Quake ‘swarm’ hits Mindanao
Credit to Author: DIVINA NOVA JOY DELA CRUZ| Date: Thu, 31 Oct 2019 16:18:13 +0000
THE series of tremors that struck parts of Mindanao, the latest of which was the magnitude 6.5 on Thursday, was part of an earthquake “swarm,” which the Philippine Institute of Volcanology and Seismology (Phivolcs) said might last until Christmas.
Phivolcs Deputy Director Bart Bautista on Thursday defined an earthquake swarm as a “series of quakes that were constantly on the same and moderate magnitude levels,” similar to what happened in Tulunan, North Cotabato.
Bautista said a usual tremor registering as a main shock would be followed by smaller magnitudes of aftershocks that diminished in frequency over time.
The major events that Phivolcs recorded in Mindanao were the magnitude 5.6 on July 9, magnitudes 5.5 and 6.3 on October 16, magnitudes 6.1 and 6.6 on October 29 and magnitude 6.5 on October 31.
“Nung [October] 16, hindi pa natin na-establish ‘yung pattern. Ang iniisip namin noon, range of aftershock sequence siya (On October 16, we haven’t established the pattern yet.
We were thinking it was only a range of aftershock sequence),” Bautista said, adding that the succeeding quakes showed a pattern of swarms.
“Ito, kakaiba siya kasi series of moderate na sunod-sunod, medyo magkakalayo ‘yung agwat This is different because it is a series of moderate and consecutive events, occurring with slightly long intervals,” he added.
In a separate interview, Phivolcs Seismology Officer in Charge Ishmael Narag said typical aftershocks were usually 1.2 magnitude lower than the main shock.
The consecutive quakes in the area, however, were observed to be of the same magnitude strength from magnitude 5.5 to 6.6.
Bautista said swarms were caused by stored energy in moving faults, which resulted in ground shaking.
“‘Yung strain energy, nai-store ‘yan dahil patuloy na nako-compress yung Philippine archipelago ng dalawang malalaking tectonic plates na nagbabanggaan, ‘yun ang Eurasian Plate at Philippine Sea plate.
So naiipit tayo ng dalawang plate na ‘yan at nagkakanda-basag-basag yung bato and then nagde-develop yung maraming active faults (Strain energy is stored because the Philippine archipelago is continously compressed by two colliding large tectonic plates — the Eurasian Plate and the Philippine Sea Plate. The compression of these two plates causes breaking of rocks and then many active faults develop),” he explained.
Through time, according to Bautista, the compression force of the plates would store strain energy because of the deformation of rocks.
He said the swarm, with moderately strong quakes, could possibly last for two to three weeks if more events would transpire.
“Ang [mangyayari] d’yan, mauubos lahat ‘yung strain energy na naka-store ‘dun sa lugar and then pagkamedyo nag-stabilize na, titigil na ‘yung activity (The activity will stop when the stored strain energy in the area has been completely released and is stabilized),” he said.
A good sign
Bautista said the swarm was actually a good sign as quakes release moderate ground-shaking as opposed to a single blow of higher magnitude single event that would be more destructive.
“Bale unti-unti niyan nire-release, which is good, kasi pag unti-unting magre-release maliliit lang, hindi gaanong destructive ‘yung earthquake, pinaka-[expected lang] moderate-sized events, kaysa sa magkaroon ka ng [magnitude] 7.5, 7.8 kagaya ng nangyari sa Central Luzon noong 1990. ‘Yun, isang beses lang niya ni-release kaya napakalaki (The energy is slowly released, which is good because gradual release would be smaller and not much destructive, only moderate-sized events are expected, rather than a magnitude 7.5 or 7.8 like what happened in Central Luzon in 1990. That was a single event so the energy released was immense),” he added.
Bautista said the earthquake swarm was concentrated and would not directly affect other faults in the country.
He, however, added that earthquakes in other parts of the country were always possible as there were many active faults across the Philippines.
5 dead
Five people died in Thursday’s quake, according to a report by the Office of the Governor of North Cotabato relayed by Lt. Gen. Felimon Santos Jr., commander of the military’s Eastern Mindanao Command.
The fatalities were identified as Cesar Bangot, a barangay (village) chairman of Batasan in the town of Makilala, who was killed when the village hall where he held office collapsed; Juve Gabriel Jauod of Buena Vida village, 7, who was pinned to death by hollow blocks; Romel Galicia and Tessie Alcaide, who died in a landslide in the village of Luayon also in the town of Makilala; and Precilla Verona, 70, whose cause of death was not given.
The Eva Hotel in Kidapawan City is on the brink of collapse, Mayor Joseph Evangelista said in a DZMM radio interview and assured families that its personnel, although injured, were safe.
In Davao City, Mayor Sara Duterte-Carpio said nine unidentified residents of the Ecoland 4000 condominium were injured when the building collapsed.
Of the nine, eight were treated on site while the other was brought to a nearby hospital, Duterte-Carpio said in a live news conference in Davao City.
She also said the Bankerohan and Bolton bridges were under inspection by the Department of Public Works and Highways (DPWH) and the City Engineer’s Office after they were found to have been damaged.
The Bolton Bridge is now limited to vehicles weighing 20 tons or less, the mayor added.
Govt response
President Rodrigo Duterte ordered “immediate” relief to the quake-affected in Mindanao.
Palace spokesman Salvador Panelo assured the public that the government, through the National Disaster Risk Reduction and Management Council and the Office of Civil Defense, was “on top of the situation.”
“The President had directed all agencies and responsible local government units to provide the necessary assistance and immediate relief to those affected,” Panelo said in a statement.
But he added that Duterte would not be able to visit the quake-hit areas “as there were ongoing operations by the local government units.”
“The President, however, deems it appropriate to refrain from personally inspecting the disaster-stricken areas and directing actions in response to the catastrophe as there are already ongoing operations by the responsible local government units, and they have so far effectively responded to the current critical situation,” Panelo said in another statement.
In a separate statement, Presidential Communications Secretary Martin Andanar said the Department of Energy was directed to check power interruptions while the DPWH was instructed to determine whether roads or highways were passable.
“He has directed the Department of Labor and Employment to report job displacements of workers in affected areas and provide necessary assistance,” Andanar added.
“The President has tasked the Department of Social Welfare and Development and the Department of Health to be in charge of relief and medical assistance,” he said.
Labor Secretary Silvestre Bello 3rd said he has dispatched labor officials and quick response teams to Mindanao to assess the extent of destruction in workplaces and possible displacement of workers in the affected areas.
He added that labor inspectors were deployed to check the compliance of establishments with occupational safety and health standards and ensure the safety of the workers.
“I have sent Undersecretary Ana Dione and Assistant Secretary Benjo Santos Benavidez to proceed to General Santos City, and Davao City to lead our regional [Labor] officials in the assessment of the situation,” Bello said.
He added he would fly to Davao City on Sunday to personally see the extent of the damage and its effect on employment in the affected areas.
Andanar said the Presidential Communications Operations Office was activating “Laging Handa: Earthquake Update” to inform the public of the government’s response and appointed Assistant Secretary Joseph Lawrence Garcia as overall head of the Laging Handa operations in Davao City.
“We advise the public to take note of advisories and announcements only from legitimate sources from the national and local government to avoid panic,” he added.
Sen. Christopher Lawrence “Bong” Go said government agencies should be strict in implementing relevant laws, particularly the National Building Code, which he planned to amend to ensure public safety during natural disasters.
The National Building Code was promulgated in 1977 when then-president Ferdinand Marcos issued Presidential Decree 1096.
Given the frequency of calamities and natural disasters in the country, Go said he was also pushing for the establishment of an “empowered, highly specialized and responsive” Department of Disaster Resilience under Senate Bill 205 or the “Disaster Resilience Act of 2019.”
The Philippine National Police (PNP) ordered all its regional offices in Mindanao to mobilize its personnel and units after a 6.5 magnitude quake hit the island.
Lt. Gen. Archie Francisco Gamboa, PNP officer in charge, gave the order in response to calls for help by affected local government units.
Full accounting
House of Representatives Deputy Majority Leader Bernadette Herrera said it was about time that the National Economic and Development Authority and the Department of Finance fully account for the $500-million loan Disaster Risk Management Development Policy Loan.
Herrera said the loan was approved in 2015.
“I am aware that our country, through our Finance department, is applying for a new $500-million loan again from the World Bank. The DoF should first report to Congress what they did with the 2015 loan. Policy loans usually run for several years,” the Bagong Henerasyon party-list lawmaker added.
Agusan del Norte Rep. Lawrence Fortun said the House Committee on Trade and Industry should conduct an inquiry in aid of legislation into the alleged continuous smuggling and proliferation of sub-standard steel products in the country that are abetted by large steelmakers and officials of the Department of Trade and Industry and the Bureau of Customs.
“This seeming grand deception may have led to the rise in the past 10 to 12 years of structurally-compromised infrastructures that may not be able to withstand high intensity quakes, thus endangering the lives of millions of Filipinos,” he added.
Until Christmas
The Phivolcs’ Narag said the quakes might last until Christmas as the faults in the area were “highly stressed.”
“We’re not dealing with one fault system na gumagalaw (moving), we’re dealing with a series of faults that are actually moving,” he added.
Narag said Phivolcs was looking at two scenarios: First, same magnitude range quakes may be generated to the north or to the south of the previous tremors or second, the longest fault in the area, Makilala-Malungon Fault, could possibly be activated and could generate up to a magnitude 7.2 quake.
Bautista said stress-loading of the Makilala-Malungon Fault is still possible but has less probability, based on its proximity to the recent events.
“Sa ngayon, hindi pa natin madi-discount na may possibility siya. May possibility siya, pero maliit (At the moment, we do not discount this possibility, but only low possibility)” he added.
“Sa ngayon, since medyo malayo siya, wala tayong nakikitang activity doon sa segment ng fault na ‘yan, so it’s possible na mababa ang probability. May chances pa rin na gagalaw, pero mababa (At the moment, since it is slightly far, we do not see any activity in that fault segment, so it’s possible but has low probability. There are chances that it may move, but low chances),” Bautista said.
Narag said if the tremors started to wane, aftershocks would be “felt” by people until December.
Safety measures
As more quakes were expected, Phivolcs advised the public to remain vigilant and prepared.
Bautista said immediate inspection of buildings by engineers was necessary to ensure that they were still safe for occupancy especially that previous tremors had weakened the structures.
Narag reminded the public to stay out and stay away from damaged buildings that could collapse, watch out for falling hazards or debris and determine safe areas in houses and buildings where they could assume the “duck, cover and hold position during an earthquake.
In mountainous areas, the public is also advised to stay clear fof slopes with tension cracks that could trigger a landslide from ground movement or heavy rainfall.
Thai trip
Panelo said the President would push through with his trip to Thailand on Friday to join other Southeast Asian leaders and dialogue partners for the Association of Southeast Asian Nations (Asean) summit this week.
“In this connection, the President shall push through with his trip to Bangkok, Thailand tomorrow (Saturday) afternoon to participate in the 35th Asean Summit and Related Summits,” he added.
WITH REPORTS FROM DEMPSEY REYES, CATHERINE S. VALENTE, WILLIAM B. DEPASUPIL, GLEE JALEA AND DARWIN PESCO