The Philippine Institute of Volcanology and Seismology (Phivolcs) reported a total of 6,456 aftershocks as of 6:00 p.m. on Sunday, but their frequency has been decreasing five days after the magnitude-6.9 earthquake jolted northern Cebu.
Of the latest figure, Phivolcs said 1,291 aftershocks were plotted and at least 29 were felt. The magnitudes ranged from 1.0 to 5.1, majority of which were traced at the quake’s epicenter in Bogo City.
The state seismic agency explained that these numbers of registered aftershocks are common, especially after a powerful earthquake. But based on their latest monitoring, the frequency of the tremors has been on a downward trend.
“Aftershocks may persist for weeks to months. It means that the part of the subsurface that moved (the main shock) is still adjusting to a stable condition/position,” Phivolcs said on Viber. “But you would notice in the chart that the number of aftershocks per day (frequency) is decreasing.”
Earlier, Department of Science and Technology (DOST) Secretary Renato Solidum said they had also expected the formation of numerous sinkholes, describing it as a natural occurrence because of the limestone terrain of northern Cebu.
He explained that limestones are easily dissolved when exposed to water, especially in areas that lacked proper drainage or road structures. Strong earthquakes such as the one that hit Bogo City last September 30 could cause thin layers covering sinkholes to collapse.
For these reasons, the Mines and Geosciences Bureau (MGB) in Central Visayas released an advisory to Bogo City, Daanbantayan, Medellin, San Remigio and Tabogon, warning residents about the threats of sinkholes and ground depressions in several villages within their respective jurisdictions.
“The recent seismic activity has destabilized ground conditions, resulting in the formation of sinkholes in certain affected areas,” according to the MGB.
San Remigio Mayor Mariano Martinez reported that more than 100 sinkholes were observed across their municipality.
On Sunday, the MGB—which is under the Department of Environment and Natural Resources (DENR)—said it has deployed an eight-member of geologists from its central office and Central Visayas regional office to conduct a comprehensive Post-Disaster Geohazards Assessment across several areas in Cebu Province.
“The initiative aims to enhance public safety and guide local government units (LGUs) in land-use planning and disaster risk reduction, particularly in communities situated on karst terrains characterized by limestone formations that are prone to dissolution and collapse,” the MGB said.
