A strong earthquake of magnitude 7.1 shook the northeast of Japan late on Thursday, and a tsunami warning was issued for the coast already devastated by last month’s massive 9.0 quake and the tsunami that crippled a nuclear power plant.

Tokyo Electric Power, operator of the crippled Fukushima Daiichi nuclear station, said initial checks on the plant had shown no new problems. Injections of coolant water into its overheated reactors, and of nitrogen gas into one unit were proceeding normally, and levels of contaminated-water in service tunnels under the plant were unchanged, it said. No workers at the plant were hurt.

Nuclear safety authorities said another nuclear power plant further to the north, Onagawa nuclear station in Miyagi, had lost external power from two of its three electricity lines, but all its safety systems were working on the remaining line. The plant, like all other nuclear stations along Japan’s northeast coast, has been off line since the country’s massive March 11 earthquake and tsunami, and its uranium fuel has been cooled to safe levels.