The Japan Meteorological Agency announced that the magnitude-6.5 earthquake that struck Kumamoto prefecture on Thursday night (April 14) was likely a foreshock to the magnitude-7.3 main quake that struck the prefecture before dawn on Saturday.

The 2011 Great East Japan Earthquake was another case in which a large quake preceded a main quake. A magnitude-7 quake hit the Tohoku region on March 9, two days prior to the massive main quake. After a series of quakes that were then believed to be aftershocks, a major magnitude-9.0 quake hit the region on March 11.

In northern Miyagi Prefecture in July 2003, a magnitude-6.4 main quake hit seven hours after a magnitude-5.6 foreshock.

In both cases, the agency initially judged the foreshocks as main quakes and did not regard them as signs that a larger quake would strike.

“When an earthquake occurs, it’s difficult to predict that a larger quake will occur later in terms of seismology,” Gen Aoki, head of the agency’s earthquake and tsunami monitoring section, said at a press conference early Saturday.

He was responding to a reporter who asked whether the agency might have failed to recognise Thursday night’s quake as a foreshock.