Earthquakes

I.  What is an earthquake?

 => a sudden shaking of the ground..........

 EX: the recent quakes in Turkey & California

- Earthquakes are a consequence of moving plates

 => distribution of EQ’s worldwide are related to plate boundaries

def.  Earthquake = vibrations (waves) from the sudden release of energy from an area (focus) along a fault.

fault = fracture in the crust along which there has been movement......

=> Movement along a fault can be either vertical or horizontal

A.  What causes the sudden release of energy?
 
 => stresses from plate movement cause movement in    different directions..

 => blocks are locked because of friction, but still move
 =  elastic energy
    "snap back" ==> elastic rebound
 => seismic waves

B.  What are seismic waves?

 seismic waves = waves that travel through the earth in all directions away from the focus (shallow, intermediate, deep)

=> the area on the surface directly above the focus = epicenter

Two types of seismic waves (1) body waves; (2) surface waves
 
 1) Body Waves = travel through the earth in all directions away  from the focus......
  * Two types = a) P-waves and b) S-waves
 

a) P-waves = primary waves
=> compression....
highest velocity

b) S-waves = secondary or shear waves
 => shear
 => slower

2) Surface Waves = travel along the surface away from the epicenter.....  slower than P- and S-waves

* Two important types = a) Love (L) waves, and b) Rayleigh (R) waves

a) L-waves => similar motion to S-wave, but side to side........

b) R-waves => forward rolling motion (similar to water waves)..…

Surface Waves = most destructive

3)How are seismic waves recorded?

=> waves received by a seismograph, and the waves are recorded on a seismogram

4) How are earthquake epicenters located?
 epicenters located using the difference in the arrival times between P- and S- waves to determine the distance from the seismograph to the epicenter.
=> time-distance graphs.
=> this method calculates distance only -> not direction

-> arcs of appropriate distance
-> intersection = epicenter

C.  Earthquake Strength

=> intensity and magnitude scales

1.  Intensity Scales => based on the damage done by an earthquake at a particular location and peoples reaction to it.…

=> Modified Mercalli Intensity Scale  I to XII

-> problem = intensity varies with distance from focus.......impossible to quantify earthquakes using this scale...
 

2.  Scales based on Magnitude => EQ ranking

=> based on the amplitude of the largest wave recorded on the seismogram.
=> calibrated for distance

  a.  Richter Magnitude Scale => Logarithmic (base 10) scale

=> amplitude = 10X:
M-6.5 is 10X 5.5 and 100X  4.5

=> E release= 30X:
M- 6.5 is 30X  5.5 and 900 X 4.5

=> can record magnitudes of < 1
largest recorded = M = 9.2

=>destruction of property begins at M = 5
total destruction occurs at M > 8.