Volcanic tremors were detected Thursday in Mount Zao, which straddles Yamagata and Miyagi prefectures, the Sendai Meteorological Observatory in Miyagi Prefecture said Thursday.
Although no warning of an impending volcanic eruption has been issued, an official at the observatory said it will closely monitor the mountain because a sudden eruption like the one on Mount Ontake last month that killed dozens of hikers cannot be completely ruled out.
According to the observatory, Yamagata University confirmed that a part of the water in the crater near the mountain summit turned muddy white on Wednesday for about 15 minutes. Such muddy white water was seen in the crater in 1940 when the mountain had a small-scale eruption.
The observatory could not see any muddy water on Thursday, but it will investigate whether it was caused by hot water or gas related to magma activities, according to the officials.
The observatory said it confirmed 18 volcanic earthquakes and three tremors between Sept. 30 and Thursday morning.