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HAZARDS GENERATED BY SEISMIC ACTIVITY - Coggle Diagram
HAZARDS GENERATED BY SEISMIC ACTIVITY
What is an earthquake
Types
Shallow focus
0-70km
old rock fractures under stress
lower energy but high energy ones are damaging
Deep focus
70-700km
deep subduction zones
much more energy
Features
Epicentre-where the earthquake is first felt on land, above the focus
Focus- point below the ground where the energy is released
Seismic hazard- focus within 700km of the earths surface
Foreshock- precede an earthquake
Aftershock - follow an earthquake
Where
Mid-ocean ridges- spreading and rifting create tensional forces
Ocean trenches and island arcs- subduction creates compressive forces
Collision zones- grinding creates compressive forces
Conservative margins- intermittent moving creates shearing forces
The role of faults
A fault is a fracture between two blocks of rock , they allow blocks to move relative to each other and can move rapidly generating an earthquake.
Normal fault
Creates dip-slips, man inclined fracture where blocks mostly shift vertically, the mass above usually moves down
Reverse fault
Rock above moves upland over the lower block in regions of subduction
Strike slip fault
Two blocks slide past each other- the displaced rock is displaced the same way when viewed from either side.
E.g San Andreas Fault
Hazards
Ground shaking
Can be vertical or horizontal, severity is dependant on distance from epicentre and geology e.g Mexico City 1988 built over dried lake bed, many layers of unconsolidated rock with high water content- extreme shaking
Hazards- building collapse, rock displacement can rip apart sewers, sever roads and railways, river rivers, disrupt water supply
Buildings collapse because the natural frequency (number of back and forth movements in a second) is directly proportional to the stiffness of a building but inversely proportional to its mass. This causes resonance where thr frequency of seismic waves matches those fo the building, each wave amplifies the buildings vibration
Liquefaction
Vibrations in areas with surfaces of a high water content or fine grain material can make them move like liquids
Hazards- Buildings can collapse as their foundations give way, Rayleigh waves are more destructive (they are amplified as they slow) liquifying saturated ground causing buildings to sink.
Landslides/Avalanches
Ground shaking and liquefication can cause slope failure. More risk if steeper, not trees, heavy rain, saturated soil
Hazards- blocked transport routes to mountainous areas, buildings destroyed, bury people, blocked rivers being released at once-flooding, dams destroyed.
E.g Nepal 2015 killed 21 on Everest after an avalanche
Tsunamis
Earthquakes release energy into water, pushing water above normal sea level, gravity pulls it back down and the energy ripples out.
Energy moves through the entire depth of the water and increases in size (shoals) as it hits shallow water as the same amount of energy is contained in smaller space, so the wave slows and rises.
Hazards- undetected by boats, can cross entire oceans in less than a day, water in front of the wave is pulled out to sea- people think its safe, drowns people, sweeps away debris, destroys buildings.
E.g Tohoku earthquake caused 40m high tsunami killing 18,000 and travelled 7 miles inland.
Measuring seismic activity
Body waves
Primary waves
First waves to arrive, moving through solid rock and fluids, move via expansion and compression
Secondary waves
Half the speed of primary waves but higher frequency, moves through rock only in a wavy movement
Surface waves
Rayleigh waves
Only travel through the surface of the crust in a rolling motion- causing the ground to move
Love waves
Move through the surface of the crust, faster than Rayleigh, move side to side
Richter scale
Corresponds to energy release, each number is a tenfold increase, no upper limit but highest recorded is 9
Moment magnitude
Most accurate, measure the energy released
Modified Mercalli scale
Measures intensity and impact via observations and descriptions