Courtesy USGS.

The minute-long quake was originally reported as a 7.4 magnitude, but that has been downgraded to 7.1.  

Believed to be an aftershock from the Mar. 11 disaster, the jolt caused Japanese authorities to issue a tsunami warning and watch the Fukushima Daiichi nuclear plant closely:

"The earthquake and tsunami alert raised fears about possible effects at the damaged Fukushima nuclear reactor, which has been leaking radio since the original earthquake. 

The Tokyo Electric Power Company said it had no reports of any new damage or casualties near the reactor site caused by Thursday's earthquake, which hit shortly before midnight."

The Wall Street Journal described the second quake vividly:

"The menace was real, and all too familiar, the tension that haunted the city in the early hours and days after March 11 snapping back immediately, as if it never left.

Some TV channels quickly shifted to by now expected earthquake programming: footage of newsrooms shaking vigorously, concerned workers holding on to desks, monitors and papers, cutting away to long shots from remote, high-placed cameras in distant towns rocking violently."

U.S. authorities said they do not expect a tsunami to make landfall here.