An expert says Tuesday's powerful earthquake that hit northern and central Japan may have been caused by unknown active faults.
The magnitude 6.7 quake struck off Yamagata Prefecture in the Sea of Japan.
Professor Hiroshi Sato of the University of Tokyo's Earthquake Research Institute analyzed the earthquake and data on active faults in the sea that have been found in past surveys.
He said the quake may have been caused by unknown active faults, judging from the location and depth of its focus.
Sato noted that it is difficult to pinpoint all existing active faults mainly because the seabed there is covered with thick sediment.
He pointed to the possibility of similar quakes in the future.
The professor also noted that tsunami hitting the Sea of Japan coast tend to erupt near coasts. He said a tsunami in the sea would come more quickly than that in the Pacific Ocean and called on residents to begin evacuating soon after they feel strong jolts.