The October 2019 Series of Earthquakes in Cotabato and Vicinity
In October 2019, a series of strong shallow earthquakes struck the province of Cotabato and vicinity. These earthquakes occurred on October 16, 7:37 PM Philippine Standard Time (PST) at magnitude 6.3, on October 29, 9:04 AM and 10:42 AM at magnitudes 6.6 and 6.1, respectively, and on October 31, 9:11 AM at magnitude 6.5. The depths of these earthquakes range from 7 to 9 km. Initial information on the hypocenters of the earthquakes and their focal mechanism solutions suggest that they are generated by the Cotabato Fault System, a system of strike-slip faults transecting the provinces of Cotabato, Davao del Sur, Maguindanao, Sarangani, South Cotabato, and Sultan Kudarat. Due to the quick succession of these events, damages to structures were severe and widespread. Geologic impacts, especially landslides, were also numerous.
The DOST-PHIVOLCS Quick Response Team (QRT) was immediately deployed on 18 October 2019 and 30 October 2019 to: 1) conduct earthquake information dissemination and education for local government units (LGUs) and locals in the affected areas; 2) conduct field surveys to determine the earthquakes’ geologic impacts and effects to structures; 3) deploy temporary seismic stations for continuous aftershocks monitoring; and 4) assist the LGUs, together with the Mines and Geosciences Bureau, in assessing selected evacuation/relocation sites in terms of earthquake hazards. Based on field investigations, the geologic impacts included numerous tension cracks and earthquake-induced landslides, especially near the epicentral region in Makilala, Kidapawan City, and Tulunan in Cotabato; and in Magsaysay and Bansalan in Davao del Sur. Liquefaction was manifested by lateral spreading, sandboils, sandblows, road buckling, ground undulation, and subsidence in low-lying and water-saturated areas of M’lang, Kabacan, Pikit, Tulunan, Makilala, and Magpet in Cotabato; Hagonoy, Magsaysay, Matanao, Digos City, Malalag, and Padada in Davao del Sur; and Pagagawan, Maguindanao. Moreover, locations of landslides and severe damage to structures after the October 31 M6.5 earthquake follow a northeast-trend and are concentrated on the central portion of Makilala and eastern portion of Kidapawan City,
Cotabato where, based on the PHIVOLCS Earthquake Intensity Scale (PEIS), Intensity VIII (Very Destructive) was observed. Other damage to structures and other geologic impacts (e.g. liquefaction and tension cracks) were observed in areas where the intensity was either PEIS VII (Destructive) or PEIS VI (Strong). Poor engineering and the use of substandard construction materials were the major contributing factors to the damages.