Aside from the constant floods and storm surges that regularly inundate Navotas City during the rainy season, the city is also at risk of being hit by earthquakes and tsunamis, the Philippine Institute of Volcanology and Seismology (Phivolcs) said.
Phivolcs Director Renato Solidum, who spoke in the recently concluded Navotas Disaster Preparedness Summit, said that the coastal city of Navotas faces threats from two tremor-generating ridges: the West Valley Fault, which runs through Parañaque, Taguig, Pasig, Makati, Marikina and Quezon City; and the Manila Trench, which passes through Luzon under the West Philippine Sea (South China Sea).
This means that Navotas residents not only face possible death and destruction from the collapse of poorly constructed structures but also from tsunamis.
“There is no fault running through Navotas, but a 7.2-magnitude earthquake triggered by the West Valley Fault will still cause strong ground-shaking and liquefaction in the city, as much of the soil [on which structures sit] is soft,” Solidum said.
He estimated that under this scenario, over 700 people may be killed and 13,000 others injured. The city would need at least P20 billion to rebuild the structures damaged in the quake, he added.
Aside from the possible damage and deaths caused by a strong quake, Navotas and the nearby city of Malabon could also be struck by a tsunami, but only if an 8.3-magnitude earthquake is triggered by movement in the Manila Trench.
“The Manila Trench is located outside Manila Bay, in the wider waters of the West Philippine Sea. But if a strong earthquake occurs there, the tsunami can still pass through the bottleneck in the mouth of Manila Bay,” Solidum said.
“The 8.3-magnitude earthquake can generate tsunami waves as high as 3 meters, or a one-story building in normal sea levels, or 5.5 meters, or a two-story building, if it is high tide. This would sweep over Navotas and even reach Malabon,” he added.
To arrive at these scenarios, Solidum said that Phivolcs analyzed the location of buildings and the composition of the ground on which they stand, the use of these structures, materials used in the building and the number of people occupying these buildings.
Solidum, meanwhile, advised the city government and residents to cooperate in preparing for devastating earthquakes.
“Later this quarter, we will be releasing a 12-item questionnaire which will help homeowners evaluate the extent to which their houses are earthquake-proof,” he said.
At the same time, he urged the city government, barangay (village) leaders and other community officials to organize an awareness campaign and earthquake drills.