April 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 5, there was a moment magnitude (Mw) 8.1 earthquake in the Kermadec Islands, which caused tsunami observed along the Pacific coasts of Hokkaido to Kanto regions and in the Ogasawara Islands.
* On March 15, there was a magnitude (M) 4.6 earthquake in the northern Wakayama prefecture. Maximum seismic intensity 5 Lower was observed in Wakayama prefecture.
* On March 20, there was an M6.9 earthquake offshore Miyagi prefecture. Maximum seismic intensity 5 Upper was observed in Miyagi prefecture and damage with injured persons and so on was caused.
* On March 3, there was an M5.9 earthquake at a depth of approximately 20 km (CMT solution), offshore east of Hokkaido. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On March 9, there was an M4.9 earthquake at a depth of approximately 75 km, offshore Miyagi prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the Pacific plate.
* On March 17, there was an M5.2 earthquake at a depth of approximately 55 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred within the Pacific plate. In the vicinity of this event, on February 13, 2021, there was an M7.3 earthquake.
* With regard to the earthquake offshore Miyagi prefecture on March 20, refer to another report (Evaluation of Earthquake Offshore Miyagi Prefecture on March 20, 2021).
* On March 16, there was an M4.9 earthquake at a depth of approximately 55 km under the southern Ibaraki prefecture. The focal mechanism showed a strike-slip fault type with a compression axis in an N-S direction. This event occurred within the Philippine Sea plate.
* On March 28, there was an M5.6 earthquake at a depth of approximately 55 km offshore east of Hachijojima Island. The focal mechanism showed a reverse fault type with a compression axis in an N-S direction.
* On March 15, there was an M4.6 earthquake at a depth of approximately 5 km in the northern Wakayama prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred within the crust. According to the result of GNSS observation, no significant crustal deformation accompanying the event was observed.
* On March 14, there was an M4.4 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an N-S direction. This event occurred within the crust.
* On March 27, there was an M6.2 earthquake at a depth of approximately 150 km, offshore north-west of Miyakojima Island. The focal mechanism showed a compression 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 great earthquakes along the Nankai Trough has relatively raised compared to the normal time.
* On March 5, there was an Mw8.1 earthquake in the Kermadec Islands. 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 Indo-Australian plates. This event caused tsunami observed along the Pacific coasts of Hokkaido to Kanto regions and in the Ogasawara Islands.
Note: GNSS is a general name of satellite positioning system such as GPS.
April 9, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during March 2021.
(Reference)
The following events for the period from March 2020 to late February 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Noto Region, Ishikawa Prefecture | M5.5 on March 13, 2020 (Depth of approximately 10 km) |
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| – Offshore East of Chiba Prefecture | M6.1 on June 25, 2020 (Depth of approximately 35 km) |
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| – Reihoku, Fukui Prefecture | M5.0 on September 4, 2020 (Depth of approximately 5 km) |
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| – Offshore Ibaraki prefecture | M5.7 on November 22, 2020 (Depth of approximately 45 km) |
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| – Offshore Iwate Prefecture | M5.6 on December 12, 2020 (Depth of approximately 50 km) |
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| – Ocean Area Near Niijima and Kozushima Islands | M5.0 on December 18, 2020 (Depth of approximately 5 km) |
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| – Offshore East of Aomori Prefecture | M6.5 on December 21, 2020 (Depth of approximately 45 km) |
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| – Offshore Fukushima Prefecture | M7.3 on February 13, 2021 (Depth of approximately 55 km) |
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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 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 western Kii Peninsula and the eastern Shikoku, observed from summer 2020, seems to have slowed down recently. This phenomenon is considered to be caused by a long-term slow slip around the Kii Channel 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 western Kii Peninsula and the eastern Shikoku, observed from summer 2020, seems to have slowed down recently. This phenomenon is considered to be caused by a long-term slow slip around the Kii Channel 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 southern Kyushu, has been observed from summer 2020. This phenomenon is considered to be caused by a long-term slow slip around the southern Hyuganada at the deeper part of the boundary between the Philippine Sea and the continental plates.
–"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.":
(This evaluation is in agreement with the views presented at the regular
meeting on April 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, April 7, 2021)
"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: