what fault are they referring to with the big one in manila

They’re referring to the West Valley Fault , the fault system tied to the Metro Manila “Big One” earthquake scenario.

What that means

The “Big One” is commonly used for a predicted major quake from a big movement on the West Valley Fault, which crosses parts of Quezon City, Marikina, Pasig, Makati, Taguig, and Muntinlupa, and extends into nearby provinces.

Why it comes up in Manila

News and disaster planning in Metro Manila often mention it because the fault runs through dense urban areas, so even a single large rupture could cause severe damage and disruption.

Important nuance

Some recent earthquakes elsewhere in the Philippines were not linked to the West Valley Fault, and officials have clarified those were generated by different faults or trenches. The “Big One” specifically refers to the Metro Manila West Valley Fault scenario, not every major quake in the country.

Bottom line

So, if someone in Manila says “the Big One,” they’re almost always talking about the West Valley Fault.