For a short while, people in Japan panicked after authorities sent an erroneous warning that a massive earthquake had hit the Tokyo area.

The Japanese Meteorological Agency sent the warning just after 5 p.m. Monday, saying a magnitude-9.1 quake had struck. The announcement, which was canceled within seconds, set off alarmed chatter on Twitter.

The agency said it was investigating why the report got sent. An agency official said the reason may have been lightning hitting a seismograph, an instrument that monitors quakes.

Japan is among the most quake-prone nations in the world. A magnitude-9 quake off the northeast coast in March 2011 set off a giant tsunami that killed more than 18,000 people and sent three nuclear reactors into meltdowns.