Seismic events

• What's an earthquake?


  sudden and violent movements of the rocks of the earths crust causing surface to vibrate
• What causes them

  pressure which builds up due to stresses in the rocks which is then suddenly released
• focus


  point within the earths crust where earthquake has originated from
• Epicenter


  point where shock waves reach the surface
• Global distribution of earthquakes

  -span all 7 continents

  - high density around the pacific, ring of fire

  -shallow focus earthquakes more prevelant

  - deepest focus found at destructive plate margin

  -shallow focus at constructive boundaries
• shallow earthquakes


  constructive,conservative
• deep earthquakes


  destructive, collision
• Body waves


  - p wave faster

  -can move through solid rocks and fluid

  -compressional and vibrate whilst travelling

  - recorded on seismograph first
• Surface Waves


  - secondary waves

  -move slower than p waves

  - can only move through solid rock

  -move ground from side to side damaging buildings

  -responsible for most of the shaking experienced during an earthquake
• magnitude


  amount of energy released
• intensity


  severity of ground shaking in a particular place based on effects on humans and environment
• How are seismic waves measured?

  seisometer
• Richter Scale


  a logarithmic scale of 1 to 10 used to express the energy released by an earthquake - energy released is proportional to its magnitude - now more common to use MMS
• Moment Magnitude Scale


  - gives an estimate of total energy released by an earthquake, can be used to rate earthquake sthat happen close or far away and of all sizes, examines distance fault has moved and force required to move it

  -goes from scale of 1-10, logarithmic
• Why is richter scale not used

  worked best for earthquakes in southern California and onoly those hitting within 370 miles of a seisometer
• Whys MMS scale preferred

  captures all different seismic waves from an earthquake and as you go up scale accuracy improves
• Mercalli Scale


  qualitative scale that uses observations of people to determine damage caused by an event ranked from 1 to 12

  1 being only felt by a few

  10+ some well built wooden structures destroyed damage is great
• primary hazards of seismic

  - seismic waves causing ground to shake
• examples

  - liquefaction

  -businessess destroyed

  - building collapse trapping people

  - gov building destroyed
• Secondary hazards


  incl tsunamis, avalanches, fires, soil liquesfaction and effects on people and built environment .
• process of soil liquefaction (4 marks)

  -occurs when compacted sediments lose strength in response to applied stress eg. earthquake

  -mostly observed in saturated loose sandy soils

  -loss of soil structure causes it to loose strength and may flow like a liquid

  - can cause building and infrastructure to collapse- acts like quick sand
• Tsunami


  - Boxing Day Indian Ocean 2004

  - sseabed rose 40 ft over 750 miles

  -magnitude 9.15

  -travel 500 mph

  - vertical wall of water 115 ft high
• Landslides and avalanches


  - Nepal 2015 avalanche

  - moment magnitude 7.8

  - focus 15km underground

  - injured climbers and Nepali guides

  -18 dead

  - tents and medical facilities destroyed at Everest basecamp

  airlift people to safety

  affect tourism season
• Effects on built environemnt

  - San Francisco

  - 6.9 moment magnitude

  - homes built on loose sandy soil

  - fires from broken gas lines

  - collapsed brisges, freeways

  - Slip on san andreas fault, shallow focus

  - unreinforced masonry buildings

  - 63 dead

  - 5 billion dollars in damage

  - no electricty for 3 days

  sewage system broken
• Soil Liquefaction


  seismic vibrations in areas of fluid saturated sand can cause the ground to behave like a liquid, soil destabilized increasing space between grain so flows like liquid

  - Palau Indonesia 2018

  - 2000 homes destroyed

  -7.5 mag quake

  - 1200 dead
• Fires


  Kobe Japan

  - gas lines ruptured

  - low water supply so hard to put out fires

  - 7.2 on richter

  - 5000 dead

  100 billion to rebuild

  - wooden housing so whole neighbourhoods levelled by fire

  - kobe port damaged which deals with 12% of japans exports
• What factors affect vulnerability

  - population density

  - level of development

  - preparedness

  building density

  - magnitude

  aftershocks

  geology

  time of day

  depth of focus

  distance from epicenter

  infrastructure- adapted