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 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.
Ground shaking is the up and down and/or sideways motion of the earth’s surface caused by the propagation of seismic waves from an earthquake.
The energy released by an earthquake is measured by Magnitude while the severity of an earthquake’s effect on people, structures, and the natural environment is measured by Intensity.
Generally, ground shaking is stronger nearer the epicenter of the earthquake and decreases in intensity farther away.
Intensity can be assessed using different scales. In the Philippines, ground shaking is measured by the PHIVOLCS Earthquake Intensity Scale (PEIS).
Strong ground-shaking impacts are widespread. It can cause building damage and collapse. Collapsed structures, falling, and toppled objects can lead to injuries and death. Fires can also happen when gas tanks, fuel pipelines, and electrical wires are damaged or flammable liquids spilled.
Strong ground shaking can trigger landslides in mountainous areas and liquefaction in low-lying areas. In areas underlain by limestone, strong ground shaking can also cause sinkholes to collapse.
An earthquake-induced landslide is the downslope movement of soil, rock, and debris due to ground shaking generated by an earthquake. Areas with steeper slopes are more prone to earthquake-induced landslides due to the greater gravitational force acting on the slope.
During the earthquake’s mainshock, landslides can occur, and tension cracks may form as well. These cracks serve as an indication of impending landslides, which could be triggered by subsequent earthquakes, or heavy and prolonged rainfall.
Landslides are mainly classified based on the type of material and movement. Materials can vary from rock, debris, or earth, and movement can range from slides, flow, spreads, topples, and falls.
An entire landslide has three parts: the source zone, transport zone, and depositional zone. The source zone is the area where the slope initially failed. The transport zone is the area where the slope material travels downslope. The depositional zone is the area where the slope material gets transported and is likely to accumulate and settle.
Earthquake-induced landslides pose threats to human life by burying households and infrastructures along the landslide path, resulting in loss of life and destruction of properties and infrastructure. Landslides can block roads which can isolate communities. Landslides may also result in temporary damming of nearby rivers, which may breach and cause flash floods downstream.
An earthquake-induced landslide is the downslope movement of soil, rock, and debris due to ground shaking generated by an earthquake. Areas with steeper slopes are more prone to earthquake-induced landslides due to the greater gravitational force acting on the slope.
During the earthquake’s mainshock, landslides can occur, and tension cracks may form as well. These cracks serve as an indication of impending landslides, which could be triggered by subsequent earthquakes, or heavy and prolonged rainfall.
Landslides are mainly classified based on the type of material and movement. Materials can vary from rock, debris, or earth, and movement can range from slides, flow, spreads, topples, and falls.
An entire landslide has three parts: the source zone, transport zone, and depositional zone. The source zone is the area where the slope initially failed. The transport zone is the area where the slope material travels downslope. The depositional zone is the area where the slope material gets transported and is likely to accumulate and settle.
Earthquake-induced landslides pose threats to human life by burying households and infrastructures along the landslide path, resulting in loss of life and destruction of properties and infrastructure. Landslides can block roads which can isolate communities. Landslides may also result in temporary damming of nearby rivers, which may breach and cause flash floods downstream.
Liquefaction occurs when sediments behave like a liquid during strong ground shaking. These sediments are typically found on lowlands near water bodies. These lowlands include beaches, deltas, riverbanks, reclaimed lands, and floodplains. For liquefaction to occur, such sediments must also be recently deposited, loosely packed, water-saturated, and sandy in composition.
Liquefaction can cause sediment and water vents (more commonly known as sand boils) to form. Liquefaction can also deform the ground and cause subsidence, swelling, undulations, and fissures. Numerous parallel fissures that form and spread toward the nearby water bodies are known as lateral spreads.
Liquefaction can also cause poorly built structures to sink or tilt, and buried buoyant structures (like water pipes, septic tanks, and fuel storage) to float.
There are two types of tsunamis that can affect the Philippines based on the sources:
Local tsunamis are generated by tsunamigenic events within the Philippine Region, typically within a hundred kilometers. Typical sources of local tsunamis are movement of offshore active faults and subduction zones surrounding the Philippines, local submarine landslides, and subaerial pyroclastic flows. Local tsunamis can reach the shoreline within 2 to 5 minutes. All coastal areas in the Philippines can be affected by local tsunamis.
Far-field or distant tsunamis are those that were generated from sources in other regions surrounding the Pacific Ocean or the Celebes Sea. Far-field tsunamis can travel from 1 to 24 hours before reaching the coast of the Philippines.
Tsunamis may consist of a series of waves continuing for hours that can cause high flood events and can reach far inland. Tsunami waves can cause destruction and movement of structures such as houses, buildings, and bridges. Vehicles, large rocks, and other debris weighing several tons may be carried far inland by the tsunami waves. Strong currents of water and the movement of these large materials can injure and kill people caught in the incoming and receding tsunami waves. Seawater may infiltrate freshwater resources making it non-potable. Agricultural lands may be rendered useless for years due to high salt content
Coastal uplift and subsidence refers to the vertical deformation of the coast in response to the upward and downward movement of the crust along a fault or subduction zone. Coastal uplift or subsidence may happen abruptly during an earthquake (coseismic) or gradually during slow progressive deformation of the crust (interseismic).
Uplift may result in widened coastal areas, seaward shift of high tide lines, and exposed coral reef systems, marine plants, and animals.
Subsidence may lead to narrowed coastlines, landward shift of high tide lines, drowned mangroves, and terrestrial plants.
