Phivolcs explains why Pampanga was hit hardest by Luzon quake

Robie de Guzman   •   April 23, 2019   •   16114

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)

PHIVOLCS may further downgrade alert level over Taal Volcano next week

Marje Pelayo   •   February 21, 2020

MANILA, Philippines – The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported only about 29 volcanic earthquakes around Taal Volcano in the past 24 hours, an indication that rocks underground are breaking.

A column of white steam also continues to form but only up to 50 meters in height.

It has been a week since PHIVOLCS downgraded alert level over the Taal volcano to level 2 from the previous level 3.

According to PHIVOLCS OIC Usec. Renato Solidum, they need to observe Taal volcano’s activities for one more week before they can decide whether to further downgrade the alert status based on the agency’s standard operating procedure.

“Basta tuloy-tuloy ang kanyang mga parametro na bumababa ang bilang pwede natin masabi na pwede nating ibaba ito sa alert level number one (If the parameters continue to reduce in number, we might lower the alert level to number one),” Solidum said.

Entry restrictions remain within the permanent danger zone in Taal Volcano Island.

Solidum said the local government unit needs to assess first if the former residential areas would be conducive for living.

PHIVOLCS does not recommended constructing houses on the volcano island due to the risk of having the ground break apart should the volcano proceed to erupt. MNP (with details from Rey Pelayo)

Magnitude 5.4 quake rattles Davao Occidental

Robie de Guzman   •   February 20, 2020

MANILA, Philippines – A magnitude 5.4 earthquake rattled parts of Davao Occidental on Thursday afternoon, the Philippine Institute of Volcanology and Seismology (Phivolcs) reported.

In its bulletin, Phivolcs said the epicenter of the quake that struck around 5:54 p.m. was traced 35 kilometers southeast of Jose Abad Santos in Davao Occidental.

It had a depth of 190 kilometers and tectonic in origin.

The tremor was felt at intensity III in General Santos City, Tupi and Polomolok in South Cotabato, and Alabel, Saranggani; intensity II in Maasin and Malapatan, Sarangani, and Davao City, while intensity I was felt in Tampakan, South Cotabato.

Phivolcs said there was no reported damage but aftershocks should be expected.

Taal Volcano status lowered to Alert level 2 – Phivolcs

Robie de Guzman   •   February 14, 2020

Aerial photos Taal Volcano eruption aftermath epa08135676 Handout aerial photo provided by the Office of Civil Defense shows the Taal Volcano crater in Batangas province, south of Manila, Philippines, 17 January 2020. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) has kept the alert level at four, following the volcano’s eruption on 12 January 2020. EPA-EFE/OFFICE OF CIVIL DEFENSE HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES

MANILA, Philippines – The Philippine Institute of Volcanology and Seismology (Phivolcs) on Friday downgraded Taal Volcano’s status from alert level 3 to alert level 2 following indications of decreased unrest in the recent weeks.

In its bulletin, Phivolcs said it lowered the alert level status of Taal Volcano after three weeks due to less frequent volcanic activity, stabilizing ground deformation of the Taal Caldera and Taal Volcano Island edifices, and weak steam or gas emissions at the Main crater.

“DOST-PHIVOLCS is lowering the alert status of Taal Volcano from Alert Level 3 to Alert Level 2 to reflect the overall decreasing trend in the level of monitoring parameters,” it said.

Alert Level 2 means there is decreased unrest, but State volcanologists said this should not be interpreted that unrest has ceased or that the threat of an eruption has disappeared.

The agency also reminds the public that at alert level 2, sudden steam-driven or phreatic explosions, volcanic earthquakes, ashfall and lethal accumulations or expulsions of volcanic gas can occur and threaten areas within the volcano island and its coast.

It also advised that entry to the Taal Volcano Island – a permanent danger zone – shall remain prohibited.

“Local government units are advised to additionally assess previously evacuated areas within the seven-kilometer radius for damages and road accessibilities and to strengthen preparedness, contingency and communication measures in case of renewed unrest,” Phivolcs said.

“People are also advised to observe precautions due to ground displacement across fissures, frequent ashfall, and minor earthquakes. Communities beside active river channels particularly where ash from the main eruption phase has been thickly deposited should increase vigilance when there is heavy and prolonged rainfall since the ash can be washed away and form lahars along the channels,” it added.

Civil aviation authorities are also advised to direct 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 said alert level 3 may be raised again should an uptrend or pronounced change in monitored parameters forewarn a potential eruption.

On January 12, Taal Volcano spewed kilometers-high ash plumes which prompted state volcanologists to raise its status to alert level 4. Thousands of residents within the 14-kilometer radius from the main crater were ordered to flee their homes due to a possible hazardous eruption.

Two weeks later, Phivolcs downgraded Taal’s status to alert level 3, which allowed displaced residents outside the seven-kilometer danger zone to return to their homes.

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