Bella Tubianosa, science specialist of the Philippine Institute of Volcanology and Seismology (Phivolcs), said the harmonic tremor occurred at 10:29 p.m. Wednesday and lasted for about 40 seconds.
Tubianosa said harmonic tremors are indicators of magma movement " which may lead to more explosions."
The volume of sulfur dioxide spewing from the volcano’s crater and western fissures also increased to 1,144 tons per day compared to 597 tons per day the other day.
Meanwhile, the Office of Civil Defense said only 76 families remain at evacuation centers in Casiguran and 40 others in Irosin.