Officials issued a tsunami warning for Japan after a 6.8 magnitude earthquake struck on June 18. The warning was lifted around two and a half hours after the quake.
Japan’s Meteorological Agency said the quake registered magnitude 6.8 and was located off the western coast of Yamagata about 47 kilometers (30 miles) southwest of the city of Sakata.
It said the quake was fairly shallow, about 16 kilometers (10 miles) below the sea’s surface. Shallow quakes tend to cause more damage on the Earth’s surface.
Officials warned of a tsunami up to one meter (3.3 feet) high along the coast of the northwestern prefectures of Yamagata, Niigata, and Ishikawa.
All seven reactors at the Kashiwazaki-Kariwa nuclear plant in Niigata were offline and no abnormalities were reported.
Japan is one of the most earthquake- and tsunami-prone areas in the world. On March 11, 2011, a magnitude 9.0 offshore quake hit the northeast coast, causing a tsunami that took more than 18,000 lives and triggered a nuclear disaster at the Fukushima power plant.
The quake struck around 6:22 p.m. local time.
Seismotectonics of Japan
The USGS gave background on the seismotectonics in the area.“The North America plate, Pacific plate, Philippine Sea plate, and Eurasia plate all influence the tectonic setting of Japan, Taiwan, and the surrounding area. Some authors divide the edges of these plates into several microplates that together take up the overall relative motions between the larger tectonic blocks, including the Okhotsk microplate in northern Japan, the Okinawa microplate in southern Japan, the Yangzee microplate in the area of the East China Sea, and the Amur microplate in the area of the Sea of Japan,” it stated.
“The seafloor expression of the boundary between the Pacific and North America plates lies 300 km off the east coasts of Hokkaido and Honshu at the Kuril-Kamchatka and Japan trenches. The subduction of the Pacific plate beneath the North America plate, at rates of 83-90 mm/yr, generates abundant seismicity, predominantly as a result of interplate slip along the interface between the plates,” it continued.
“The 1958 M 8.4 Etorofu, 1963 M 8.6 Kuril, 2003 M 8.3 Tokachi-Oki, and the 2011 M 9.0 Tohoku earthquakes all exemplify such megathrust seismicity. The 1933 M 8.4 Sanriku-Oki earthquake and the 1994 M 8.3 Shikotan earthquake are examples of intraplate seismicity, caused by deformation within the lithosphere of the subducting Pacific plate (Sanriku-Oki) and of the overriding North America plate (Shikotan), respectively.”
The agency added: “At the southern terminus of the Japan Trench the intersection of the Pacific, North America, and Philippine Sea plates forms the Boso Triple Junction, the only example of a trench-trench-trench intersection in the world. South of the triple junction the Pacific plate subducts beneath the Philippine Sea plate at the Izu-Ogasawara trench, at rates of 45-56 mm/yr. This margin is noteworthy because of the steep dip of the subducting Pacific plate (70° or greater below depths of 50 km depth), and because of its heterogeneous seismicity; few earthquakes above M 7 occur at shallow depths, yet many occur below 400 km. The lack of large shallow megathrust earthquakes may be a result of weak coupling at the plate interface, or simply a reflection of an incomplete earthquake catalog with respect to the length of typical seismic cycles.”
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