Japan's Geographical Survey Institute is watching Iwojima carefully for a possible volcanic eruption as it has recorded a 12-cm elevation in the land on the southeastern region of the island in the last month, the biggest increase since it began taking measurements in 1997.
The land is rising faster than the speed of the elevation observed just before an eruption on the island in September 2001, the institute said.
"It is unclear whether the move will immediately lead to an eruption, but careful observation is needed," it said.
The total elevation has surpassed 20 cm since the land began rising in August, it said.
The island, 1,000 km south of Tokyo, saw an eruption in 2001 for the first time since 1982. There have been 10 other small-scale eruptions since the late 19th century.