By Keith Bacongco DAVAO CITY - The Philippine Institute of Volcanology and Seismology (Phivolcs) has yet to determine the exact fault line that triggered the magnitude-6.9 earthquake in Davao del Sur last Sunday. (GOOGLE IMAGE / MANILA BULLETIN) Phivolcs chief Dr. Renato Solidum said that while the epicenter of the earthquake was located near the Tangbulan Fault, which runs across the coastal towns of Davao del Sur, their team was still conducting ground assessment to find out which fault triggered the quake. “We have a team now conducting ground assessment if it was the Tangbulan Fault or the Makilala-Malungon fault that triggered the recent earthquake,” he told The Manila Bulletin in a phone interview. But he did not discount the possibility that the series of earthquakes that have hit the region since October were related. He said these could have been a stress transfer since these are active faults. Solidum also admitted that, with the latest earthquake, it was the first time in recent history that a succession of above-magnitude 6 earthquakes occurred within a short time of each other, and in close proximity to each other. While most of the attention was focused on Padada after the destruction of commercial establishments left three persons killed, the Phivolcs briefer noted that the epicenter of the earthquake on Sunday was in the neighboring town of Matanao. Matanao is a neighboring municipality of Magsaysay, one of the hardest-hit towns during the series of tremors last October. On October 16, a magnitude-6.4 earthquake hit 22 kilometers southeast of Tulunan, North Cotabato, rocked neighboring provinces, with its intensity also felt as far as Dipolog City in Zamboanga Del Norte, some 400 kilometers away. The tremor left a trail of destruction in North Cotabato, as well as in the towns Bansalan, Kiblawan Magsaysay and Matanao, all in Davao del Sur. Thirteen days later, a magnitude-6.6 tremor at 9:04 am, with epicenter in almost the same area in Tulunan tremor, jolted the region. Like the October 16 earthquake, aftershocks of weaker intensity also rippled in some parts of Zamboanga region. Less than two hours later, a magnitude-6.1 aftershock hit. Then on October 31, a magnitude-6.5 earthquake hit with the epicenter recorded in Makilala, North Cotabato. This triggered massive landslides in the mountainous villages leaving at least eight persons missing to date. According to the Phivolcs, the series of powerful temblors last October were triggered by the movement of Makilala-Malungon fault that cuts across the provinces Cotabato, Davao del Sur and Sarangani. The briefer also added that the Davao region is one of the seismically active regions in the country because of the presence of several active faults that include the Tangbulan Fault, Central Digos Fault and Cotabato Fault System (CFS): Makilala-Malungon, Makilala, M’lang, North Columbio, South Columbio and Balabag. Based on the seismicity map released by the Phivolcs on November 8, the epicenters of the four previous powerful earthquakes apparently sat just within a 25-kilometer radius (aerial distance). Recorded aftershocks pepper around the three previous epicenters, covering the entire topographic map of the towns of Makilala, M’lang, Tulunan and the towns of Datu Paglas, Gen. Salipada K. Pendatun and Pandag in Maguindanao province.