Low-frequency harmonic quakes rocked Mount Mayon accompanied by a sudden drop in its steam emission caused by a partial "blockade" of fresh magma at a portion of the volcano's conduit, indicating a high probability of an explosive eruption, scientists said Thursday.

According to the Philippine Institute of Volcanology and Seismology, seismic instruments detected 51 low frequency quakes compared to the 16 quakes recorded on Wednesday. PHIVOLCS said the tremors were brought by the active degassing process by the volcano for the past days, indicating that "viscous" magma is ascending toward the crater summit.

Instruments also recorded 343 tremors caused by lava tumbling down the volcano's slopes.

PHIVOLCS resident volcanologist Ed Laguerta, said the rate of sulfuric steam disgorged by the volcano continues to fluctuate from high to low levels, signifying an active degassing is taking place as a new batch of magma enters the volcano's vent.

"The presence of low-frequency volcanic earthquakes triggered by the active degassing and the partial "blockade" of viscous magma would either lead to a continued lava flow or an explosive eruption."

The PHIVOLCS advisory said the drop in the sulfur dioxide (SO2) on Thursday at 4,760 tons a day compared to the 12,548 rate the previous day still shows active degassing.

"The sudden drop in gas emission due to the ‘blockade’ in Mayon's conduit cause a significant slowdown of degassing magma at the summit," Laguerta said.

He added the fast and slow degassing rate suggests that the volcano is building up pressure that may eventually lead to an explosion.

Lava oozing from the volcano crater since its mild eruption last July 14, had accumulated to about 18 million cubic meters. The bulk of the lava was first funneled into the Mabinit channel south-southeast of the crater while the subsequent flows have entered the gullies at the southeast upper and middle slopes from Miisi channel in Daraga to Buyoan channel.

The lava front that is as big as a four-story building is reaching the six-km Permanent Danger Zone (PDZ) and destroying hectares of coconut plantation and other farm lots in Barangay Mabinit.

Farmers in the village have cut hundreds of coconut trees planted along the pathways of the lava so that they can make use of it.

Alert Level 3 is still up, meaning sudden explosion and lava avalanche may occur anytime.