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A2 Earthquakes (Predicting Earthquakes (FORESHOCKS- large earthquakes are…
A2 Earthquakes
Predicting Earthquakes
FORESHOCKS- large earthquakes are preceded by small tremors but this is not always reliable as they don't always occur. Only 6% chance that a major tremor occurs after an earthquake of a magnitude of 6 on the Richter scale (California)
- HAICHENG EARTHQUAKE- evacuation of 1 million citizens due to changes in groundwater, unusual animal behaviour and foreshocks
SEISMIC GAP THEORY- is a segment of an active fault has not slipped recently, it is more likely to experience movement e.g 1989 Loma Prieta in California
GROUND DEFORMATION- built up of stress causes rock to slide along a fault. measured with a strainmeter (measures changes in the horizontal) or a tiltmeter (changes in surface gradient)
GROUNDWATER- earthquakes caused by an increase in groundwater pressure along a fault zone which reduces friction. monitored in a borehole using a piezometer.
12 months before- a gradual lowering of water levels
several weeks- accelerated lowering of water as cracks open
hours before- rapid rise in groundwater levels
Tangshan 1976, gradual drop several years prior then rapid rise hours before
RADON GAS- radioactive decay of Uranium-238, spikes in concentrations prior to an earthquake. 125 reports of changes in gas prior to 86 earthquakes since 1966
EARTHQUAKE LIGHTS- bright and colourful lights seen in the atmosphere before, during or after an earthquake from
- Piezoelectric effect- materials produce electrical charge in response to stress
- Small pockets of trapped natural gas are ignited by friction
- water molecules separate into atoms of hydrogen and oxygen and recombine
ELECTROMAGNETIC FIELDS- release large electric currents and zones are usually more conductive e.g 1989 Loma Prieta
ANIMAL BEHAVIOUR- abnormal behaviour prior to quakes as P-waves have a frequency that may be audible to animals
Earthquake Engineering
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RETRO-FITTING- this is where a brittle material such as concrete is replaced by steel supports. e.g Cypress Stress Viaduct in LA
ELEVATION CONTROL- a pyramid shaped structure helps to reduce the upwards movement of vibration e.g Transamerica Building in San Fransisco
VIBRATION DAMPERS- act as a shock absorber and reduce the transfer of vibrations from the ground to the structure above
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FLEXIBLE MATERIALS- materials will bend and twist in response to seismic vibrations but they won't fall
secondary effects
LIQUEFACTION- occurs when seismic waves shake saturated sediment which reduces the load pressure and cases groundwater pressure to become greater. The sediment liquefies as a result of loss of strength. Can result in subsidence and collapse of buildings
LANDSLIDES AND ROCKFALLS- unstable sections of rock may be destabilised by violent ground tremors and then collapse
TSUNAMI- energy wave caused by the displacement of water following a submarine earthquake or landslide. May travel at speeds of 500km per hour but slows down as it moves into a shallow coastal area and deforms the sea surface
DEEP OCEAN
- wave length- 10s to 100s km
- velocity- up to 800kmhr-1
- height- >0.5m
SHALLOW OCEAN
- wave length- several kms
- velocity- 30-40kmhr-1
- height- 5-10m
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prevention
stress release on faults- theoretical and has never been tried
- triggering a number of small earthquakes by pumping pressurised water or oil to lubricate the fault
earthquake education- Japan National Disaster prevention day on sept. 1st
- has phone apps for P wave detection alerts
- children are taught what to do from an early age e.g under desk
- earthquake simulator
- adults educated through TV programmes- earthquake kits
- immediately after earthquakes, all television and radio switch to official coverage
primary effects
GROUND SHAKING- movement from 2 types of waves
LOVE WAVES- shake the ground from side to side
RAYLEIGH WAVES- up and down movement (elliptical motion of sea wave)
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CONSTRUCTIVE MARGINS- narrow zones
- slumping of crustal blocks along normal faults
- splitting of rock due to pressurised injection of magma
- strike slip movement of oceanic crust along transform faults
- usually measure 4-5 but can be 8 along transform faults
DESTRUCTIVE MARGINS- wide zones
- occur along the upper margin of the subducting plate and are triggered by strike-slip motion of the descending oceanic crust
- area they occur is the benioff zone
- occur at great depths
CONSERVATIVE MARGINS
- strike slip motion of two plate
- e.g san andreas
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urban areas more vulnerable because:
- PROXIMITY TO THE EPICENTRE- nearer the greater magnitude
- ECONOMIC DEVELOPMENT- greater loss of life in less developed as lower building standards and poorer infrastructure
- EARTHQUAKE MAGNITUDE- greater magnitude makes more intense ground shaking
- ROCK AND SOIL TYPE- weak rocks and soil amplifies the seismic waves as greater ground shaking
- FOCUS DEPTH- shallow focus makes more destruction as less distance between
return period- average time for an earthquake of a given magnitude or more to happen again- measure the likelihood of an earthquake event
mitigating against earthquake
- forecasting where they may happen
- predicting the possibility
- looking for the signs leading to an event
Kobe Earthquake
primary impacts
5500 deaths- 60% were people over 60 who struggled to reach safety
35000 injuries
300 major fires
104,000 buildings destroyed
300,000 people left homeless
secondary impacts
20,000 people lost their jobs
only 7% of population had insurance
2900 people died of suicide or neglect
62 high rise were demolished and only 19 rebuilt
Japan- Eurasian, Philippine and Pacific Plate
Great Hanshin Earthquake occurred at 5:46am on Tuesday, January 17th 1995 and had a magnitude of 7.2
Epicentre located under Awaji-Shima, 20km from Kobe
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shocking because Kobe was not known for large earthquakes since it was far away from intersecting plates and it was considered highly prepared due to strict building codes and public education programmes
- soil liquefaction due to the ground being soft sediment so buildings collapsed
- older housing was made of wooden frames with heavy clay tile roofs which collapsed
- ruptured gas pipes creates over 300 fires
- collapse of the Hanshin Expressway
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