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EARTHQUAKE HAZARD

WHAT IS AN EARTHQUAKE?

  • An earthquake is defined as a weak to violent shaking of the ground.

  • It is produced by the sudden movement of rock materials located below the earth’s surface.

  • Earthquakes occur primarily along tectonic plate boundaries and active faults.

WHAT IS A FAULT?

  • A fault refers to a break, fracture, fissure, or zone of weakness where movement or displacement has occurred or may occur again.

  • Faults can extend hundreds of kilometers across the earth’s surface and reach tens of kilometers downward.

WHAT ARE THE TYPES OF EARTHQUAKES?

  • Volcanic Earthquakes:

    • These are produced by the movement of magma beneath active volcanoes.

  • Tectonic Earthquakes:

    • These occur due to sudden movements along faults and plate boundaries.

EARTHQUAKE GENERATORS

  • An active fault is characterized by movement that has occurred within the last 10,000 years, showing evidence or having a documented history of recent activity.

  • A trench refers to the deepest portion of the sea floor. It manifests as a subduction zone where one tectonic plate moves beneath another.

  • Major Active Fault Lines in the Philippines:

    • Western Philippine Fault

    • Eastern Philippine Fault

    • South of Mindanao Fault

    • Central Philippine Fault

    • Valley Fault System (includes Marikina Valley Fault and East Valley Fault)

WHERE DO EARTHQUAKES ORIGINATE?

  • The focus is the point inside the earth where the earthquake originates, also known as the hypocenter.

  • The epicenter is the point on the surface of the earth directly above the focus.

TYPES OF FAULTS

  • Normal Fault:

    • A geologic fault where the hanging wall has moved downward relative to the footwall, occurring where two rock blocks are pulled apart by tension.

  • Thrust or Reverse Fault:

    • A fault in which the hanging wall moves upward relative to the footwall, typically resulting from two rock blocks being forced together by compression.

  • Strike-slip Fault:

    • A fault where the surfaces on either side of the fault plane move horizontally and parallel to the strike of the fault.

WHY DO EARTHQUAKES OCCUR IN THE PHILIPPINES?

  • The Philippines is located in a tectonically active region known as the Pacific Ring of Fire, where numerous earthquakes and volcanic eruptions are prevalent.

  • The archipelago is surrounded by subducting tectonic plates:

    • Philippine Sea Plate (to the east)

    • Eurasian Plate (to the west)

  • Offshore earthquakes are often associated with:

    • Philippine Trench

    • East Luzon Trough

    • Manila Trench

    • Negros Trench

    • Sulu Trench

    • Cotabato Trench

  • Inland earthquakes often stem from the movement along the Philippine Fault Zone, which spans approximately 1,300 km from the Ilocos Region to eastern Mindanao.

EARTHQUAKE HAZARDS

  • Ground Rupture:

    • Refers to the deformation on the ground marking the intersection of the fault with the earth’s surface. It includes fissuring and displacement of the ground due to fault movement.

  • Ground Shaking:

    • Disruptive vibrations of the ground during an earthquake, which can lead to damage or collapse of structures.

    • Can result in hazards such as liquefaction and landslides.

  • Liquefaction:

    • Phenomenon where sediments, particularly near bodies of water, behave like liquid, similar to quicksand.

    • Consequences may include sinking or tilting of structures above, sand boils, or fissuring.

  • Landslide:

    • Down slope movement of rocks, soil, and debris, often triggered by strong shaking.

    • Can cause erosion, burial, and blockage of roads and rivers.

  • Tsunami:

    • A series of waves typically caused by underwater earthquakes.

    • Associated effects include flooding, coastal erosion, drowning of individuals, and property damage.

EARTHQUAKE MEASUREMENT

  • Magnitude:

    • Proportional to the energy released by an earthquake at its focus, calculated using a seismograph. Represented by Arabic numbers (e.g., 4.8, 9.0).

  • Intensity:

    • Strength of an earthquake as perceived by people, based on relative effects on people, objects, environment, and structures. Intensity is typically higher near the epicenter and is represented by Roman numerals (e.g., II, IV, IX).

PHILIPPINE EARTHQUAKE INTENSITY SCALE

  • Intensity I: Scarcely Perceptible

    • Perceptible to some under favorable conditions; delicate objects disturbed slightly.

  • Intensity II: Slightly Felt

    • Felt indoors by few individuals; hanging objects swing slightly.

  • Intensity III: Weak

    • Felt by many, especially in upper floors; resembles passing light truck.

  • Intensity IV: Moderately Strong

    • Generally felt indoors; light sleepers awakened; effects noticeable on objects and liquids.

  • Intensity V: Strong

    • Commonly felt indoors and outdoors; many sleeping people awakened; significant shaking.

  • Intensity VI: Very Strong

    • Many frightened; heavy objects may shift; old structures may show slight damage.

  • Intensity VII: Destructive

    • Most frightened; people find it difficult to stand; old structures suffer considerable damage.

    • Liquefaction can be observed.

  • Intensity VIII: Very Destructive

    • Panic ensues; significant damage to well-built structures; many natural hazards like landslides occur.

  • Intensity IX: Devastating

    • People thrown to the ground; most buildings totally damaged; widespread liquefaction and landslides.

  • Intensity X: Completely Devastating

    • Nearly all structures destroyed; massive landslides and considerable geological changes observed.