Download c. Section 4.3 Seismology (powerpoint)

Unit 4: Earth Science
Section 4.3 Seismology
4.3 Seismology – The Study of Earthquakes
• An earthquake is a shaking of the ground as
the result of a sudden release of energy in
Earth’s crust
Focus – the location where there
is the energy is released, below
the surface of the earth.
Epicentre – point on the surface
that is directly above the focus.
• Earthquakes occur at various depths,
depending on the plates involved.
• Earthquakes near the surface tend to cause
more damage.
I. Causes of Earthquakes
• Rock masses on either side of a
fault are pushing past one
another.
• Rock masses bend and compress
until there is enough stored
energy to overcome friction
between the rock masses;
• this energy is then quickly
released as kinetic energy during
an earthquake – Elastic Rebound
Theory.
II. Earthquake Waves
• P Waves – primary wave
• S Waves – secondary wave
• Love Waves – surface wave
Primary waves (P waves)
• pressure wave;
• particles move in the same direction as the
wave.
• Almost twice as fast as S waves.
• Travel through solids and liquids (crust, mantle
and core)
Secondary waves ( S wave)
• transverse wave;
• particles move perpendicular to the direction
of the wave.
• Travel slower than P waves.
• Travel only through solids (lithosphere)
Surface waves (Love waves):
• ground motion is a rolling action, like ripples
on a pond.
• Travels along surface of the Earth.
• Slowest and last waves to arrive after an
earthquake
• Usually cause the most structural damage
III. Locating an Earthquake
The seismometer
• Instrument used to measure the ground
motion resulting from earthquakes
• Produces a zigzag tracing on a paper by a pen
• Tracing is called a seismogram
Determining the Distance to the Epicentre
• P waves are faster than S waves; therefore, P
waves arrive at a seismograph station before
the S waves.
• The time interval between the
arrival of P and S waves at a
seismograph is used to determine
the distance to epicenter.
• Time-distance graph is used as a
tool to calculate distance to
epicenter
Locating the Epicentre
• Need THREE different locations using the
time-distance graph to calculate the epicenter.
• Draw a circle around each seismograph with a
radius equal to the distance to the epicenter.
• The location where the three circles meet is
the epicenter.
• Depth to the focus is found by the lag time of
the L wave – the longer the lag time, the
deeper the focus.
IV. Measuring an Earthquake –
Earthquake Magnitude
• The Richter Scale is a standardized scale of
earthquake magnitude (amount of shaking)
• Each point represents vibrations that are 10
times greater than the point below it.
• Each tenfold increase in
vibrations means 30
times more energy
released.
V. Earthquake Hazards
1. Ground shaking
• The result of the waves set in
motion by the earthquake
• Some vibrations move up and down while
others move side to side
• Most buildings can withstand large up-anddown vibrations, but not side-to-side (cause
collapse)
2. Liquifaction
• Vibrations cause groundwater to rise, turning
solid ground into a liquid-like material.
• Buildings built on
solid rock experience
little damage;
buildings located on
bog muds or soft fill
suffer severe
damage
3. Tsunamis
• Large ocean waves produced when
earthquakes occur under water.
• Caused by a large displacement in the water
column
VI. Vancouver’s Next “Big Earthquake”
• The Juan de Fuca Plate,
Explorer Plate, and Gorda Plate
are all subducting under the
North American Plate
• Each of the three plates have
transform fault lines
• Will most likely be produced by
stress and strain generated at a
transform fault or along the
subduction zone under the
North American Plate.