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Earthquakes Release Stored Potential Energy
When rocks break and an earthquake occurs a significant amount of
stored energy is released.
• The energy travels through Earth in the form of waves.
• These waves are called seismic waves.
• The waves start in the crust where the rocks break and travel
outward in all directions from this point.
• This outward traveling energy causes Earthquake damage.
Focus vs. Epicenter
• Focus:
• The focus is the starting point of an earthquake.
• It is where the rock actually snaps and motion occurs.
• Most of the time it is underground (as least 65 km down).
• Epicenter:
• The spot on Earth’s surface
directly above the focus.
• This spot experiences the
most energy during the
earthquake.
• After an earthquake, the
earthquakes position is
given by the epicenter
location.
Seismic Waves
• When there is movement at a fault, energy is
released in the form of seismic waves:
• The shaking occurs as the energy travels out from the focus.
• Like throwing a stone in a pond, the ripples of water move out in all
directions.
• There are two types of waves released
• Body Waves – travel through Earth
• Surface Waves – travel on Surface
Seismic Waves – Body Waves
• P-Waves – A fast moving compressional body wave that can travel through
both solids and liquids.
– Because they travel through liquids and solids, P-waves can travel through the liquid and solid interior of Earth and affect
locations on the other side of Earth.
Seismic Waves – Body Waves
• P-Waves (Primary – Think Slinky)
1. Primary waves: Arrive at a location before any other
wave.
2. Phastest waves: Travel at the highest speed (6km/sec.)
3. Push-Pull waves: Pushes & Pulls at rock as it travels through
it.
4. Pass Through solids & liquids – can travel through magma.
Seismic Waves – Body Waves
• S-Waves – A slower moving body wave that travels at right angles to the
primary wave and cannot travel through liquids.
– Because they travel through solids only, the S-waves do not travel
to the other side of Earth.
– When they hit the liquid magma interior of Earth, they reflectback eventually stopping.
Seismic Waves – Body Waves
• S-Waves (Secondary – Think Shaking a Rope)
1. Secondary waves: Waves that arrive second at a
location.
2. Slow waves: Travel at the slow speeds (about ½ of P wave
- 3km/sec.)
3. Shake waves: Vibrate and move up and down.
4. Solids-only waves: travel through solid rock only.
Seismic Waves – Surface Waves
• Surface Waves (Think ripples in water)
1. Waves that propagate through Earth’s surface
2. Created when P and S waves reach the surface
3. The surface rolls like an ocean wave.
4. They are the slowest type of waves.
• Surface waves are the most destructive!
Seismic Waves
We measure all of these seismic waves using a
seismograph
• It draws a zigzag pattern that depicts the seismic waves
motion.
• It creates a print out called a seismogram.
Seismic Waves
By looking at a seismogram, two conclusions can be made.
1. Speed of the waves.
2. Distance to the Earthquakes epicenter.
Seismic Waves
• P-waves are the first to arrive
• S-waves are the second to arrive
• Surface waves are the last to arrive
The greater the distance
between the S and P
waves, the further the earthquake epicenter.
Examples: Race car
racing a station
wagon…the longer the
race, the larger the distance between the
two cars
Seismic Waves
• What if you want to find the distance to an epicenter?
• Use a time-travel graph
• It shows the average travel times for P and S waves.
1. Find the time between
the P and S wave (either
from the seismogram, or
it may be given to you).
2. Use the time-travel graph
and find the spot on the
graph where the S and P
waves are separate by the
time you found in step 1
above.
3. Drop a line straight down
at the location and read
the distance off the graph.
Seismic Waves
• What if you wanted to find where the earthquake occurred?
• Once an earthquake occurs, you can determine its location.
• 3 seismograph stations must detect the seismic waves.
• Using the arrival time of the waves, the station can say that the earthquake
occurred within a certain radius (distance).
• Once 3 stations map out the radius, the exact epicenter is where the 3 circles
intercept each other.
This is referred to as
Triangulation:
Commonly used by
geologists to find the
locations of Earthquakes.
Seismic Waves
• Time lag between P-wave and S-wave indicate how far the
earthquake is from a seismograph station.
• The lag time tells us how far we are from an earthquake but not from
which direction.
Epicenter can be
any where on the
radius of the circle.
Seismic Waves
• At least 3 seismograph stations are needed to pinpoint the epicenter
of an earthquake.
Infinite number of
possible epicenters
Seismic Waves
• At least 3 seismograph stations are needed to pinpoint the epicenter
of an earthquake.
Two seismograph
stations narrow
possible epicenters
down to two.
Possible Epicenter
Possible Epicenter
Seismic Waves
• At least 3 seismograph stations are needed to pinpoint the epicenter
of an earthquake.
Epicenter
3 seismograph
stations narrow
possible epicenters
down to one.