An earthquake produces several distinct hazards. Two of them are commonly confused, and confusing them leads people to misread the fault distance on a hazard report.

Ground rupture

What it is: the ground surface actually tearing and moving along the fault line. One side shifts relative to the other, horizontally, vertically, or both.

Where it happens: in a narrow band along the fault trace. Metres to tens of metres wide in most cases.

What it does: anything built across the trace is torn apart. There is no structural design that survives the ground under a building moving several metres in opposite directions.

What you can do about it: avoid it. This is the one hazard where the answer is genuinely "do not build here", and it is why PHIVOLCS and LGUs apply buffer zones along known fault traces.

The report layer: the distance from your exact coordinates to the nearest mapped active fault. See the active faults brief and how to check if a property is in a fault zone.

Ground shaking

What it is: the seismic waves that travel outward from the rupture and shake everything.

Where it happens: over a very wide area. Hundreds of kilometres for a large earthquake.

What it does: damages and collapses buildings. This is what kills the great majority of people in earthquakes.

What you can do about it: design and build for it. Shaking is an engineering problem with an engineering answer, governed by the National Structural Code and applied by a licensed structural engineer.

The report layer: none directly. Fault distance is a weak proxy, and the liquefaction estimate flags ground that amplifies shaking and can fail under it.

Why this changes how you read the number

A buyer sees "nearest mapped active fault: 850 metres" and reads it as a danger rating. What it actually says is:

• You are probably outside the rupture band, but check the LGU's buffer requirement, because 850 metres is close enough that the exact trace location matters.
• It says nothing useful about how hard the ground will shake, because that depends on the earthquake's magnitude, depth, and your soil.

And a buyer who sees "nearest mapped active fault: 40 kilometres" reads it as safety. What it actually says is:

• Ground rupture is very unlikely at this point.
• Shaking is entirely possible. Baguio was more than a hundred kilometres from the 1990 rupture and buildings collapsed. See July 1990.

Three more earthquake hazards worth naming

Liquefaction. Soft, saturated, sandy ground losing strength and letting buildings sink or tilt. See what liquefaction is.

Earthquake-induced landslide. Strong shaking on steep ground, especially wet steep ground. This is why slope and fault distance should be read together in mountain provinces. See buying in Abra or Kalinga.

Tsunami, for coastal properties, which is not a mapped layer on a screening and is PHIVOLCS's domain. See tsunami and storm surge.

What to actually do

1. Screen the coordinates and note the fault distance and the liquefaction estimate.
2. If the fault distance is small, ask the LGU whether the property falls within any declared buffer, and ask PHIVOLCS about the mapped trace.
3. Whatever the distance, hire a licensed structural engineer for an existing building.
4. On soft ground, commission a soil investigation before designing.
5. On steep ground, read slope and landslide susceptibility alongside the fault distance.
6. Do not read a large distance as a clearance, and remember that fault maps record what is known. See Bohol, 2013.

Checklist

1. Screen the exact coordinates.
2. Separate the rupture question from the shaking question in your notes.
3. Check the LGU buffer requirement if you are anywhere near a mapped trace.
4. Get a structural assessment regardless of distance.
5. Note the liquefaction estimate and act on it if the ground is soft.
6. Read slope with the fault distance in hill country.
7. Give all of it to your engineer rather than interpreting it yourself.

The honest limit

PHIVOLCS is the authority on faults, rupture, intensity, and tsunami. A hazard screening reports the distance to the nearest mapped active fault and a liquefaction estimate at your point, and nothing more about earthquakes. Everything that determines whether a building survives shaking is a licensed structural engineer's work.

CheckHazard does not replace a professional geotechnical or engineering survey.