Earth's Surface Cracks and Faults

Earth's Surface Cracks

• Earthquakes: Sudden releases of energy in Earth's crust cause shaking and trembling.
• Faults: Fractures or breaks in Earth's crust.
• Fault zones: Series of associated faults.
• Stress: Force applied to rocks.
• Types of Stress:
  • Tensional stress: Rocks stretched apart.
  • Compressional stress: Rocks pushed together.
  • Shear stress: Rocks pushed perpendicular to the axis.

Faults and Movements

• Dip: Angle of a fault plane.
• Dip-slip faults: Faults formed by the dip, including normal and reverse faults.
• Hanging wall: Part of the fault above the fault plane.
• Footwall: Part of the fault below the fault plane.
• Normal Fault: Hanging wall moves down relative to the footwall due to tensional stress.
• Reverse Fault: Hanging wall moves up relative to the footwall due to compressional stress.
  • Thrust fault: Subtype of reverse fault with a dip less than 45°.
• Strike-slip fault: Fault movement along the strike (horizontal).
  • Shear: Parallel but opposite movements along the fault plane.
    • Left-lateral strike-slip fault: Leftward movement of one side relative to the other.
    • Right-lateral strike-slip fault: Rightward movement of one side relative to the other.
• Oblique-slip fault: Movement along both dip and strike directions, combining shearing and compression/tension.

Active and Inactive Faults

• Active faults: Generated earthquakes within the last 10,000 years and may continue to do so.
  • Shallow earthquakes occur on active faults.
• Inactive faults: No record of generating earthquakes in the last 10,000 years, but may generate earthquakes in the future.
• Creep fault: Fault movement without associated earthquake activity.
• Trenches: Long, narrow, and steep-sided depressions in the ocean floor.
  • Philippine Trench: Deepest trench in the Philippines, third deepest globally.
• Philippine Fault Zone (PFZ): Major tectonic feature crossing the Philippines (northwestern Luzon to southeastern Mindanao), 1200 km long, composed of many faults.
  • Central Mindoro Fault: Right-lateral fault marking a slope break.
  • Lubang Fault: Left-lateral fault offshore between Batangas and Mindoro.

Anatomy of an Earthquake

• Focus/Hypocenter: Point where rocks first break, releasing seismic energy.

• Fault plane: Breaking surface underground.

• Fault line: Edge of the fault plane.

• Fault scarp: Exposed fault plane above the ground, indicating a fault.

• Epicenter: Point on Earth's surface directly above the focus; the strongest earthquake point.

• Types of Earthquake According to Depth of Focus: (related information omitted in the text).

All About Tsunamis

• Tsunami: Large volume of water reaching the surface, triggered by underwater events.
• Causes of Tsunamis:
  • Underwater earthquakes.
  • Volcanic eruptions.
  • Nuclear explosions.
  • Meteors.
  • Underwater landslides.
• Strong Tsunamis:
  • Tohoku, Sendai (Japan) - March 11, 2011: 9.0 magnitude.
  • Southern Chile - May 22, 1960: Strongest earthquake, 9.5 magnitude.
  • Northwest Sumatra (Indonesia) - December 26, 2004: 9.3 magnitude underwater earthquake.