noto region, Ishikawa Prefecture] was the regional name of the epicenter used in the JMA announcement.
#2: [the Osumi region, Kagoshima Prefecture] was the regional name of the epicenter used in the JMA announcement.
Note: GNSS is a general name of satellite positioning system such as GPS.
December 11, 2023
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during November 2023.
(Reference)
The following events for the period from November 2022 to late October 2023 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Under the Southern Ibaraki Prefecture | M4.9 on November 9, 2022 (Depth of approximately 50 km) |
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| – Offshore of Kushiro | M6.0 on February 25, 2023 (Depth of approximately 65 km) |
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| – Offshore of the Noto Peninsula | M6.5 on May 5, 2023 (Depth of approximately 10 km) |
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| – Under the Southern Chiba Prefecture | M6.5 on May 5, 2023 (Depth of approximately 10 km) |
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| – In the Ocean Area near Tokara Islands (Near Kuchinoshima and Nakanoshima Islands) |
M5.1 on May 13, 2023 | ||
| – In the Ocean Area near Niijima and Kozushima Islands | M5.3 on May 22, 2023 (Depth of approximately 10 km) |
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| – Offshore East of Chiba Prefecture | M6.2 on May 26, 2023 (Depth of approximately 50 km) |
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| – Offshore of Tomakomai | M6.2 on June 11, 2023 (Depth of approximately 140 km) |
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| – In the Ocean Area near Torishima Island | Maximum M6.5 October 2 ~ 9, 2023 | ||
There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
There is no supplementary information for Kanto-Chubu region.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from about spring 2019. This phenomenon is considered to be caused by a long-term slow slip around the central Shikoku at the deeper part of the boundary between the Philippine Sea and the continental plates.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu has been observed from early 2023. This phenomenon is considered to be caused by a long-term slow slip around the southern Hyuganada Sea at the deeper part of the boundary between the Philippine Sea and the continental plates. Recently this crustal deformation seems to be slowing down.
– "There has been observed no unusual change which indicates that the possibility of occurrence of a great earthquake along the Nankai Trough has relatively raised compared to the normal time.":
(This evaluation is in agreement with the views presented at the regular meeting on December 7 of the Assessment Committee for Earthquakes Along the Nankai Trough (reference below).)
(Reference)
Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, December 7, 2023)
"In the present, there has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time. (Excerpted)"
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: