TOKYO—Japan's government said Thursday it is considering extending the evacuation zone around its hobbled Fukushima Daiichi nuclear complex, as the government recalculates the risk of radioactivity that continues to issue from the plant four weeks after Japan's devastating earthquake and tsunami.

Later on Thursday, Japan suffered its strongest temblor since the March 11 quake, highlighting the ongoing uncertainty over whether further aftershocks could damage the reactors further and release more radiation, or put other nuclear...