Phivolcs explains why Pampanga was hit hardest by Luzon quake

Robie de Guzman   •   April 23, 2019   •   13211

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 downgrades Quezon town quake to magnitude 5.3

Robie de Guzman   •   September 13, 2019

MANILA, Philippines – The Philippine Institute of Volcanology and Seismology (Phivolcs) has downgraded the magnitude of an earthquake that hit Burdeos town in Quezon Province to magnitude 5.3 on Friday afternoon.

Phivolcs originally placed the magnitude at 5.5 before eventually revising it to 5.3.

Based on the latest earthquake information posted on Phivolcs website, the tectonic quake struck at 4:28 p.m. with a depth of 62 kilometers. Its epicenter was traced 39 kilometers northeast of Burdeos.

Phivolcs recorded varying quake intensities felt in the following areas:

  • Intensity 5 – Burdeos and Infanta, Quezon; Los Baños and Pakil, Laguna
  • Intensity 4 – Jose Panganiban, Camarines Norte; Quezon City; Marikina City; Sta. Cruz, Laguna; Alabat and Gen. Nakar, Quezon
  • Intensity 3 – Guinayangan, Quezon; Muntinlupa City; Manila City; San Jose Del Monte, Bulacan; Bacoor, Cavite; Maddela, Quirino; San Mateo, Rizal; Baler, Aurora; Lucena City
  • Intensity 2 – Dingalan, Aurora; Mandaluyong City; Apalit, Pampanga; Taguig City; Lucban, Quezon
  • Intensity 1 – San Isidro, Nueva Ecija; Gapan City; Meycauayan, Bulacan

Instrumental Intensity were recorded in the following areas:

  • Intensity 5 – Polillo, Quezon
  • Intensity 4 – Jose Panganiban, Camarines Norte; Quezon City; Alabat and Mauban, Quezon
  • Intensity 3 – Guinayangan, Quezon; Marikina City; Baler, Aurora; Tagaytay City; Quezon City; Navotas City
  • Intensity 2 – Mulanay, Lucban and Gumaca, Quezon; Malolos and San Ildefonso, Bulacan; Guagua, Pampanga; Las Pinas City; Pasig City; San Juan City; Caloocan City; Malabon City; Muntinlupa City; Calumpit, Bulacan
  • Intensity 1 – Talisay, Batangas; Olongapo City; San Jose, Nueva Ecija

Phivolcs said that due to the magnitude of this quake, the islands of Polillo, Jamalig, and Burdeos may suffer damages.

Aftershocks are also expected and may be felt in areas near the epicenter.

Based on earthquake information posted on Phivolcs website, aftershocks of varying magnitudes have been recorded: magnitude 2.4 at 5:02 p.m., magnitude 4.7 at 5:18 p.m., magnitude 2.9 at 5:20 pm.; and magnitude 1.5 at 6:27 p.m. – All were felt in Burdeos, Quezon.

The temblor caused the temporary suspension of railway services and classes in several schools in Metro Manila.

PHIVOLCS: Damaged structures in Itbayat may have used porous limestone

Marje Pelayo   •   July 31, 2019

Photo by Dominic de Sagon Asa

MANILA, Philippines – The municipality of Itbayat and other island towns in the province of Batanes in Northern Luzon are landmasses that emerged from the ocean, according to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

The area’s soil composition manifests its origin, according to Science and Technology USec. Renato Solidum.

“Doon sa Itbayat ang makikita mong mas maraming bato doon ay limestone (You notice in Itbayat, most of the stones there are limestone),” the PHIVOLCS Director said.

“Ang limestones ay galing sa corals na namuhay sa ilalim ng tubig. Kaya lang napapaangat ito dahil sa pagkilos ng fault, (Limestones are formed from corals on the sea bed. They surface to the ground when the fault moves),” he added.

In July 27, a series of three moderately strong earthquakes rocked the municipality of Itbayat – magnitudes 5.4, 5.9 and 5.8, respectively.

It was in 2015 when Itbayat experienced its latest intensity 5 tremor.

But after 60 or 70 years, it was only on Saturday (July 27) that intensity 6 was again experienced in the area.

According to Solidum, most of the houses or structures suffered damages because the materials used in their constructions were mostly limestone.

Though limestone-based structures could stand strong tropical cyclones, they couldn’t hold strong against earthquakes.

Also, some structures had no steel frames that supposedly add strength and resilience.

“Dahil ang limestone ay madaling matunaw o naaagnas habang tumatagal dahil sa ulan, nagiging marupok ang limestone, (Limestone is a porous material and easily absorbs liquid when drenched in rainwater. Limestone easily breaks),” Usec. Solidum explained.

Limestone was also traced in old churches that were destroyed during a magnitude 7.2 earthquake that struck the province of Bohol in 2013.

Other structures that were damaged also showed traces of limestone that’s why they were not able to stand the strength of the tremors. – (MNP with reports from Rey Pelayo)

Know possible hazards in your area with web app ‘Hazard Hunter’

Marje Pelayo   •   July 16, 2019

MANILA, Philippines – Want to know potential hazards within your area?

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) on Tuesday (July 16) launched a web application dubbed as ‘Hazard Hunter’ which provides the public with information on the possible dangers that may happen in a specific area like earthquake, flood or even volcanic eruption.

‘Hazard Hunter’ is a web-based application that can be accessed on desktop, laptop or in smart phones.

To access, just load https://hazardhunter.georisk.gov.ph/ on your web browser.

A welcome message will lead you to the Hazard Hunter official page.

You will be asked to proceed until you see the full map of the Philippines.

Click on the menu bar then several options on the type of hazards that may be present in your area will appear – seismic for earthquakes, volcanic for volcanoes and hydro-meteorological for flood and storm-related hazards.

Click on the type of hazard then select an area by pointing the cursor to the place of inquiry on the map.

Double click on the area of choice.

The system will immediately load the results that will flash on the right side of the screen.

For instance, Baseco compound in Manila has a population of about 60,000 people.

Based on the app’s reading, Baseco is about 12 kilometers away from the West Valley Fault.

Residents in Baseco may feel ground shaking of up to intensity 8 once the West Valley Fault moves in an event called the Big One.

The area is prone to liquefaction and could suffer from up to four-meter-high tsunami as it is near Manila Bay.

Based on the app, Baseco is less likely to be affected if ever the nearest active volcano, Mount Taal in Batangas, erupts.

Taal Lake is about 58 kilometers away from Baseco.

Baseco is a flood-prone area and floodwater can go as high as two meters and can take up to three days to subside.

During the onset of a severe tropical cyclone, the area could suffer from storm surge as high as 4 meters.

But there is no possibility of a landslide in the area.

The app also has detailed recommendations or suggestions for a plan of action during a specific hazard.

Science and Technology Undersecretary and PHIVOLCS OIC Renato Solidum believes ‘Hazard Hunter’ will enable the public to prepare and plan ahead of a natural disaster.

Government agencies will also be guided for appropriate actions to be taken to minimize or prevent the loss of lives and damage to properties.

“Hindi naman masama na may hazards diyan. What is important is that the hazard is recognized so that the developer can develop approaches to lessen the impact, to mitigate the possible impact,” Solidum noted.

Solidum said even his own house sits on a location that is hazard-prone but recognizing the dangers enabled him to plan for his and his family’s protection.

“Tinaasan ko yung bahay. Ginawa kong three floors para hindi ako mamatay sa baha (What I did was I elevated my house. I made it into three floors to keep me safe from the flood),” he said.

“Inayos ko yung foundation ng building para sa shaking ng liquefaction. So mga ganung klaseng real life application magagamit (ang Hazard Hunter) (I aligned the foundation of the building in case of shaking during liquefaction. During those real-life applications, [Hazard Hunter] can be useful),” he added.

Before the end of the year, PHIVOLCS plans to launch the mobile version of the website. – with details from Rey Pelayo

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