May 14, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
There was no remarkable activity.
* On April 18, there was a magnitude (M) 5.8 earthquake at a depth of approximately 50 km, offshore Miyagi 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.
There was no remarkable activity.
* On April 18, there was an M3.6 earthquake at a depth of approximately 5 km in the northern Hiroshima prefecture. This event occurred within the crust.
* On April 9, the seismic activity became high under the ocean area near the Tokara Islands (near Kodakarajima Island), and till April 30 there occurred 265 earthquakes with seismic intensity 1 or over observed. Of these there were 25 earthquakes with seismic intensity 3 or over observed. The largest were M5.3 earthquakes on April 10 and 12. The focal mechanisms showed a strike-slip fault type with a tension axis in an NW-SE direction. These events occurred within the continental plate. In the vicinity of the recent activity, there was a moderate seismic activity in 2000.
* 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 April 18, there was an M6.1 earthquake at a depth of approximately 15 km near Taiwan. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction.
* On May 1, there was an M6.8 earthquake at a depth of approximately 50
km, offshore Miyagi prefecture. Maximum seismic intensity 5 Upper was observed
in Miyagi 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.
The recent event occurred at about 40 km southeast of the epicenter of the earthquake offshore Miyagi prefecture on March 20 (M6.9). Afterwards there is a moderate seismic activity in the area of about 40km east-west and about 20km north-south, including the recent event. Note that the earthquake offshore Miyagi prefecture on April 18 (M5.8) occurred at about 20 km northeast of the recent event.
According to the result of GNSS observation, the crustal deformation associated with the recent event was observed in a wide area, centering on Miyagi prefecture: a little more than 1 cm east-south-eastward movement at S Ishinomaki-makihama station, Ishinomaki City, and at Yamoto station, Higashi-matsushima City and a little more than 1 cm subsidence at Onagawa station, Onagawa City.
From the distribution of seismic activity and the analyzed results of GNSS observation and seismic waves, the source region is the area extending north from the epicenter, offshore Oshika Peninsula. The added source region of the recent event and the earthquake on May 20, overlaps a part of the western source region of the 1978 Miyagi-ken-Oki Earthquake (M7.4), but it doesn't cover the whole source regions of the 1978 Miyagi-ken-Oki Earthquake and the earthquake offshore Miyagi prefecture in 2005 (M7.2).
The recent event occurred within the aftershock area of "the 2011 off the Pacific coast of Tohoku Earthquake" (hereinafter referred to as the Tohoku-Oki Earthquake). Synthetically judging from the following facts, the state continues that the number of earthquakes per year is larger than that of the era before the Tohoku-Oki Earthquake, examples of the other gigantic earthquakes, and [Long-term Evaluation of the Seismic Activity Along the Japan Trench (published on February 26, 2019)], we should pay attention for a long time, to the probability of a large earthquake occurring in the aftershock area of the Tohoku-Oki Earthquake and the surrounding area including the inland, and of suffering from strong shaking and high tsunami.
* On May 5, there was an M5.1 earthquake at a depth of approximately 35 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in an N-S direction.
* On May 6, there was an M4.0 earthquake at a depth of approximately 15 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a normal fault type with a tension axis in an N-S direction. This event occurred within the crust.
* On May 14, there was an M6.0 (preliminary) earthquake at a depth of approximately 40 km, offshore Fukushima prefecture. The focal mechanism (preliminary) 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.
Note: GNSS is a general name of satellite positioning system such
as GPS.
May 14, 2021
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during April 2021.
(Reference)
The following events for the period from April 2020 to late March 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Offshore East of Chiba Prefecture | M6.1 on June 25, 2020 (Depth of approximately 35 km) |
||
| – 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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| – In Kermadec Islands | Mw8.1 on March 5, 2021 | ||
| – In the Northern Wakayama M4.6 on March 15, 2021 Prefecture | M4.6 on March 15, 2021 (Depth of approximately 5 km) |
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| – Offshore Miyagi Prefecture | M6.9 on March 20, 2021 (Depth of approximately 60 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– There is a moderate seismic activity beginning on March 18 in the ocean area near Izu-Oshima Island, and till April 30 there occurred 19 earthquakes with seismic intensity 1 or over. The largest was an M4.3 earthquake on April 21. In the vicinity of the recent activity, there was a moderate seismic activity in 2003.
– There is a moderate seismic activity beginning on March 21 in the northern Nagano prefecture, and till April 30 there occurred 20 earthquakes with seismic intensity 1 or over. The largest was an M3.6 earthquake on April 13.
– 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, which 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, seems to have slowed down recently.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the northern Kyushu, observed from summer 2020, which is considered to be caused by a long-term slow slip around the northern Hyuganada at the deeper part of the boundary between the Philippine Sea and the continental plates, seems to have slowed down recently.
– 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 May 12 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, May 12, 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)"
* There is no supplementary information for other regions.
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: