The Ta'u Island underwater volcano causing earthquakes in American Samoa's Manu'a island group will not produce an eruption like that seen at Hunga Tonga Hunga Ha'apai in January, says a volcanologist.

The earthquakes have been reported in Manu'a since 26 July.

Volcanologist Natalia Deligne, who is in American Samoa, said unlike the volcano which erupted so violently in Tonga, the gas in the Ta'u Island volcano was not under major pressure.

She likened the pressure that had built up at Hunga Tonga Hunga Ha'apai prior to its January blast as being like that inside a can of soda that had been shaken up.

"Whereas here, the gas can escape without being pressurised and so you will not have the same kind of large explosive eruption."

American Samoa's governor, Lemanu Peleti Mauga, has declared a state of emergency for the entire territory as the earthquakes are continuing.