earthquake hazards

• Earthquake


  Shaking of earth caused by waves on and below earth's surface
• Earthquakes


  • Cause surface faulting
  • Cause aftershocks
  • Cause tsunamis
  • Cause tremors
  • Cause vibrations
  • Cause liquefaction
  • Cause landslides
• Earthquakes are mainly caused by internal movements in earth
• Direct cause is rocks underground rupture when affected by powerful external stress greater than strength
• Approx. 5 mil earthquakes occur worldwide in a year
• Crust


  Outermost layer of earth consisting of rigid oceanic and continental tectonic plates
• Mantle


  Layer of heated viscous rock between crust and core
• Epicenter


  Point on earth surface above point at depth in earth crust where earthquake begins
• Focus


  Point at which earthquake rupture begins, usually deep within the earth on a fault
• Fault


  Fracture/crack along which two blocks of rock slide past one another. Movement may occur rapidly (earthquake) or slowly (creep)
• Seismogram


  Record made by seismograph
• Seismograph


  Instruments that make an automatic record of earthquake
• Tectonic plate


  Earth's outer shells. Movements on faults that define plate boundaries produce more earthquakes
• Tectonic subsidence


  Downdropping and tilting of a basin floor on downdropped side
• Foreshock


  Earthquakes that precedes mainshock of an earthquake sequence, may occur seconds to weeks before mainshock, not all mainshocks have foreshocks
• Mainshock


  Largest quake of an earthquake sequence
• Magnitude


  Determined from measurements on seismographs
• Intensity


  Measures strength of shaking produced by earthquake at certain location, determined from effects on people, structures, and environment
• Earthquakes have one magnitude but many intensities
• Notable earthquakes in Philippines

  • Luzon 1990 earthquake
  • Bohol 2013 earthquake
  • Surigao 2017 earthquake
  • 8.0 magnitude earthquake in Mindanao (1976)
  • 7.5 magnitude earthquake in Luzon (1645)
  • 7.3 magnitude earthquake in Casiguran (1968)
• Ground shaking


  Primary cause of earthquake damage to man made structures, shaking of earth caused by waves on and below earth's surface
• Impacts of ground shaking

  • Damage to structures
  • Fires
  • Injuries/death
  • Spill of hazardous chemicals if not properly secured
• Agents of damage


  Effects of earthquakes that can cause damage, initial agent is displacement of plates along a fault
• Fault/ground rupture


  Movement of ground along one side of fault relative to other side, possible impacts include damage to structures, broken pipes/utility lines, death/injury
• How to reduce vulnerability to fault/ground rupture

  • Select a location far from fault line
  • Government should provide zones where structures can be built
  • Proper structural and geotechnical engineering designs
• Liquefaction


  Process where water saturated sediment temporarily loses strength and acts as a fluid, aftermath are flood and leaving areas covered in layer of mud
• Areas prone to liquefaction

  • Near fault zone
  • Located in coastal zone (loose sand deposit)
  • Area has shallow ground water table
• Impacts of liquefaction


  • Buildings and other structures can sink into ground/tilt over
  • Underground pipes and tanks may rise to surface
• How to reduce vulnerability to liquefaction

  • Ground improvement engineering
  • Use locations where soil is not predominantly sand
• Earthquake induced ground subsidence

  Lowering of land surface, can occur rapidly due to sinkhole/underground mine collapse/major earthquake
• Impacts of earthquake induced ground subsidence

  • Cracked infrastructure
  • Changes in elevation and gradient of channels
  • Broken pipes and utility lines
  • Injury/death
• How to reduce vulnerability to earthquake induced ground subsidence


  • Public information programs
  • Maps
  • Land-use management
• Landslides/mass wasting


  Downslope movement of rocks and soils under effect of earthquake, important secondary earthquake hazard
• Signs of area prone to landslides


  • Springs, seeps/saturated ground, street, or sidewalks
  • New cracks/unusal bulges in ground
  • Soil moving away from foundations/tilting or cracking of concrete floor and foundations
  • Sunken/down-dropped road beds
• Impacts of landslides


  • Damage to properties
  • Disruption of transportation
  • Loss of agricultural sources
  • Death/injury
• How to reduce vulnerability to landslides

  • Local risk reduction activities - communities trained to recognize potential land instabilities and avoid siting houses in hazardous locations, constructing structures with strong foundations
  • General risk reduction strategies - avoid hazardous areas being used for settlements, create shallower slope angles in hillsides through excavation of top layers of earth
• Tsunami


  Harbour wave, originates from undersea/coastal seismic activity, waves travel long distances at high speeds
• Signs of tsunami


  • Strong ground shaking from earthquake
  • Unusual sea level fluctuations
  • Abnormally huge wave
  • Loud ocean roar
• Causes of tsunami damage

  • High velocity impact of incoming waves
  • Inland distance of wave runup
  • Vertical height of wave runup
  • Inadequate resistance of buildings
  • Flooding inadequate horizontal and vertical evacuation
  • Proximity to source of tsunami
• How to reduce vulnerability to tsunami

  • Land use management
  • Planting and environmental preservation
  • Structural designs
  • Hazard awareness
  • Tsunami warning