Wall of rock rises out of the ground in the Philippines: new fault triggered large quake

 

Residents stand next to a long earthen wall which formed following the Oct. 15 earthquake in Barangay Anonang in Inabanga, Bohol. Phivolcs scientists said the wall is the face of a previously unknown fault line which caused the magnitude 7.2 quake

October 24, 2013 – PHILIPPINES – As the magnitude 7.2 earthquake ended on Oct. 15, residents of Sitio Kumayot in Barangay Anonang heard an explosive sound like a thunderclap. Villagers watched in horrified disbelief as the ground cracked open and, with smoke and the stench of sulphur spreading, one side started to rise. The emerging wall of rock and earth missed by a hairline the toilet of baker Menecia Bautista Aparecio, 43. “We will be living forever in fear, being so close to the fault line,” said Aparecio, who fears returning to her home and now bakes her “pan Bisaya” or “pan kinamot,” a local bread, in the village chapel. The rock face, about three meters high and two kilometers long, raised fears among villagers that more cracks would appear on the ground and swallow them up. Scientists, who may declare a 300-meter permanent danger zone around the fault, described the appearance of the ground rupture as a “eureka” moment in their search for what they have long suspected was an active earthquake fault in the area. Government scientists said the appearance of the yet unnamed fault, which does not exist on the country’s map of fault lines, triggered the powerful earthquake in Central Visayas. “We are 100 percent sure that this is the generator (of the earthquake),” Teresito Bacolcol told GMA 7 as he noted that the rock face appeared near the quake’s epicenter at the boundary of Sagbayan and Catigbian towns. “When we saw (the fault), eureka! This is it.” Bacolcol led a team from the Philippine Institute of Volcanology and Seismology (Phivolcs), which inspected the rock face last Monday. “We recommend that no structures should be built on top of a fault and within the five-meter buffer zone on both sides of the fault,” Phivolcs director Renato Solidum told The STAR. He also urged the local government of Bohol to revise its land use policy around the fault. –Philstar

Geologists from the Philippine Institute of Volcanology and Seismology (Phivolcs) found “ground zero” or the fault that triggered the devastating magnitude 7.2 earthquake. The discovery on Monday confirmed the existence of a previously unknown fault in western Bohol. The Oct. 15 earthquake claimed around 200 lives and caused major infrastructure damage, including Spanish-era churches in Bohol and Cebu. Immediately after the earthquake, Phivolcs said the tremor was caused by the movement of the East Bohol Fault. The epicenter, first reported as Carmen town, was later adjusted to the general area between Sagbayan and Catigbian towns in north central Bohol based on the distribution of aftershocks. Geologists found a 5-kilometer surface rupture that extended from barangay Anonang in Inabanga town to barangay New Anonang in Buenavista town. “That fault motion caused the earthquake,” Renato Solidum, director of Phivolcs told ABS-CBN News Channel’s Talkback yesterday morning. The ground rupture pushed the ground upward from two to three meters, cutting through mountains, roads and possibly settlement areas. Phivolcs teams are still studying the extent of the fault line which could extend 100 kilometers or longer.

Meanwhile, aftershocks recorded by Phivolcs indicate a general distribution from north Bohol to the offshores of Dalaguete, Alcoy and Boljoon towns in southern Cebu. The data was plotted on a map by Jessie Alaivar Floren, a private Geographic Informatino System expert in Cebu. Aside from structural damage, residents in southern Cebu towns of Alegria and Badian and Carcar City have complained that ground water turned to “milo” brown. “The earthquake may have disturbed the clay-limestone land formations in these areas, affecting their water table and underground water channels,” said Jun Lucero, senior geologist of the DENR Mines and Geosciences Bureau (MGB). Lucero told Cebu Daily News that they are now checking reports of possible liquefaction in several areas in Cebu and Bohol. Severe liquefaction could result in the sinking or submersion of lands, he  said. Areas near the river and those reclaimed from the sea are more prone to liquefaction, Lucero said. Structures in reclaimed areas, he said, should have stronger foundations and must use materials that can withstand strong ground shaking and liquefaction.  –Inquirer