Earthquake Hazards: Ground Shaking

• Earthquake hazards are potential damages, risks, and dangers associated with seismic events caused by earthquakes, affecting people's daily lives, animals, and the environment.

Ground Shaking

• Ground shaking refers to the vibration of the ground that occurs during an earthquake.
• Seismic waves passing beneath structures cause this vibrating.
• Ground shaking is a primary cause of earthquake damage and can trigger other hazards.
• Ground shaking is more intense closer to the earthquake's epicenter and decreases in intensity farther away.
• Magnitude measures the energy released by an earthquake, while intensity measures the earthquake's effect on people, structures, and the natural environment.
• The Philippines uses the PHIVOLCS Earthquake Intensity Scale (PEIS) to measure ground shaking.
• Velocity measures how fast the ground moves during an earthquake and is the speed at which seismic waves travel through the Earth's crust in kilometers per second.
• Acceleration measures the rate at which the ground's velocity changes during shaking.
• Frequency defines how often an earthquake of a certain magnitude occurs, measured in hertz (Hz).
• Duration is the length of time, in seconds, that the ground shakes during an earthquake.

Ground Rupture

• Ground rupture is a hazard involving active faults, leading to visible ground fracturing or cracking.
• Ground rupture can cause significant damage to structures built across the fault line due to ground movement.
• Faults are fractures on the Earth's crust where blocks of rocks move relative to one another.

Three Main Types of Fault Movement

• Normal faults are fractures where the hanging wall moves downward relative to the footwall, occurring when two blocks of rock are pulled apart by tension.
• Reverse faults are fractures where the hanging wall moves upward relative to the footwall, occurring when 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-slip faults occur when the ground on the other side of the fault moves horizontally to the left.
  • Right-lateral strike-slip faults occur when the ground on the other side of the fault moves horizontally to the right.

Liquefaction

• Liquefaction is a process that turns saturated soil into a liquid-like state due to vibration, causing the soil to decrease in density.
• Liquefaction occurs when the ground shakes rapidly, increasing pore pressure and reducing effective stress in the soil.
• Wet, sandy, and silty soils are most likely to liquefy.
• Man-made landfills, especially those built on former bay floors, are at high risk of liquefaction.
• Larger stream channels can be hazardous because they contain loose, young soils.
• Buildings and roads can sink or tilt as an effect of liquefaction.
• Increased water pressure due to liquefaction can trigger landslides and cause dams to collapse.
• Cars can be swallowed into the ground, and roads and bridges can crack and fail due to liquefaction.

Earthquake-Induced Ground Subsidence or Uplift

• Earthquake-induced ground subsidence or uplift describes the upward and downward movement of the crust along a fault or subduction zone, causing the coast to deform vertically.
• Subsidence may lead to narrowed coastlines, landward shift of high tide lines, drowned mangroves, and terrestrial plants.
• Uplift may result in widened coastal areas, seaward shift of high tide lines, and exposed coral reef systems, marine plants, and animals.

Tsunami

• Tsunamis consist of a series of waves continuing for hours, which can cause high flood events and reach far inland.
• Freshwater resources may become unfit for human consumption due to seawater seeping in and the high salt content of agricultural lands can make them unusable for years.
• Tsunamis can tear down and move buildings, bridges, and other structures.
• Tsunami waves may carry vehicles, big rocks, and other debris weighing several tons far inland.
• People caught in incoming and receding tsunami waves may be killed or seriously injured by strong water currents and moving large objects.

Two Types of Tsunamis that Can Affect the Philippines

• Local tsunamis are produced by tsunami-causing events that typically occur within a hundred kilometers of the Philippine Region and can reach the shoreline within 2 to 5 minutes.
• Far-field/distant tsunamis are those originating from sources in areas other than the Celebes Sea or the Pacific Ocean and can travel from 1 to 24 hours before reaching the coast of the Philippines.

Earthquake-Induced Landslide

• Earthquake-induced landslide involves the downslope movement of rocks and other debris, commonly triggered by strong shaking.
• Areas with steeper slopes are more prone to earthquake-induced landslides due to the greater gravitational force acting on the slope.
• The source zone is the location where the slope first failed.
• The transport zone is where the downslope movement of the slope material occurs.
• The depositional zone is the location of the slope material's transportation, where it is likely to gather and settle.