Faults are fractures on the Earth’s crust where blocks of rocks move relative to one another. It can be small and localized or hundreds of kilometers long.
Active faults are faults that moved within the last 10,000 years and will move again in the future.
Potentially active faults are faults that have evidence of deformation, but their activity within the last 10,000 years is unclear. However, the possibility of future movement along these types of faults should not be discounted. Potentially active faults may be reclassified into active faults when additional information becomes available.
There are three main types of fault movement:
- Normal faults are fractures in which the hanging wall has moved downward relative to the footwall. Normal faults occur where two blocks of rock are pulled apart by tension.
- Reverse faults are fractures where the hanging wall has moved upward relative to the footwall. Reverse faults occur where two blocks of rock are forced together by compression.
- Strike-slip faults are fractures where the ground moves horizontally past each other. Left-lateral strike-strip faults are where the ground on the other side of the fault moves horizontally to the left while right-lateral strike-strip faults are where the ground on the other side of the fault moves horizontally to the right.
Ground rupture is the hazard associated with active faults. It refers to the displacement of the Earth's surface along active faults resulting in visible fracturing or cracking of the ground.
Figure 1. Types of fault movements
Figure 2. The Digdig Segment of the Philippine fault generated a MS 7.8 earthquake on 16 July 1990, producing a 125-km long ground rupture extending from Dingalan, Aurora to Kayapa, Nueva Vizcaya. The photo shows left-laterally displaced rice paddies in Imugan, Nueva Vizcaya.
Figure 3. Ground rupture of the 15 October 2013 MS 7.2 Bohol Earthquake in Brgy. Anonang, Inabanga, Bohol. The ground moved up by ~2 meters due to reverse faulting along the North Bohol Fault.
Hyatt Terraces Hotel in Baguio City collapsed during the 16 July 1990 Luzon Earthquake.
