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Seismic Hazards (Management (Prediction (Very difficult, regions at risk…
Seismic Hazards
Management
Prediction
Very difficult, regions at risk can be identified, fault lines can be monitored, local magnetic fields can be measured
Prevention
Impossible to prevent EQs
Protection
Essential everyone is prepared, modify human and built environment to decrease vulnerability
Hazard resistant design
Buildings to be designed in 3 ways- (1) large concrete weight on top of building, (2) large rubber shock absorbers, (3) cross-bracing to hold together structures
Education
EQ drills at schools, instructions on EQ hazards and drills
Fire prevention
"Smart metres" cut off gas if sufficient EQ magnitude occurs
Emergency services
Careful organisation and planning, equipment available for use, Frist aid training
Land use planning
Most hazardous identified areas regulated for land use, schools and hospitals in low risk areas, sufficient open space
Insurance
Richer areas urged to take out insurance to cover losses very expensive
Aid
Poorer countries given aid= medical services, tents, water purification, equipment, search and rescue, etc, Reconstruction of buildings, recovery of economy
Tsunami protection
Not entirely predicted, automated systems can be installed to give warnings, high risk areas use risk, systems to warn population before reaches land, sea walls
Impacts
Primary
Ground shaking, Aftershock, Ground rupture, Ground displacement
Secondary
Fires, Liquefaction, Tsunamis, Landslides, Damage, Death, Aftershocks
Ground shaking
Depends upon magnitude of EQ, depth of focus and distance from epicentre, Local geological conditions also contributre
Ground rupture
Visible breaking and displacement of surface, probably along line of fault, poses risk for large engineering structures
Ground dispalcent
Uplift and subsidence of land
Liquefaction
Soils with high water content loose mechanical strength and start to behave as a liquid
Landslides/ Avelanches
Pose problem in upland, rural areas as can block roads etc
Fires
Gas pipes in urban areas can cause fires when they break, collapsed electricity systems also cause fires
Nature and distribution
Types
Earthquakes
Tsunamis
Shallow focus
0-70km, account for 75% of all energy released, Constructive and conservative
Deep focus
70-700km, destructive and collisional
Tsunamis
Formation
Generated by shallow focus EQ underwater, Upper plate of subduction zone snapped upwards, displaced water moves out as ripples 1m high, sea pulls back as reaches land and causes huge wave
Characteristics
Long wavelength, low height, travel quickly (up to 700km/h), height increases in shallow water= slows wave so back= faster than front= compresses into vertical height
Damage
Japan 2011- 70km offshore, Magnitude 9, 40m height, travelled 10km inland, 16,000 deaths, 6,000 injured, 2,500 missing, 300,000 displaced from houses, 127,000 buildings destroyed, Fukushima power plant affected, Economic cost= $235 billion
Shock waves
Body waves
P waves
Fastest, expand and compress rock, travel through solids, liquids and gasses, Longitudinal waves, little destruction
S waves
Slower, solids and semi solids only, up and down force, destructive, cannot travel through outer core
Surface waves
Love
After body waves, named after mathematician, side to side movement, lot of damage and destruction
Rayleigh
Very destructive, forwards and backwards and up and down, like rolling sea wave
Frequency, magnitude and predictability
Small EQ occur frequently, many happen around world every day, several EQ that affect British Isles on regular basis but these are so small as to be felt by very few people
3 Scales used to measure magnitude
Richter
Most famous, inaccurate, Measure amplitude using seismometer, Difference between arrival time of P and S waves is recorded, Line drawn through scales hits Magnitude scale at an interval= Magnitude of EQ, Logarithmic scale, Magnitude 7 = 30 times more powerful than Magnitude 6
Developed by Charles Richter 1935, based his work on EQ in California which occurred at Conservative margins so tend to have shallow focus (up to 16km), Not developed to allow for accurate measurement of deeper focus EQs, later adapted and scientists= concerned about use for some time
Moment Magnitude
Replacement for Richter scale, complex to calculate but based on energy released and amount of displacement rock experiences during the EQ, can be deduced from amplitude of seismic waves, values 1-10, higher= more powerful, abbreviated Mw, used more commonly than Richter scale as time goes on, considered more reliable at measuring larger scale EQ more accurately
Mercalli
Based on intensity measured by damage caused, 12 point scale, higher score= more damage, no mathematical basis and based on visual observations making it subjective
I= Not felt
II= Weak (few people felt)
III= Weak (quite noticeably indoors)
IV (Mw=3)= Light (felt indoors by many, cracks made)
V= Moderate (felt by nearly everyone, broken windows)
VI= Strong (felt by all, damage slight)
VII (Mw=5)= Very strong (Damage negligible in good structures, Considerable in poorly built)
VIII= Severe (Heavy furniture overturned, chimneys broken)
IX= Violent (Damage considerable, building collapse)
X (Mw=7)= Extreme (buildings destroyed)