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EARTHQUAKE
Earthquake causes
• An earthquake is the shaking of the Earth’s
crust caused by a release of energy
• The movement of the Earth’s plates causes
most earthquakes
• EQs happen along cracks in the ground
called FAULTS
Elastic rebound theory
• As Earth’s plates move past one another, friction causes the
plates to get “stuck”
• Stress and pressure builds up and causes the plate to become
deformed (bend)
– (the plates are elastic-they can change shape)
• Eventually, the pressure overcomes the friction and the plates
slip past one another
• The plate movement is the earthquake. The greater the plate
movement, the larger the earthquake.
• The plates are in a new location, but they go back to their
original shape (rebound).
Earthquake depth
• Earthquake depth depends on the type of plate
boundary where the movement occurs
• Diverging boundaries (mid-Atlantic Ridge), and
sliding boundaries (San Andreas Fault) tend to have
earthquakes which are closer to the surface
(shallow focus)
• Subduction boundaries tend to have very deep
earthquakes as the plate plunges into the mantle
(deep focus)
Shallow focus
Intermediate focus
Subducting plate
Deep focus
• The point where the actual movement of
the plates takes place, and where the
energy is released from is called the focus
• The point on the Earth’s surface that is
directly above the focus is called the
epicenter
• When an earthquake occurs, energy waves
are released and move outward from the
focus
Types of faults
• NORMAL - one side of a fault slips down relative to
another
• REVERSE (& Thrust) - one side of a fault is driven
up and over the other
• STRIKE-SLIP – occur where plates meet evenly and
slip past each other horizontally. (The angle at
which a fault cuts through the earth is referred to as
the strike, so a strike-slip fault happens when plates
slip along the strike).
– most common fault type in California
FAULT ANIMATIONS
seismographs
• Earthquakes generate
seismic waves which
can be detected with a
sensitive instrument
called a seismograph
3 types of waves
P-WAVE
S-WAVE
L-WAVE
P-WAVES
• PRIMARY WAVES
• COMPRESSIONAL WAVES (push)
• cause the rocks in the crust to be squeezed and
stretched
• the first waves received by a seismograph
station because they move the fastest
• These waves can move through solids, liquids,
and gases
S-waves
• SECONDARY WAVES
• SHEAR WAVES
• Cause the rocks to SNAKE, causing a ripping
(shearing) motion
• Slower than p-waves
• Can only move through solids
****S waves = Snake-like, Slow, Solids
P & S Wave Animations
L-waves
• LONGITUDINAL WAVES
• SURFACE WAVES
• when P & S-waves reach the surface, they
cause waves like ripples on the water when you
throw a rock in a pond
Wave speed
• Determined by the characteristics of the
object the earthquakes wave is moving
through!
• The more solid the material, the faster the
waves can move through it (but remember,
P-waves will always move faster than Swaves)
Movement of Waves Inside the
Earth
• The velocity of the P-waves and S-waves increase as
they travel deeper into the Earth
• The boundary between the crust and the mantle is a
thin layer of denser rocks. Because of this increase in
density, the P & S-waves increase in velocity.
• This layer is called the Mohorovicic discontinuity or
Moho
…Wave movement…
• When the waves reach the outer core, the P-waves
slow down and the S-waves stop.
• S-waves can’t travel through liquids, and P-waves
move easiest through solids.
• **The change in velocity is evidence that the outer core
is liquid
• After passing through the outer core, the P-waves
increase in velocity as they pass through the solid
inner core to the other side of the Earth
…Wave movement…
• Seismograph stations on the opposite side of the earth
from the epicenter only receive P-waves
• Some seismograph stations do not receive any waves.
• This is because the P-waves bend as they travel through
the liquid outer core, and the S-waves don’t make it
through
• This area where no waves reach is called the SHADOW
ZONE
• The location of the shadow zone depends on the location
of the epicenter
Locating an Epicenter
• The location of an earthquake epicenter can be found by
using the measurement of seismic waves from 3
seismic stations and a P-wave/S-wave Time Travel
Graph
– (Reference Tables pg 11)
• Epicenters are found by drawing circles around each
seismograph station that are the exact distance that
was determined using the Time Travel Graph.
• Where all three circles meet is the location of the
epicenter (LAB)
Things to remember
• P-waves always arrive before S-waves
• The farther a seismograph is from the epicenter of
the earthquake, the greater the time difference
between the arrival of the P-waves and S-waves
• Knowing the distance from the epicenter of one or
two seismographs will only give you a rough area of
where the earthquake occurred. To find the exact
position of the epicenter, you must have three
seismograph station recordings
Earthquake Strength and Damage
• Seismographs can also be used to determine
the magnitude (strength) of an earthquake
• The Richter Scale is used to measure the
amount of energy released by an earthquake
Richter Scale
• rating scale from 1 to 10
• Each increase on the Richter Scale is an increase of
10 times in size of the earthquake
– Ex. an earthquake measuring a 5 is 10 times
stronger than a 4, and 100 times stronger than a 3
• Each increase is also 32 times greater in energy
Damage &
prediction
• ground shaking causes earthquake damage
– The shaking, depending on severity, can cause building
foundations to fail, gas and water lines to rupture,
electrical disruptions, avalanches, etc
• Earthquake prediction is important to determine
areas where damage is likely to occur
– This allows emergency services to develop evacuation
plans, building codes to build structures to withstand an
earthquake, etc
Partial collapse of the 'Million Dollar Bridge' into the Copper River in
Alaska following the 1964 earthquake. (9.2 magnitude)
Evidence of LIQUIFACTION –
The ground literally becomes liquid
due to the amazing force of the
earthquake waves. Happens in
areas that have clay and silt soils.
This was a communications building in Spitak during the 1988 Armenian
earthquake.
Elementary School torn apart by a landslide that occurred during the 1964 Alaska
earthquake. Notice the large crack in the foreground that also was caused by the
earthquake. (9.2 magnitude)
Epicenter of the Manhattan earthquake on 17 January
2001 is plotted on an aerial photograph
US Earthquakes 1973 - 2002