April 10, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for March 2018

1 Major Seismic Activities

On March 1, there was a magnitude (M) 5.6 earthquake near Iriomotejima. Maximum seismic intensity 5 Lower was observed in Okinawa prefecture.

• Seismic Activities for All of Japan in March 2018 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There was no remarkable activity.

• Hokkaido Region

to supplementary reports

(2) Tohoku Region

* On March 23, there was an M5.1 earthquake at a depth of approximately 45 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.

• Tohoku Region
• Earthquake Offshore Miyagi Prefecture on March 23

to supplementary reports

(3) Kanto-Chubu Region

* On March 13, there was an M4.9 earthquake at a depth of approximately 50 km under the northeastern Chiba prefecture. The focal mechanism had a compression axis in a WNW-ESE direction.

* On March 25, there was an M5.8 earthquake offshore east of Hachijojima. 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 30, there was an M5.1 earthquake at a depth of approximately 55 km, offshore Ibaraki prefecture [under the northern Ibaraki 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.

• Kanto - Chubu Region
• Earthquake under the Northeastern Chiba Prefecture on March 13
• Earthquake Offshore East of Hachijojima on March 25
• Earthquake Offshore Ibaraki Prefecture on March 30

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 March 1, there was an M5.6 earthquake at a depth of approximately 15 km near Iriomotejima. The focal mechanism had a tension axis in an NE-SW direction. This event occurred within the continental crust.

• Kyushu Region
• Okinawa Region
• Earthquake near Iriomotejima on March 1
• Earthquake near Iriomotejima on March 1 (Seismic swarm activities in 1991 and 1992)

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

Supplement

* On April 9 at 01:32 (JST), there was an M6.1 earthquake at a depth of approximately 10 km in the western Shimane prefecture. Maximum seismic intensity 5 Upper was observed in Shimane prefecture. This event occurred within the crust. The focal mechanism showed a strike-slip fault type with a compression axis in an NNW-SSE direction (preliminary). The seismic fault is estimated to be a left-lateral strike-slip one extending in an NNW-SSE direction. In the vicinity of this event, there were 4 earthquakes with maximum seismic intensity 4 until April 10 at 15 o'clock, including an M4.8 (preliminary) earthquake which occurred on April 9 at about 02:10 (JST).

• Earthquake under the Western Shimane Prefecture on April 9, 2018 (Detail of the earthquake location)
• Seismic Activity in the Western Shimane Prefecture - Table of the Number of Earthquakes classified in Maximum Seismic Intensity
• Earthquake under the Western Shimane Prefecture on April 9, 2018 [Situation of seismic activity (As of April 10 at 10:00 (JST))]
• Earthquake under the Western Shimane Prefecture on April 9, 2018 [Situation of seismic activity (As of April 9 at 24:00 (JST))]
• Earthquake under the Western Shimane Prefecture on April 9, 2018 (Past seismic activity of the surrounding area)
• Examples of the Successive Earthquakes within the Inland Crust in the Vicinity of the Earthquake under the Western Shimane Prefecture

Note: [ ] is the regional name of the epicenter used in the Japan Meteorological Agency announcement.

Supplementary Information to the Evaluation of Seismic Activities for March 2018

April 10, 2018
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 March 2018.

• M4.0 or greater: 73 (107 events in February)
• M5.0 or greater: 11 (22 events in February)
• M6.0 or greater: 0 (4 events in February)

(reference)

• M4.0 or greater: 73 / month (Median number of earthquakes over the past 10 years from 1998 through 2007)
• M5.0 or greater: 9 / month (Median number of earthquakes over the past 35 years from 1973 through 2007)
• M6.0 or greater: 1.4 / month, approximately 17 / year (Average number of earthquakes over the past 84 years from 1924 through 2007)

The following events for the period from March 2017 to late February 2018 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

to the text

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(3) Kanto-Chubu Region

There is no supplementary information for Kanto-Chubu region.

to the text

(4) Kinki-Chugoku-Shikoku Region

– Near the boundary between the Philippine Sea and the continental plates, from Tokushima prefecture to the Bungo Channel, deep low-frequency earthquakes (tremors) were observed in the period from February 21 to March 31. Also due to the strain-inclination data, slight crustal deformation was observed in Ehime and Kochi prefectures and its surroundings from late February to late March. Also GNSS data showed a slight crustal deformation from late February. These phenomena are considered to be caused by the short-term slow-slip at the boundary between the Philippine Sea and the continental plates.

• Deep Low-Frequency Seismic (Tremor) Activity and Short-Term Slow Slip in the Area from Tokushima Prefecture to the Bungo Channel
• Short-Term Slow Slip Activity in Shikoku Region (February to March, 2018) (1)
• Short-Term Slow Slip Activity in Shikoku Region (February to March, 2018) (2)
• Observed Results of Strain, Tilt, and Underground Water in Shikoku (2018/2/5 00:00 – 2018/3/28 00:00 (JST)).
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2[A]) 2018/2/21 – 2/25
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2[B]) 2018/2/26 – 2/28
• Fault Model Explaining the Variation of Strain, Tilt, and Underground Water(Fig.2[C]) 2018/3/1 – 3/5
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2[D]) 2018/3/6 – 3/9
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2[E]) 2018/3/10 – 3/17
• Time Variation of Strain and Tilt in Shikoku (2018/2/5 00:00 – 2018/3/28 00:00 (JST))
• Fault Model Explaining the Variation of Strain and Tilt (Fig.2[F]) 2018/3/18 – 3/24
• Non-Stationary Crustal Deformation in Shikoku Region (1)
• Non-Stationary Crustal Deformation in Shikoku Region (2)

to the text

(5) Kyushu-Okinawa Region

–";On March 1, there was an M5.6 earthquake at a depth of approximately 15 km near Iriomotejima. (Exerpted)":
After this event, the seismic activity became high and there were 40 earthquakes with maximum seismic intensity 1 or over until March 31.

to the text

(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 April 6 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 6, 2018)
"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)"

to the text