LEGAZPI CITY, Philippines—Mayon Volcano spewed ash anew on Wednesday morning, indicating fresh sign of heightened restiveness that could result in a major eruption, a month and 13 days after its first ash explosion episode last September 15.

The event prompted authorities to issue a new 24-hour alert to residents of communities surrounding the volcano.

This, amid preparations for a possible storm, which the weather bureau has named "Santi" once it enters the Philippine area of responsibility.

"At 5:32 a.m. today, one minor ash explosion occurred at the summit crater of Mayon volcano. This event lasted for about one minute," said resident volcanologist Eduardo Laguerta in a phone interview with the Philippine Daily Inaquirer.

Laguerta said the explosion produced a brownish ash column that rose to a maximum height of about 600 meters above the crater and drifted toward the northeast direction.

For the past 24-hour observation period, the seismic network recorded 13 volcanic earthquakes. Steam emission was at moderate level, creeping downslope toward southwest.

Sulfur dioxide emission rate, however, was measured Monday at only 250 tons, way below the baseline data of 500 tons while crater glow was not observed last night due to poor visibility.

The Philippine Institute of Volcanology and Seismology (Phivolcs) said Mayon blew ash starting past 5 a.m. on Tuesday.

Residents living near the slopes claimed hearing what they said was a "thud" followed by a display of whitish cauliflower-like plume above the crater.

The minor explosion prompted Albay Governor Joey Salceda, chairman of the provincial disaster coordinating council, to direct smaller disaster councils to be on 24-hour high level of alert.

Salceda directed the councils to prepare to evacuate residents if things got worse.

Phivolcs, however, kept alert level 2 in place, which meant more ash explosions were likely.

Phivolcs reiterated a strict off-limits order inside a six-kilometer permanent danger zone (PDZ) and a seven-km extended danger zone (EDZ).

Villagers were also advised to stay away from active river channels and areas tagged as lahar-prone.

Last August, scientists kept watch for a feared collapse of a portion of Mayon's crater, which holds an estimated 200,000 cubic feet of rocks.

"We still cannot raise the alert level to status 3 because we have yet to check if there is a new lava deposit at the crater and the scheduled aerial survey on Tuesday was canceled due to thick clouds hovering around the summit area," Laguerta explained.

But the volcanologist admitted that if in case the interval of ash explosions would draw closer and closer, the institute might recommend raising status to alert level 3, which will mean the provincial government has to evacuate some 30,000 residents living along the seven-kilometer EDZ. "We can't predict yet when it will erupt but the possibility is always there."

Laguerta said the Phivolcs would need to install at least two more seismographs to enhance readings of four units currently at work around the volcano.

Two digital tilt meters were also installed last Oct. 6 to complement the leveling instruments to measure the ground swelling of Mayon.

"The two tilt meters are already installed at the Mayon Resthouse and in Lidong, Sto. Domingo flanks and both are working smoothly to spot any changes in the 'bulging' of the edifice," Laguerta said.

Keeping the status of Mayon Volcano at Alert Level 2 means that a state of unrest, which could lead to more ash explosions or eventually to a hazardous magmatic eruption, is possible anytime but Laguerta said it is very hard to tell when it could happen.

The Phivolcs strongly recommended that the six-km radius PDZ around the volcano and the seven-km EDZ at the southeast flank of the volcano are off-limits due to the threat from sudden explosions and rockfalls from the upper slope.

Active river channels and those areas perennially identified as lahar-prone in the southeast sector should also be avoided, especially during bad weather condition or when there is heavy and prolonged rainfall.

In August, scientists kept watch for a feared collapse of a portion of Mayon's crater.

Mayon, 300 km southeast of Manila and famous for its nearly perfect conical shape, has a height of over 2,462 meters above sea level.

The volcano, said to be the most active in the country, has erupted at least 50 times since 1616. Its worst eruption buried Cagsawa town and killed 1,200 people in 1814. An eruption in 1993 killed more than 70 farmers.

Mayon erupted again in 2006.