A magnitude 7.3 earthquake struck off the east coast of Honshu, Japan, at 11:36 p.m. (local time) yesterday, resulting in significant ground shaking near the epicenter, much structural damage, at least four deaths, and a power outage affecting more than 2 million households, including some 700,000 in Tokyo. (According to Japanese utility Tepco, power was restored to all households by 2:52 a.m. on Thursday.)
The seismic event also triggered a tsunami advisory for Japan’s Fukushima and Miyagi Prefectures and stirred up painful memories of the massive March 2011 earthquake and its attendant tsunami, which led to the disaster at the Fukushima Daiichi nuclear plant. The advisory was lifted early Thursday. Waves of approximately 20 centimeters (about 8 inches) above normal tide levels were reported in Sendai and the Port of Ishinomaki, both in Miyagi Prefecture.
“To generate a significant tsunami, you need uplifting of the seafloor,” explained Andrew Whittaker, a civil engineering professor at the University of Buffalo, in a Wednesday interview with Nuclear Newswire. “Think of water in a bathtub. You suddenly raise up one half of the bathtub vertically with respect to the other half, and you create a wave. . . . From the information that’s available now from the U.S. Geological Survey website, the hypocenter of the magnitude 7.3 earthquake where the fault rupture initiated was very deep, approximately 60 kilometers [about 37 miles] below sea level. The likelihood of significant movement of the seafloor for a magnitude 7.3 earthquake with such a deep hypocenter is much lower than, say, for the magnitude 9 event of 11 years ago.”
The Fukushima plants: According to a news bulletin from Tepco, owner of the two Fukushima plants, no radiological, environmental, or other operational abnormalities were observed at Fukushima Daiichi following the quake. “All cooling systems were maintained, and the ALPS [advanced liquid processing system] water treatment system was manually suspended as a precaution in accordance with standard procedures and without any irregularity,” Tepco stated. “The cooling systems for the fuel debris at Units 1–3 remain functioning, there is no abnormality for the cooling systems for the spent fuel pool for all units, as well as the common pool, and there are no abnormalities in the water levels of storage tanks containing contaminated water.”
The situation at Fukushima Daini was largely the same, Tepco added, “with the exception of a brief automatic stoppage of the cooling pump for the spent fuel pool at Units 1 and 3. The pumps were restarted after careful examination, and there was no risk to the environment or to workers.”
Tepco stressed that it would continue to closely monitor the status of the two plants and would commence a more detailed investigation Thursday morning.