Tectonism

Plate Tectonics/Tectonism

Earthquakes

What is an earthquake?

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

 EX: the recent quakes in Turkey & the Pacific Northwest

- 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......

Faults
• Parts
­ Focus & epicenter
­ Fault plane
­ Hangingwall & footwall

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

**Dip-slip = vertical movement

 a. Normal Fault => the result of tensional forces
 b. Reverse Fault=> the result of compression

**Strike-slip = horizontal movement
 =>Transform Fault => the result of shear forces
 

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

Seismic Waves
• 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

How are seismic waves recorded?

Seismographs and seismograms
Seismic wave velocities

• 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

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...

EQ magnitude
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.

Moment-magnitude scale - (Mw)
 - based on the amount of energy released by an EQ

 - more accurate for comparing large EQ’s
 

Earthquake Damage

The amount of damage caused by an EQ is a function of…
 
a) duration and magnitude of quake
 b) type of substrate under structures
c) building design
d) population density & TOD

EQ destruction
Types of destructive behavior

a. earth movements
 
1) shaking & ground displacement

2) liquefaction

3) landslides and subsidence or slump
 
4) earth fracture or rupture

Other types of destructive behavior…
a. Fire
b. Tsunami or seismic sea wave

Earthquake Prediction

1. Long Term => based on amount of relative movement of plates and historical records.

=> statistical probabilities for earthquakes in 10s of years....  seismic risk maps

Seismic-Gap Method.

=> based on fault slip and historical records of seismic activity.

=> areas along fault with very little slip and seismic activity

= stored energy

=> high probability of producing a destructive quake
Short Term....still being refined

-> careful monitoring

     = precursors.

a) bulges and changes in elevation

b) foreshocks

c) changes in the magnetic field

d) ultra low frequency radio waves

EQ engineering & zoning
• Building design
• education