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Mw 6.2 Earthquake Strikes Western Honshu, Japan

21 Oct 2016

In the afternoon of 21 October 2016, a moderate earthquake (Mw 6.2) struck the city of Kurayoshi, located on the northern coast of western Honshu Island, Japan. The earthquake occurred approximately two hours after a Mw 4.1 foreshock struck the same area. The tremors from the main quake were strong enough to be felt throughout the entire western Honshu Island, including the city of Osaka and Hiroshima.

Photographs of the city of Kurayoshi, shared on social media, show that most of the damage are in the form of fallen plaster and collapsed old buildings. (Source: https://twitter.com/i/moments/789340811406090240)

According to the United States Geological Survey, the earthquake’s epicentre is located 8 kilometres (km) west of the city of Kurayoshi  (35.358°N 133.801°E). The magnitude of this earthquake (Mw 6.2) is about 16 times greater than the recent Mw 5.4 earthquake in South Korea that, interestingly, is still generating aftershocks more than one month after the mainshock event.

The seismic analysis of the Mw 6.2 earthquake, conducted by Japan’s National Research Institute for Earth Science and Disaster Resilience (NIED), suggests that the quake was caused by the rupture of a strike-slip fault within the shallow layers of the Earth’s crust, where the ground on both sides of the fault move horizontally in opposite directions.

Earthquake distributions, from 14-21 Oct 2016, near the epicentre indicate that the N-S running fault trace is likely to be responsible for today’s Mw 6.2 earthquake. (Source: NIED Hi-net)

Given the location of the epicentre and its strike-slip focal solution, it is believed that this earthquake is likely to have been generated by one of the N-S running left-lateral active faults near the northern coast of the western Honshu Island. Further west, the active E-W running active faults is responsible for the Mw 7.0 Tottori earthquake, on 10 September 1943, that killed more than 1,000 people and brought down close to 7,500 houses.

We can expect the area near the mainshock’s epicentre to be struck continually by aftershocks in the following weeks. Most of these aftershocks are likely to be less than 6.0 in magnitude, and are not expected to cause great damage.