March 11, 2019
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for February 2019

1 Major Seismic Activities

* On February 21, there was a magnitude (M) 5.8 earthquake in the central and eastern Iburi regions, Hokkaido. The maximum seismic intensity 6 Lower was observed in Hokkaido and damage was caused by this event.

• Seismic Activities for All of Japan in February 2019 (Magnitude 4.0 or over)

to supplementary reports

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On February 21, there was an M5.8 earthquake at a depth of approximately 35 km in the central and eastern Iburi regions, Hokkaido. The focal mechanism had a compression axis in an E-W direction. This event occurred within the continental plate. This event is considered as one of the aftershocks of the 2018 Hokkaido Eastern Iburi Earthquake, which was an M6.7 earthquake occurring on September 6, 2018. The seismic activity after the 2018 Hokkaido Eastern Iburi Earthquake confined in the area of about 30 km length in NS, still continues. The recent event is situated in the northern part of the aftershock area. According to the GNSS observation and the images of the interferometry analysis of PALSAR data by the Advanced Land Observing Satellite-2 "DAICHI-2," any significant crustal deformation accompanying the recent event is not observed.

• Hokkaido Region
• Earthquake in the Central and Eastern Iburi Regions on February 21 (1)
• Earthquake in the Central and Eastern Iburi Regions on February 21 (2)
• Earthquake in the Central and Eastern Iburi Regions on February 21 (3)
• Earthquake in the Central and Eastern Iburi Regions on February 21 (4)
• The 2018 Hokkaido Eastern Iburi Earthquake - Details of Seismic Activity (as of February 28, 2019 at 24:00)) -
• Earthquake (M5.8) at 21:22 (JST) on February 21, 2019 and the Distribution of Aftershocks of the 2018 Hokkaido Eastern Iburi Earthquake
• The Distribution of Aftershocks of the 2018 Hokkaido Eastern Iburi Earthquak
• Observed Data Before and After the Earthquake in the Central and Eastern Iburi Regions (February 21, M5.8) (provisional)
• Earthquake in the Central and Eastern Iburi Regions (February 21, M5.8) - Result of Interferometry Analysis of SAR Data by "Daichi-2" -

to supplementary reports

(2) Tohoku Region

There was no remarkable activity.

• Tohoku Region

to supplementary reports

(3) Kanto-Chubu Region

* On February 19, there was an M4.7 earthquake at a very shallow depth in the northern Nagano prefecture (in the vicinity of the boundary between Nagano and Toyama prefectures). The focal mechanism showed a strike-slip fault type with a compression axis in an E-W direction. This event occurred within the crust.

• Kanto - Chubu Region
• Earthquake in the Northern Nagano Prefecture (near the Boundary Between Nagano and Toyama Prefectures) on February 19

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 February 10, there was an M4.9 earthquake at a depth of approximately 35 km in the ocean area near Amami Oshima. The focal mechanism of this event showed a normal fault type with a tension axis in an NW-SE direction.

• Kyushu Region
• Earthquake in the Ocean Area near Amami Oshima on February 10
• Okinawa Region
• Crustal Deformation in Sakishima Islands (provisional) (1)
• Crustal Deformation in Sakishima Islands (provisional) (2)

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 March 2, there was an M6.2 earthquake at a depth of approximately 30 km (due to CMT solution), offshore southeast of Nemuro Peninsula. The focal mechanism showed a strike-slip fault with a tension axis in an N-S direction. This event occurred within the Pacific plate.

* On March 7, there was an M4.6 earthquake at a depth of approximately 70 km, offshore Miyagi prefecture. This event occurred within the Pacific plate.

• Earthquake Offshore Southeast of Nemuro Peninsula on March 2
• Earthquake Offshore Miyagi Prefecture on March 7

Note:GNSS is a general name of satellite positioning system such as GPS.

Supplementary Information to the Evaluation of Seismic Activities for February 2019

March 11, 2019
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 February 2019.

• M4.0 or greater: 79 (76 events in January)
• M5.0 or greater: 14 (11 events in January)
• M6.0 or greater: 0 (1 event in January)

(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 February 2018 to late January 2019 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

–"On February 21, there was a magnitude (M) 5.8 earthquake in the central and eastern Iburi regions, Hokkaido. (Excerpted)"
The serial seismic activity of the 2018 Hokkaido Eastern Iburi Earthquake is decaying as a whole but still continues. The serial activity continues for a while, and it is necessary to pay attention.

to the text

(2) Tohoku Region

There is no supplementary information for Tohoku region.

to the text

(3) Kanto-Chubu Region

– Deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates from February 2 to 10 in the Tokai region. According to strain, tilt and GNSS data, a slight crustal deformation was observed almost synchronously to deep low-frequency earthquakes (tremors). These 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 Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (1)
• Deep Low-Frequency Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (2)
• Deep Low-Frequency Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (3)
• Deep Low-Frequency Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (4)
• Deep Low-Frequency Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (5)
• Deep Low-Frequency Tremor Activity in Kii Peninsula and Tokai Region (February 2019) (6)
• Slow-Slip in Tokai Region Synchronizing with Deep Low-Frequency Tremor, Estimated from GNSS Data (provisional).

to the text

(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

to the text

(5) Kyushu-Okinawa Region

– According to the GNSS observation, a crustal deformation different from the so far trend has been observed in the northern Kyushu since about spring of 2018, and also in the western Shikoku since about autumn of 2018. These phenomena are considered to be caused by a long-term slow slip at the deep part of the boundary between the Philippine Sea and the continental plates in the neighborhood of the northern Hyuganada and Bungo channel.

– According to the result of the GNSS observation, crustal deformation such that Tarama observatory displaced to the south about 1 cm (provisional) was observed in the middle February in Sakishima Islands.

• Non-Stationary Horizontal Crustal Deformation of Northern Kyushu and Western Shikoku (Linear trend, annual component, semi-annual component removed)
• Time Series of GNSS Continuous Observation: Northern Kyushu and Western Shikoku
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (1) - Distribution of Estimated Slip -
• Long-Term Slow Slip in the Hyuganada and Bungo Channel Estimated from GNSS Data (provisional) (2) - Comparison Between Observed Values (black) and Calculated Values (white) -

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 March 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, March 7, 2019)
"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