June 9, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for May 2021

1 Major Seismic Activities

On May 1, there was a magnitude (M) 6.8 earthquake offshore Miyagi prefecture. Maximum seismic intensity 5 Upper was observed in Miyagi prefecture and was caused damage with injured persons and so on.

• Seismic Activities for All of Japan in May 2021 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On May 14, there was an M4.6 earthquake at a depth of approximately 20 km under the central Hidaka region. The focal mechanism showed a reverse fault type with a compression axis in an NE-SW direction. This event occurred within the crust.

* On May 16, there was an M6.1 earthquake at a depth of approximately 10 km, offshore Tokachi (#). The focal mechanism had a compression axis in an E-W direction. This event occurred within the crust of the continental plate.

• Hokkaido Region
• Earthquake Under the Central Hidaka Region on May 14
• Earthquake Offshore Tokachi on May 16

to supplementary reports

(2) Tohoku Region

* On May 1, there was an M6.8 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.
The recent event occurred at about 40 km southeast of the epicenter of the earthquake offshore Miyagi prefecture on March 20 (M6.9), and at about 20 km southwest of the epicenter of the earthquake offshore Miyagi prefecture on April 18 (M5.8). Afterwards there is a moderate seismic activity in the area of about 40km east-west and about 20km north-south, including 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 March 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. This event occurred within the crust.

* On May 14, there was an M6.3 earthquake at a depth of approximately 45 km, offshore Fukushima 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 vicinity of the recent event, on February 13, there was an M7.3 earthquake within the Pacific plate.

• Tohoku Region
• Earthquake Offshore Miyagi Prefecture on May 1
• Earthquake Offshore Miyagi Prefecture on May 1 (Seismic Activity of the Recent Event)
• Distribution of Hypocenters Surrounding the Earthquake Offshore Miyagi Prefecture on May 1, 2021 Determined Using 3 Dimensional Structure of Seismic Wave Velocity
• Observed Data Before and After the Earthquake Offshore Miyagi Prefecture (May 1, M6.8) (provisional)
• Observed Data Before and After the Earthquake Offshore Miyagi Prefecture (May 1, M6.8) (provisional)
• Distribution of Slip of the Earthquake Offshore Miyagi Prefecture on May 1, 2021
• Earthquake Offshore Miyagi Prefecture on May 1, 2021 –Analysis of Focal Process by Near-Field Strong Motion Waveforms (provisional)–
• Focal Process of the Earthquake Offshore Miyagi Prefecture on May 1, 2021 (provisional)
• Earthquake Offshore Miyagi Prefecture on May 1, 2021 –Distribution of Seismic Slip Estimated by the Waveform Inversion–
• Long-Term Evaluation of the Seismic Activity Along the Japan Trench –Points–
• A One Order Small Inter-Plate Earthquake That Occurs Repeatedly in the Landward Area of the Offshore Miyagi Prefecture (Earthquake Offshore Miyagi Prefecture)
• Earthquake Offshore Fukushima Prefecture on May 5
• Earthquake Offshore Fukushima Prefecture on May 14

to supplementary reports

(3) Kanto-Chubu Region

There was no remarkable activity.

• Kanto - Chubu Region

to supplementary reports

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

• Kinki-Chugoku-Shikoku Region

to supplementary reports

(5) Kyushu-Okinawa Region

* 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.

• Kyushu Region
• Earthquake in the Kumamoto Region, Kumamoto Prefecture on May 6
• Okinawa Region

to supplementary reports

(6) Around the Nankai Trough

* 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.

• Vertical Movement of GNSS-Based Control Stations - Omaezaki
• Vertical Movements of GNSS-Based Control Stations - Kii Peninsula and Around Muroto Cape

to supplementary reports

 #: "Offshore Kushiro" is the region name used in the announcement of Japan Meteorological Agency
 Note: GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for May 2021

June 9, 2021
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during May 2021.

• M4.0 or greater: 96 (80 events in April)
• M5.0 or greater: 10 (9 events in April)
• M6.0 or greater: 3 (1 event in April)

(Reference)

• M4.0 or greater: 81 / month (69-104)
• (Median monthly number from 1998 to 2017, half of data are included in the interval shown in parentheses)
• M5.0 or greater: 10 / month (7-14)
• (Median monthly number from 1973 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 1 / month (0-2)
• (Median monthly number from 1919 to 2017, half of data are included in the interval shown in parentheses)
• M6.0 or greater: 16 / year (12-21)
• (Median annual number from 1919 to 2017, half of data are included in the interval shown in parentheses)

The following events for the period from May 2020 to late April 2021 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

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2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

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(2) Tohoku Region

There is no supplementary information for Tohoku region.

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(3) Kanto-Chubu Region

– From May 7 till 16 in the northern Kii Peninsula, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. Almost synchronous with the deep low-frequency earthquakes (tremors), slight crustal deformations were observed on strain and tilt data in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the deep boundary between the Philippine Sea and the continental plates.

• Deep Low-Frequency Earthquake (Tremor) Activity and Short-Term Slow Slip in the Northern Kii Peninsula
• Fig.2 Observed results of Strain, Tilt and Underground Water in the Kii Peninsula (2021/05/01 00:00 - 2021/05/25 00:00 (JST))
• Fig.3 Observed results of Strain and Tilt in the Kii Peninsula (2021/05/01 00:00 - 2021/05/25 00:00 (JST))
• Fig.4 Observed results of Strain and Tilt in the Tokai Region (2021/05/01 00:00 - 2021/05/25 00:00 (JST))
• Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.2-4[A]) During 2021/05/09PM-12
• Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.2-4[B]) During 2021/05/13-14AM
• Fault Model Explaining the Variation of Strain, Tilt and Underground Water (Fig.2-4[C]) During 2021/05/14PM-17

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(4) Kinki-Chugoku-Shikoku 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, 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.

• Non-Stationary Horizontal Crustal Deformation of the Central Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Central Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Central Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Central Shikoku Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of the Western Kii Peninsula and the Eastern Shikoku (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (1) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Western Kii Peninsula and the Eastern Shikoku (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip in the Kii Channel Estimated from GNSS Data (provisional)

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(5) Kyushu-Okinawa Region

– 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.

* In the Monthly Evaluation of Seismic Activities from December 2020 to April 2021, it was reported that the crustal deformation different from the ordinary trend in the northern Kyushu was observed from summer 2020, and a long-term slow slip amount around the northern Hyuganada at the deeper part of the plate boundary was estimated. By the re-examination of the analysis method, considering the effect of after-slip of the 2016 Kumamoto earthquakes, it was concluded the above mentioned crustal deformation and the estimated long-term slow-slip were noises.

• Non-Stationary Horizontal Crustal Deformation of the Kyushu Region (After Linear Trend, Annual Component, and Semi-Annual Component Removed)
• Time Series of GNSS Continuous Observation: the Kyushu Region (1) Graphs After Linear Trend, Annual Cmponent, and Semi-Annual Component Removed
• Time Series of GNSS Continuous Observation: the Kyushu Region (2) Graphs After Linear Trend, Annual Component, and Semi-Annual Component Removed
• Long-Term Slow Slip Around the Hyuganada Estimated from GNSS Data (provisional)
• Non-Stationary Horizontal Crustal Deformation of the Northern Hyuganada - Example of the Time Series of East-West Component -
• Figure of Accumulated Moment of SSE Around the Hyuganada (Trial Calculation) (2020/6/1 - 2021/5/4)

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(6) Around the Nankai Trough

–"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 June 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, June 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)"

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