seismic waves and measuring earthquakes

primary and secondary waves, and measuring earthquakes 

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Seismicity
Seismic waves
Greater the magnitude of earthquake, rarer they are- negative correlation
The point within the crust where pressure release occurs- focus/hypocentre
Closest distance from focus
Seismic waves don't all travel at the same speed. The rock type will effect velocity
Epicentre are worked out by looking at different wave speeds
Body waves
Travel through interior of the earth
o P waves- compress and dilate
o S waves- waves that go up and down
P waves- primary waves
o Compressional
o First waves that are produced by the earthquake
o Can travel through any material
o Twice the speed of S waves
S waves- secondary waves
o Transverse in nature
o Displace the ground perpendicular to the direction of propagation
o Can travel only through solids
o Slower than P waves
Surface waves
They move the surface of the earth up and down
Surface waves- Rayleigh waves
Travel much nearer the surface and more slowly than P or S
waves- more destructive than either
L waves- cause ground to move sideways
R waves- make it go up and down in a cycle, perpendicular
motion
Measuring earthquakes

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Mercalli scale
Measures and refers to likely damage
Ground observations
Measured on a scale of 1- 12, 1 being lowest and 12 being highest
1900's first created
Now called modified Mercalli scale (MMI)
Generally higher MMI near epicentre
Richter scale
Logarithmic
Measures magnitude (energy released from earthquake)
Scientific
10x difference in each scale 2 is 10x worse than a 1
Created 1936 by Charles Richter
Richter scale- scientific (more reliable?), doesn't say effects
Mercalli scale- human point of view (more useful?), variation due to location…read more

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