Phivolcs explains why Pampanga was hit hardest by Luzon quake

Robie de Guzman   •   April 23, 2019   •   19706

MANILA, Philippines – The center of the magnitude 6.1 earthquake that rattled parts of Luzon on Monday was traced in Castillejos, Zambales, but the province of Pampanga recorded the hardest hit.

The Philippine Institute of Volcanology and Seismology (Phivolcs) explained that aside from being a neighbor of Zambales, the province sits on soft sediment and alluvial soil.

Alluvial, in geology, means made up of or found in the materials that are left by the water of rivers, floods among others. An area with softer soil is prone to strong shaking during an earthquake.

Although the temblor was not considered major, Phivolcs said, it was felt by many because it was a bit shallow.

Monday’s earthquake had a depth of 21 kilometers.

“Kaya nagkaroon ng damages sa Pampanga, bukod sa malapit siya sa episentro ng lindol ay malambot yung area na yan. Alluvial area po yung Pampanga kaya malakas yung shaking na mararamdaman,” said Phivolcs Senior Science Research Specialist Melchor Lasala.

(Pampanga recorded heavy damages because aside from being near the quake’s epicenter, the province has soft soil. It is an alluvial area so the shaking was amplified.)

Among the areas that suffered heavy damages were Porac, Angeles and Lubao.

The National Disaster Risk Reduction and Management Council (NDRRMC) on Tuesday reported 16 fatalities in the quake, 15 of which were from Pampanga.

Five people were killed in the collapse of a four-story supermarket in Porac, seven in different barangays in Porac, two in Lubao and one in Angeles.

READ: Luzon quake death toll rises to 16

The earthquake that struck Castillejos, Zambales on Monday was also felt in Metro Manila, parts of Bataan, Tarlac City, Batangas and Cavite.

Phivolcs said it has yet to identify if the recent quake in Luzon originated from an existing active fault in Zambales.

“Yung dalawang fault system na tinitingnan namin yung Iba Fault at East Zambales Fault,” Lasala said.

(We are looking at two fault systems, the Iba and East Zambales fault.)

Lasala added that the movement felt on Monday was called “strike slip”.

“‘Pag strike slip may dalawa tayong slab yung fault—yung left at yung right. Sa behavior, horizontal yung movement niya. ‘Yung isa papunta sa kaliwa yung isa sa papunta sa kanan,” Lasala explained.

Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally, according to the United States Geological Survey (USGS) earthquake glossary.

If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral. 

Phivols also believes that the magnitude 6.1 quake that jolted Zambales will not trigger a movement of the West Valley Fault as it is 100 kilometers away from the epicenter traced in the Castillejos town.

According to a 2004 study, the West Valley Fault runs through the cities of Marikina, Pasig and Muntinlupa. It is capable of generating a magnitude 7.2 quake that could kill up to 34,000 people and injure 100,000 others. — Robie de Guzman (with details from Rey Pelayo)

383 volcanic earthquakes recorded in Taal Volcano in past 24 hours

Aileen Cerrudo   •   April 14, 2021

MANILA, Philippines— The Philippine Institute of Volcanology and Seismology (Phivolcs) on Wednesday maintained Alert level 2 over Taal Volcano as it recorded 383 volcanic earthquakes in the past 24 hours.

Based on the latest bulletin, upwelling of hot volcanic fluids in the main crater lake occurred Tuesday morning (April 13) and 383 volcanic earthquakes, including 238 episodes of volcanic tremor lasting from one to 12 minutes occurred in the past 24 hours.

Phivolcs also said it continues to observe “a very slow and steady inflation and expansion of the Taal region that began after the January 2020 eruption.”

The agency reminds local government units around the area to be vigilant and continuously assess the situation. Civil aviation authorities must advise pilots to avoid flying close to the volcano.

“DOST-PHIVOLCS is closely monitoring Taal Volcano’s activity and any new significant development will be immediately communicated to all stakeholders,” the bulletin said. AAC

PHIVOLCS: 270 quakes recorded at Taal Volcano in past 24 hours

Robie de Guzman   •   March 26, 2021

MANILA, Philippines — A total of 270 quakes, including 193 episodes of volcanic tremor having durations of up to 12 minutes, were recorded in Taal Volcano in the past 24 hours, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) said Friday.

In its latest bulletin, PHIVOLCS said it also monitored 77 low-frequency volcanic earthquakes, which is higher than the 25 recorded on the previous day.

The agency said it also observed weak emission of steam-laden plumes from fumarolic vents that rose up to 10 meters. Sulfur dioxide emission that averaged 925 tons per day was measured on Thursday.

“Ground deformation parameters from electronic tilt, continuous GPS and InSAR data analysis indicated a very slow and steady inflation and expansion of the Taal region since after the January 2020 eruption,” PHIVOLCS said.

“These parameters may indicate increased magmatic activity at shallow depths beneath the edifice,” it added.

Despite increased seismic activities, PHIVOLCS said that Taal Volcano remains under alert level 2 which means “increased unrest.”

The agency noted that the unrest has been “elevating” and that it is under constant evaluation.

PHIVOLCS reminds the public that under alert level 2, “sudden steam-driven or phreatic explosions, volcanic earthquakes, minor ashfall, and lethal accumulations or expulsions of volcanic gas can occur and threaten areas within and around Taal Volcano Island.”

Entry into the island and its permanent danger zone, especially in the vicinities of the main crater and the Daang Kastila fissure, must be strictly prohibited, the agency stressed.

Civil aviation authorities are also advised to inform pilots to avoid flying close to the volcano as airborne ash and ballistic fragments from sudden explosions and wind-remobilized ash may pose hazards to aircraft.

Phivolcs maintains Alert Level 2 in Taal as possibility of magmatic eruption increases

Aileen Cerrudo   •   March 24, 2021

MANILA, Philippines — The Philippine Institute of Volcanology and Seismology (Phivolcs) on Wednesday (March 24) warned about an increasing possibility of a magmatic eruption in Taal Volcano following an elevated increase in seismic activity.

In an advisory, Phivolcs said that since the Taal Volcano unrest in February, the overall volcanic activity has increased. The agency has recorded 259 volcanic earthquakes in Taal Volcano in the past 24 hours.

In the previous week, sulfur oxide gas emission also reached a high peak compared to the previous month. 

There has also been an indication of magma migrating across shallow depths of the Taal Volcano island which increased the possibilities of a magmatic eruption, according to Phivolcs.

“Alert Level 2 (increased unrest) is currently maintained over Taal Volcano but that unrest has been elevating and is under constant evaluation,” according to Phivolcs.

The agency maintains an Alert Level 2 in Taal. It also reminds local government units to continuously assess and strengthen their preparation. Entry is prohibited on the Taal Volcano Island, Taal’s permanent danger zone, especially the vicinity of the main crater and the Daang Kastila fissure. -AAC

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