Magnitude 5.7 earthquake jolts central Japan, injuring more than 20

A magnitude-5.7 earthquake struck western Japan before dawn on Sunday, injuring more than 20 people across Aichi and Shizuoka prefectures and prompting a weeklong aftershock warning in a region that sits astride one of the country's most closely watched fault systems.

The quake hit at 1:04 a.m. local time at a depth of about 40 kilometers, centered in western Shizuoka Prefecture, according to Japan's Meteorological Agency, which recorded upper intensity 5+ on the national seismic scale in parts of Aichi Prefecture, the strongest shaking it has registered there in roughly three decades. The US Geological Survey put the shock at magnitude 5.5, a routine discrepancy between national and international monitoring networks that measure slightly different seismic properties.

Shaking felt across the Tokai region

Shaking rippled outward from the city of Hamamatsu through the ancient capitals of Kyoto and Nara and into Yamanashi, home to Mount Fuji, and neighboring Gifu Prefecture, though it was not felt in Tokyo, Al Jazeera reported, citing the Associated Press. Store windows shattered in the hardest-hit towns, and several of the more than 20 people hurt were injured falling on stairs as they fled buildings.

No tsunami warning was issued. The operator of the Hamaoka nuclear plant, which sits near the projected source area of a feared Nankai Trough megaquake, said it had found no abnormalities at the facility. Japanese authorities said they had so far detected no change in the trough itself and put the odds that Sunday's quake was linked to a Nankai Trough rupture as low, Japan Today reported, citing Kyodo News. Still, the agency urged residents to stay alert for roughly a week, and especially over the next two to three days, for the possibility of stronger aftershocks as well as landslides and rockfalls on slopes loosened by the shaking.

Rail service slowed, then resumed

Some local train lines were suspended immediately after the quake, and the operator of the Tokaido Shinkansen slowed bullet trains between Shizuoka and Gifu-Hashima stations while crews inspected track and overhead power lines; normal speeds had resumed by midday. A separate, unrelated magnitude-4.3 quake also shook Kumamoto Prefecture in southern Japan the same morning, a reminder of the aftershock activity that has continued there since a magnitude-7.1 quake hit the region in July.

Japan sits atop four tectonic plates and logs thousands of earthquakes a year, the vast majority too small to feel. Sunday's quake was notable less for its size than for where it struck: directly astride the Tokai industrial corridor and inside the long-monitored Nankai Trough zone, where seismologists have for decades warned that a far larger, deadlier rupture is overdue. No deaths had been reported as of Sunday evening local time.