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Earthquake faults and cracks
« on: November 14, 2019, 09:40:06 AM »
Written by Germelina Lacorte (Facebook)

The story I wrote yesterday after
an interview with an award winning geologist of Phivolcs who gave me a copy of his paper on the Surigao fault which ruptured in 2017 to convince me that these cracks aren't what laymen think. I post this here because it's always been so heartwarming to interview such an intelligent person, always so willing to answer questions and explain. Also, I'm saying this because scientists and experts are always wary of journalists who never take the time to understand what they're doing because of the pressing deadline; and those who are only after the soundbites.

Geologist explains large cracks are not faults

KIDAPAWAN CITY—Boulders as big as a hut came rolling down the mountain, loosened by the quake that rattled the villages along mountain range bordering the Cotabato and Davao del Sur provinces. But what scared villagers the most was the appearance of large cracks—at times two to three feet wide and stretching kilometers long—on the ground after the series of magnitude 6 earthquakes that jolted Cotabato province and left a trail of destruction in neighboring Davao del Sur province and other areas in Mindanao.
Carlito Onor Jr., 33, a rubber tapper in Luayon village, Makilala town of Cotabato, said they left their village for fear that they would be trapped inside by landslides as large cracks began to appear on the ground and stretched all the way to the village of Santo Niño about four kilometers away. “We could have stayed but our place has turned dangerous,” Onor, whose whole family was among the 600 villagers from Luayon taking shelter at the Malasila Elementary School in Makilala town after the quake.
In Davao del Sur’s Magsaysay town, villagers of sitio Labidangan, Upper Bala mistook these cracks as the earthquake fault itself. “The fault passed through the concrete flooring of my house, traveled through the yard where it sliced open the grave of my child and traversed the entire village,” Amando Attic, a resident, told the Inquirer in Cebuano.
But geologist Jeoffrey Perez, supervising science research specialist of the Philippine Institute of Volcanology and Seismology (Phivolcs), said there was no fault that ruptured on the surface during the series of quakes that happened along the Cotabato fault system in October this year. Surface rupture is the term that geologists use to describe when movement on a fault deep within the earth breaks through to the surface.
Perez said the big cracks that people saw were tension cracks brought about by the earth’s shaking.
“For this earthquake, we did not see a fault that ruptured on the surface, most of these cracks are tension cracks if it’s on the mountain range, or lateral spread or liquefaction—if it’s on a flat ground near the water or swampy area,” said Perez, referring to the cracks found in the upland areas of Makilala, Kidapawan City, Tulunan in Cotabato province; and in Magsaysay, Davao del Sur that people mistook as fault.
“But what’s dangerous about these cracks, they might finally give way when another earthquake strikes or when there’s heavy rain, they might cause a landslide,” he explained.
He said a team of scientists from Phivolcs and the Mines and Geosciences Bureau (MGB) had already assessed quite a number of areas where there were tension cracks on the ground and in cases, where there were potential threats, had recommended to the local government to order forced evacuation of residents. Among the towns that issued forcible evacuation after the quake in identified landslide sites were Magsaysay in Davao del Sur; Makilala, Tulunan and Kidapawan City in Cotabato. The areas declared as no-build zone were also places where the large cracks were found.
Perez explained that the series of quakes that rocked Cotabato province and affected large area in Mindanao last month occurred within the Cotabato fault system, caused by the movement of two local faults; namely, the Mlang and Makilala faults.
The Hazard Hunter Philippines, an app that plots the faults in the country and can be downloaded online, shows five faults within the Cotabato fault system; which include the Mlang fault, stretching about 20 kilometers from Pulang Lupa village of Mlang down to Banayal village of Tulunan town; the Makilala fault, stretching less than 20 km from Kawayanon village of Makilala down to the area near Paraiso village in Tulunan town; and the Makilala to Malungon fault, which extends 65 km from Makilala to Sarangani province; the North Columbio fault and the South Columbio fault.
Perez said they were closely watching the Makilala-Malungon Fault as this fault, which is the longest in the Cotabato fault system, had the potential to generate some 7.2 magnitude quake.
He said the 6.3 magnitude quake on October 16 and the 6.6 magnitude quake on October 29, both occurred within the Mlang fault; while the third quake, the 6.5 magnitude quake on October 31, occurred within the Makilala fault.
He said the October 31 quake could not be considered an aftershock of the previous quake because it occurred within the Makilala fault, which is a separate fault.
He also said the October 29 quake was bigger in magnitude than the October 16 quake that preceded it because the latter was the foreshock of the October 29 quake; and that, he saw nothing boggling about it.
Perez, who had been in the area for weeks to observe the effects of the quake and to help explain to affected residents the phenomenon of the earth shaking, said Makilala town appeared to be most affected because the fault that moved during the last earthquake was close to it and the town had the most number of landslide-prone mountainous communities, which was also factor.
He also said that in cities and urban centers, no building should have collapsed below an 8 magnitude earthquake, if builders both private and public, had strictly adhered to the National Building Code.
Following the first quake on October 16, Magsaysay town mayor Arthur Davin ordered the forced evacuation of people in the landslide-prone sitios of Labidangan and Kabuhuan in Upper Bala and in another landslide prone village of Tagaytay. In Makilala, people trapped in Luayon village because of landslides were earlier rescued by helicopter. In Kidapawan City, some portion of Ilomavis, Balabag and Perez villages were declared no-build zone because of threats of landslides, according to Psalmer Bernalte, Kidapawan City disaster risk reduction and management officer.
At least 11 people have been reported missing because of landslides, three of them in Magsaysay town and eight in Makilala. In Magsaysay town’s Upper Bala village two people who fixed the water system that bogged down after the first quake got buried by a landslide triggered by the second quake.
“Kahit na walang lindol, pag tinitingnan natin ang (Even without an earthquake, if we check the) geohazard map, these areas are landslide prone and mountainous areas and slope has been a big factor,” Perez said.
Below is a crack found on the road to Perez village, Kidapawan City. PHOTO FROM PSALMER BERNALTE, Chief of Kidapawan CDRRMC.

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