Mw 6.9 Earthquake Strikes Off the Coast of Fukushima, Japan

Mw 6.9 Earthquake Strikes Off the Coast of Fukushima, Japan

  • EOS News
22 Nov 2016

The Mw 6.9 earthquake was located near the east coast of Honshu, Japan (Source: USGS))

A strong earthquake measuring Mw 6.9 (MJMA 7.4) struck Japan’s northeastern coast at about 6 am (Japan local time) on 22 Nov 2016. According to the United States Geological Survey (USGS), the earthquake’s epicentre was located about 40 km northeast of Namie (37.392°N 141.403°E), a small town north of the Fukushima Daiichi Nuclear Power Plant, which was evacuated soon after the 2011 Mw 9.0 Tohoku-Oki earthquake which had resulted in the plant sustaining a nuclear leak.

The main shock was followed by a series of aftershocks off Japan’s eastern coast. Most of these aftershocks were less than 5.0 in magnitude, and felt only along the Fukushima coastline. The tremors from the main quake were felt in the western part of Honshu Island. The seismic intensity recorded in Fukushima’s coastal towns was strong enough to reach intensity levels VI to VII on the Modified Mercalli Intensity Scale (MMI). The MMI is a scale used to measure the strength of ground shaking from an earthquake as felt by people. Intensity levels VI to VII are usually powerful enough to move heavy furniture and cause minor damage to some buildings.

The schematic block diagram shows the location of today’s Mw 6.9 earthquake. The earthquake occurred within the overriding plate above the subduction zone interface that ruptured during the 2011 Tohoku-Oki earthquake.

There are a few important things to note about today’s earthquake: At Mw 6.9, it is of a much smaller magnitude than the 2011 megathrust earthquake (Mw 9.0); it appears to have occurred on a different structure; and is likely to be an aftershock from the former event. According to Assistant Professor Wei Shengji’s research group at the Earth Observatory of Singapore, the findings from this morning’s earthquake event suggest that it is likely to have occurred on a normal fault above the subduction interface.

“It is possible that the 2011 Mw 9.0 Tohoku-Oki earthquake increased the stress on the fault that caused this morning's Mw 6.9 earthquake,” said Asst. Prof Wei. “However, it is important to note that the Mw 6.9 earthquake did not occur on the same fault as the 2011 quake. It took place on a different fault at a shallower depth.” Scientists from Japan’s National Research Institute for Earth Science and Disaster Resilience (NIED), as well as USGS, have also arrived at similar conclusions.

Asst. Prof Wei added that aftershocks can be expected to follow this morning’s Mw 6.9 event. “However, the likelihood of having another mega-thrust earthquake similar to the 2011 event is slim,” he explained, “as it would take hundreds of years to re-accumulate the same amount of stress on the main plate boundary.”

Because the earthquake was generated by a shallow rupture of a normal fault along Honshu’s eastern coast, it had generated a local tsunami on Japan’s eastern coast. Most of the resulting tsunami waves observed along Japan’s coastline were less than 1-metre (m) in height, except for the Sendai area where the tsunami waves reached about 1.4 m. A magnitude-6.9 earthquake usually would not produce significant tsunami hazards along the coastline. This is because the rupture would normally cause only a minor displacement (up to few metres) of the seafloor. However, if the earthquake causes a submarine landslide to occur on a slope, higher tsunami waves may then be produced in nearby coastal regions.

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