Tectonic Summary

The September 5, 2018, M 6.6 earthquake east of Tomakomai, on the island of Hokkaido, Japan, occurred as the result of shallow reverse faulting. Focal mechanism solutions for the earthquake indicate faulting occurred on either a moderately dipping reverse fault striking northwest, or on a shallow-to-moderately dipping fault striking southeast. At the location of this earthquake, the Pacific plate is moving towards the west-northwest at a velocity of about 87 mm/yr relative to the North America plate, subducting beneath Japan and Eurasia at the Japan and Kuril-Kamchatka Trenches to the east of the September 5th earthquake. Note that some authors divide this region into several microplates that together define the relative motions between among the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are part of North America and Eurasia, respectively. Given the depth and focal mechanism solutions of this earthquake, the event likely represents rupture of a fault within the North America (upper) plate (or Okhotsk microplate), rather than on the subduction zone plate boundary interface between the Pacific and North America plates, which is at a depth of approximately 100 km at the location of this earthquake. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

This reverse-faulting earthquake occurred in a collisional belt whose tectonics are driven by the convergence of the Pacific, Eurasia and North America plates. Hokkaido is made up of accreted terrain that roughly parallels the plate boundary to the east, and today’s earthquake is located in the Oshima Belt region. Japanese seismic hazard maps indicate this earthquake was located in close proximity to the Nohoro-Kyuryo fault zone, with high hazard and an expected maximum magnitude of M~7.

While large earthquakes are common in Japan, and regularly cause damage, few moderate-to-large shallow intraplate earthquakes have occurred in the vicinity of today’s earthquake. Over the preceding century, 70 earthquakes of M 6 or larger have occurred within 250 km of the September 5th earthquake, though only 6 of these occurred at shallow depths beneath Hokkaido. The most recent of these six upper plate events was a M 6.4 earthquake about 100 km to the east of today’s event in January, 1970.

Over the first 14 hours following the September 5th earthquake, the USGS have located 7 aftershocks of M 4.3 and larger. Two of these were larger than M 5, and the largest was M 5.4. News reports indicate that significant landsliding was triggered by the M 6.6 mainshock, burying houses in the epicentral region. At the time of writing, there have been 7 reported fatalities, hundreds of injuries, and dozens of people reported missing.

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)