The Marikina Valley is prone to both severe flooding and destructive earthquakes because it sits in a basin created when a block of earth dropped between the East and West Valley Fault systems, making it a natural collection point for water from Sierra Madre while simultaneously sitting in one of the most seismically active zones in Metro Manila.
Mines and Geosciences Bureau (MGB) Senior Geologist Michelle Mendoza made the statement on DZRH News program The Situation Report on September 11, explaining the geology of the Marikina River basin to residents who experience both flooding and ground shaking without fully understanding why.
Mendoza said the valley was created by tectonic activity, with a block of earth dropping between the two branches of the Marikina Valley Fault System, creating an inland basin through which the Marikina River now flows.
“Yung Marikina Valley Fault System natin, dalawa po kasi yan, may eastern part and western, so East Valley Fault and West Valley Fault. Sa gitna po niyan parang nag-drop down na block of earth, and yan po yung basin, parang inland basin wherein situated po yung Marikina River basin natin,” the MGB senior geologist said.
Mendoza said the dropped basin is a natural catchbasin, meaning rainwater from Sierra Madre and the surrounding mountain ranges flows by gravity into the valley, which then channels that water into the Marikina River and eventually into Metro Manila’s lower-lying areas.
The same fault system that created the valley also makes it seismically dangerous, because active faults capable of generating strong earthquakes run along both sides of the valley where millions of people now live and work.
Mendoza said the dual hazard of flooding and earthquakes in the Marikina area is not a coincidence but a direct consequence of its geological formation, and understanding this should inform both land use planning and disaster preparedness decisions in the area.
She said the good news is that the central ridge areas of Metro Manila, which sit on harder volcanic rock, are relatively more resistant to both ground shaking and landslides, though construction quality and land management still determine actual safety outcomes.
