Natural hazards seen in Naga City
More communities in Naga City are seen to be at risk of natural hazards, said Environment Secretary Roy Cimatu on Monday.
The September 20 landslides that killed 78 and displaced more than 7,000 reportedly exposed the natural hazards in the city and both the city mayor and residents are unaware.
At a press conference at the Naga City Hall, state geologist Liza Manzano revealed that the city is predominantly a karst terrain with numerous underground caves and sinkholes.
She said what happened on September 20 was actually a karst subsidence that was followed by a landslide or a planar block slide, describing it a “complex mass movement.”
She defined karst subsidence as the gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials.
“Subsidence in karst is one of the most dangerous geohazards due to its extreme unpredictability,” she said.
Manzano said one of the causes of subsidence in karst is ground shaking due to earthquake, which she pointed as the main “culprit” of the tragic incident.
She said strong earthquakes and the swarm of seismic activities in the past started to shift the subterranean flow of underground water and eventually weakened the karst terrain.
She said low magnitude earthquakes were frequent in 2017 and 2018 that had epicenters reported within Naga City and nearby localities such as Asturias, Toledo, and Balamban.
But the most notable one was the movement of the North Bohol Fault that generated a 7.2 magnitude earthquake in October 2013, affecting central Cebu with Naga City experiencing Intensity 7. The coastal Naga City faces Bohol Island.
Manzano said the nearest active fault from the Central Cebu fault system is 5.3 kilometers away from the landslide area.
“The shift and ponding of subterranean flow may have caused the build-up of hydrostatic pressure that leads to the landslide,” read the findings of the geo-hazard assessment and investigation on the September 20 incident.
She said there was pressure of groundwater held between the soil or rock layers.
“Sobrang lakas kaya nagflow siya towards a different direction. Kaya iyon, gumalaw ang lupa na parang nag-surfing iyong lupa sa ibabaw. Due to pore water pressure dahil sa sobrang tubig sa ilalim,” she explained.
Her theory is that underground cave absorbed the energy of the series of earthquakes that weakened the support in between that was eventually collapsing. And because water flow is blocked, pressure started building up.
But the degrading of karst is not only due to natural phenomenon but also because of human-induced activities.
Aside from the geology or topography of Naga and the earth tremors occurring there, other contributory factors can be attributed to anthropogenic processes such as urbanization, industrialization, and quarrying, Manzano said.
“Human activities including quarry operation that affect the change of the natural landscape are contributory factors to the complex landslide. The urbanization and industrialization have caused the degradation of the karst landscape such as the diversion of natural drainages, filling-up of closed depressions or sinkholes, disposal of acidic wastewater, cutting of slopes and removal of vegetation,” her report cited.
It also noted that other upland barangays underlain by limestone may also be susceptible to similar natural hazard in Barangay Tina-an.
Barangays, which are outside the active quarry operations, such as Cabungahan, Cantaoan, Upper Naalad, Mainit, Inayagan, and Inoburan, showed signs of active subsidence such as spalling and heaving of karst hills and ridges, and development of hairline cracks and slope face and roads.
With this, other households may also be recommended for evacuation, Cimatu said.
“Pero merong mga lugar na makakasama na sila kasi nakita natin na ito ay hindi nakalagay sa danger zone noong una pero nasa danger zone pala,” he said.
Cimatu said households within the danger zone will never be allowed to return and will have to be relocated.
MGB has also delineated a regulated zone, which surrounds the danger zone that will still undergo further study to examine if it is safe for habitation.
Cimatu said that by Thursday, DENR-MGB Region 7 will identify who among those that were evacuated can return to their homes and who needs to remain at evacuation centers while the relocation program is being worked out.