Earthquakes
By Danielle Stroup
Introduction
• An earthquake is one of the most terrifying and destructive naturally
occurring phenomena on our planet.
• An earthquake is essentially a vibration through the earth’s crust,
caused by movement of the Earth’s plates along a fault line.
• A fault is a crystal fracture, or deformation of the rock structure, along
which movement occurs, and thus, with this movement, an earthquake
occurs and elastic energy is released.
• In this presentation, I would like to discuss the four different kinds of
faults, emphasizing strike-slip faults, the effects after movement has
occurred between the fault boundaries, the energies released within the
expelled primary and secondary waves, how the magnitude of an
earthquake is determined, and how developments in prediction of an
earthquake are being made.
Faults
• There are four different kinds of faults; normal, reverse,
thrust, and strike-slip.
• Normal and reverse faults take place between two
diverging plates, but are specified by their direction of
plate movement.
• Thrust faults occur at converging plate boundaries, and act
in the same way as reverse faults, but are nearly horizontal.
Strike-slip faults
• In a strike-slip fault, the plates move in opposite directions
away from each other.
• When the rock structure cannot take any more frictional
stress, the plates slip, causing an earthquake, and then the
fault “strikes” again, coming to rest, and the whole process
then repeats.
• When the friction level is high enough, the two plates can
actually become locked.
• With the temperature and pressure increasing, these
components can actually overcome the force of friction.
Strike-Slip Fault zones
• Most earthquakes take place at plate boundaries.
• Pressure and strain is so intense, fault zones, or
groups of interconnected faults are created.
• In these fault zones, release of kinetic energy from
one fault, may actually increase pressure, strain,
and therefore potential energy at another fault.
• This effective pressure causes other earthquakes to
occur, or aftershocks.
Seismic Waves
• Energy is released from an earthquake in
the form of seismic waves.
• Two main types of waves:
– Body waves
– Surface waves
Body and surface waves
• Body waves
– Travel through inner
part of the earth
– Two main types:
• Primary, or compression
waves
• Secondary, or shear
waves
• Surface waves
– Travel over the earth
– Stem from body waves
that have made it to the
surface
– Cause the most damage
Primary and Secondary Waves
• Primary waves
– Move around 1 to 5 miles
per second
– Fastest type of waves
– Travel through all states of
matter
– Rock particles oscillate
back and forth
– Accompanied by an abrupt
thud
• Secondary waves
– Lag behind primary waves
– Do not move straight
through the earth
– Cannot travel through solids
– Rocks are displaced
outwards
– First period of rolling
Earthquake’s Magnitude
• Seismograph: calculates maximum deflection of
seismic waves from their amplitude
• From this amplitude, an earthquake’s magnitude
can be determined
• Charles Richter, inventor of the magnitude scale,
measured maximum amplitudes determined on
seismograms of earthquakes at various distances
• Richter thus developed a logarithmic, base-ten,
scale
Magnitude scale
• Richter plotted logarithms of the maximum
trace amplitudes against distance
M = log A – log Ao
Defining:
M = Richter local magnitude
A = recorded trace amplitude
Ao = amplitude of mildest tremor
Defining Magnitudes
• Richter did not assign a bottom or ceiling to the
scale
• Earthquakes larger than a 9, usually do not occur
• Largest earthquake ever recorded is an 8.6
• Magnitude of 5.5 is considered the “threshold of
damage”
• Magnitude of 10 would correspond to zone failure
that would encircle the earth
Magnitude uses
• Establishes the size of an earthquake
• Calculates energy, or capacity to do work
• Work for an earthquake is defined as how
much crustal material is displaced
• Seismologists relate earthquakes to another
known energy source: nuclear explosions
• The energy scale must also be logarithmic
Conclusion
• Many developments have been made
towards understanding earthquakes
• Seismologists still cannot predict when
earthquakes will occur
• Seismic gaps will be the best bet for future
prediction