October 12, 2018
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of the 2018 Hokkaido Eastern Iburi Earthquake

* On September 6 at 03:07 (JST), there was a magnitude (M) 6.7 earthquake at a depth of approximately 35 km in the central and eastern Iburi regions, Hokkaido. The maximum seismic intensity 7 was observed in the Iburi region and damage was caused by this event. The focal mechanism showed a reverse fault type with a compression axis in an ENE-WSW direction. This event occurred within the continental plate.

* The seismic activity after the M6.7 event is confined in the surrounding area of about 30 km length in NS. The seismic activity is decaying but still continues to be high. In addition to the maximum seismic intensity 5 Lower observed at the time of occurrence of earthquakes on September 6 at 06:11 (JST) and on October 5 at 08:58 (JST), there occurred 20 earthquakes with maximum seismic intensity 4 till October 12 at 10 o'clock. The largest until October 12 at 10 o'clock was an M5.5 earthquake occurring on September 6 at 03:20 (JST).

* Large accelerations were observed such as 1,796 gal (synthesis of 3 components) at K-NET Oiwake station in Abira town.

* According to the GNSS observation, crustal deformation accompanying this event is reported: southward displacement of about 5 cm at Monbetsu station in Hidaka town and eastward displacement of about 4 cm at Tomakomai station in Tomakomai city. The interferometry analysis of PALSAR data by the Advanced Land Observing Satellite-2 "DAICHI-2" showed uplift of maximum 7 cm (provisional) at the epicentral area and eastward displacement of maximum 4 cm (provisional) at the east side of the uplift area.

* The area extending from the eastern Iburi region and Hidaka region to offshore Urakawa has a characteristic that there occur many crustal earthquakes which are deeper than the ordinary depth ones in land. The recent seismic activity occurs in the area with such a characteristic.

* The distribution of the seismic activity and the crustal deformation shows the possibility that the upper edge of the seismic fault reaches the depth of 15 km or so. Also in the vicinity of the surface, west of the recent seismic activity, there exists the trace of Ishikari-teichi-toen fault zone extending in an NS direction.

* As the decaying seismic activity still continues to be high and in October there occurred an earthquake with maximum seismic intensity 5 Lower. It is necessary to be prepared for the continuing seismic activity.

Note: GNSS is a general name of satellite positioning system such as GPS.
   K-NET is the strong motion seismograph network installed by the National Research Institute for Earth Science and Disaster Resilience.