What Causes this?
San Francisco 1906
San Francisco 1989
Before and after
What is at fault?
• We’ve talked about the movement of tectonic
• We have also mentioned the different types of
•Can anyone tell me what happens between the plates
at each of these boundaries?
• When these tectonics plates move, the rocks
adjacent to these boundaries changes shape.
– Deformation: is the bending, tilting and breaking
of the Earth’s crust; the change in the shape of
rock in response to stress.
– Rock layers bend under stress, but enough stress
can cause these layers to reach their elastic limit
• Folding: is the bending of rock layers due to
Ever notice how some materials bend
under pressure while others break?
There are two types of stress
involved in the collision
of tectonic plates.
• Compression- is the type of stress an object
experiences when it is being squeezed.
• Tension- is the type of stress that occurs when
forces act to stretch an object.
• We have established that some rock layers
cannot withstand the pressure caused by the
movement of tectonic plates.
– Fault: a break in a body of rock along which one
block slides relative to another.
•There are three types of faults:
•Normal faults: occur when rocks are pulled apart because of
•Reverse faults: occur when rocks are pushed together by
• The third type of fault is the strike-slip fault,
which occurs when rocks are moved
horizontally by opposing forces.
– One example of this type is the San Andreas Fault.
• It is at these faults that most earthquakes
occurs. Seismology or the study of
earthquakes is a branch of science devoted
strictly to the study of earthquakes.
How many earthquakes do you
think happen every year?
• Following a period of
Earth’s crust around
a fault “bounces”
back to its original
shape. This sudden
return of elasticity is
• Seismic waves: are waves of energy that travel
through the Earth, away from an earthquake
in all directions.
– There are three types of seismic waves.
• “P” or Primary wave
• “S” or Secondary wave
• “L” or Longitudinal wave (a.k.a. surface wave)
• P-wave or Primary Wave
– First to be felt
– Least amount of damage
• S-wave of Secondary Wave
•Side to side motion
• L-wave/Longitudinal wave or Surface wave
– Up and down motion
– Most damage.