Buying a house is probably the biggest purchase you will ever make. An active fault running under the lot is one of the few hazards that no renovation can fix later. So it is worth ten minutes to check before you sign anything.

This guide explains what an active fault is, how close is too close, and how to screen any Philippine address yourself.

What an active fault actually is

An active fault is a crack in the earth's crust that has moved in the recent geologic past and can move again. When it slips, the ground on each side shifts in opposite directions. A building sitting directly on the trace gets torn apart, not just shaken. This is different from general earthquake shaking, which spreads over a wide area. Fault rupture is a narrow, sharp line of total damage.

In Metro Manila the big one is the West Valley Fault. It runs through several cities and crosses places like Marikina. PHIVOLCS, the government volcano and earthquake agency, maps these fault lines and warns against building directly on top of them.

How close is too close

There is no single magic number, but the distance to the nearest active fault sorts a property into clear risk bands. CheckHazard uses the same distance bands that PHIVOLCS and Philippine engineering practice rely on:

• Within 100 meters: very high risk. This is the fault-trace rupture zone. PHIVOLCS advises against building here at all. If the ground slips, the structure is on the tear line.
• Within 500 meters: high risk. This is the critical-facility advisory band. Government guidance says schools, hospitals, and other important buildings should be sited carefully or avoided this close.
• Within 2 kilometers: moderate. You are inside the strong-shaking band used in the Metro Manila earthquake studies. The ground will shake hard in a big quake, but you are outside the direct rupture line.
• Beyond 2 kilometers: low. The fault is no longer the headline concern at this distance.

The point is not to panic at "moderate." Almost all of Metro Manila will shake in a major quake. The point is to know whether you are on the rupture line itself, which is the one situation you genuinely cannot build your way out of.

How to screen an address

You do not need to be an engineer to do a first check.

1. Get the exact address or pin. A fault line is narrow, so a rough barangay-level guess is not enough. You want the actual lot.
2. Measure the distance to the nearest mapped active fault. This is the core check. CheckHazard does this automatically: type the address and the report measures the straight-line distance from the pin to the nearest active fault in the PHIVOLCS and GEM fault dataset, then tells you which band you fall into.
3. Read it alongside the other hazards. A property can be far from a fault but still sit in a flood zone or on a steep slope. A single hazard rarely tells the whole story, which is why our report covers eleven layers, not just one. You can read more about how we handle this layer on our active faults brief.

What a screening can and cannot do

A distance check tells you how close the nearest mapped fault is. It is a strong, fast first filter. What it cannot do is confirm there is no unmapped fault strand on the lot, or tell you how the soil under the house will behave in a quake. Those answers come from a licensed engineer doing an on-site assessment.

So treat the screening as the thing that tells you whether you even need that conversation, and how urgent it is. If the address lands in the very-high band, you walk into the engineer's office already knowing the most important question to ask. If you are a first-time buyer, our homebuyer guide walks through the wider checklist.

CheckHazard does not replace a professional geotechnical or engineering survey. It is preliminary screening that surfaces the right questions to ask before you commit.