Geologic hazards results from geologic processes acting on or beneath the earth’s surface. These include movement of plates in the earth’s crust or from local concentrations of heat and are a source of hazards to people and their natural and built-up environment on the earth’s surface. The types of geologic hazard which the Province of Cotabato is exposed to are: earthquake and earthquake-induced hazards such as ground shaking, liquefaction, and earthquake-induced landslide (EIL).
An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface. Earthquakes are caused either by the sudden movement along faults and trenches (tectonic), or by movement of magma beneath volcanoes (volcanic).
There are active faults that traverse the Province of Cotabato. There are likewise numerous active faults in the main island of Mindanao which may affect the Province. The municipalities of President Roxas, Antipas, Magpet and Arakan are proximate to the Central Mindanao Fault. Faults and trenches are also known as the “earthquake generators”.
Active Faults and Trenches Map in Mindanao
The main hazard created by seismic earth movements is ground shaking. This term is used to describe the vibration of the ground during an earthquake.
Liquefaction is a process where particles of loosely-consolidated and water-saturated deposits of sand are re-arranged into a more compact state. This results in the squeezing of water and sediments towards the surface in the form of “sand fountain” and creating a condition resembling a “quicksand”. In this phenomenon, the strength of the soil is reduced to a point where it is unable to support structures.
The severe shaking in an earthquake can cause natural slopes to weaken and fail, resulting to landslides. Earthquake-induced landslides can be divided into three main categories: disrupted slides and falls, coherent slides, and lateral spreads and flows.
The Philippines uses the PHIVOLCS Earthquake Intensity Scale (PEIS) to help explain the intensity assigned to a specified location based on observations made on the consequences from the earthquake event.
The preparation of earthquake maps was based on the selection of five (5) worst case scenarios if the faults and trenches (“earthquake generators”) within and near the province would move to generate destructive earthquakes.
The Rapid Earthquake Damage Assessment System (REDAS) of the PHIVOLCS was the main software used to simulate the five (5) worst case scenarios which are the following: (a) Central Mindanao Fault; (b) Mindanao Fault Daguma Extension (MFDE); (c) Banga Fault; (d) Davao River Fault and, (e) Mindanao Fault 1.
Shown below is the information about the epicenter, magnitude and depth of the five (5) worst case scenario earthquake which was simulated in the REDAS software:
Geologic Hazard Maps
1. Earthquake (Groundshaking) Hazard Maps
2. Earthquake (Liquefaction) Hazard Maps
3. Earthquake Induced Landslide (EIL) Hazard Maps
Source: Disaster Risk Assessment (DRA) Report-Cotabato