Download S waves

An Earthquake is a rapid
vibration or shaking of the
Earth’s crust created by a release
in energy from sudden movement
of a part of a plate along a fault.
Energy released radiates in all directions
from its source, the focus.
So the Focus is the point at which the
first movement occurs during an
earthquake.
(Where the earthquake originates from).
The epicenter is the point on Earth’s
surface directly above the focus of an
earthquake.
Energy propagates in the form of
seismic waves.
So seismic waves are the energy that is
released from the earthquake.
A Fault is a fracture or break in
rock (in the lithosphere) where
movement is occurring or has
occurred.
Faults can be active or inactive, and
can be associated with either
current or old plate boundaries.
Label the focus,
epicenter and
fault for each
pictures.
Where do Earthquakes occur?
Earthquakes Occur at Different Depths:
•Shallow depths along transform boundaries
•Shallow to Medium depths (up to 30km bgs)
along divergent boundaries
•Deep depths (up to 700km bgs) along subduction
boundaries
What causes an Earthquake??
So, how does energy released by
slippage at a fault travel through
the ground?
Energy from
an Earthquake
travels in
seismic
waves.
2 Main Types of Waves:
• Body Waves – earthquake waves that travel
from a earthquake through the Earth.
• Surface Waves – earthquake waves that
travel along the Earth’s surface.
Body Waves
• P waves – (Primary Waves) compression waves
that squeeze and stretch rock materials as they move
– Can travel through any material - solid rock, magma, ocean
water, air
• S waves – (Secondary Waves) shear waves that
cause the particles of rock material to move at right
angles to the direction the waves are traveling.
– Can travel through solid material only (CANNOT travel
through liquids or gas)
– S waves travel a little more than ½ the speed of a P wave (so,
s-waves are slower than P waves)
P Wave (Compression Waves) move like a slinky
S Wave (Shear Waves) seem more like a worm or snake
S-Wave Motion – “Side-to-Side” Motion
Surface Waves – earthquake waves that
travel along Earth’s surface
• Travel more slowly than P or S waves and
can move:
– side to side (Love Wave)
– Elliptical pattern (Rayleigh Wave)
• Can cause considerable damage even far
from the earthquake’s epicenter
Surface Waves – earthquake waves that travel along Earth’s surface
Two Types: Love & Rayleigh
How the seismic waves move
through Earth’s Interior:
• Depths of Earth’s Interior are different
densities, wave velocities corresponds to
interior of Earth
– Velocity of waves increases when waves travel
through dense material
– Velocity of waves decreases when waves travel
through less dense material.
So think back to our layers of the Earth…
Solid = inner core, lithosphere,
(upper parts of mantle), crust
Liquid/melted = outer core,
Asthenosphere, (lower parts
of mantle)
How the seismic waves move
through Earth’s Interior:
– Boundary of core and mantle is at 2,900km below
the surface – Here P waves are slowed and S waves
stop.
• So material below 2,900km must be liquid
(because S waves CANNOT travel through
liquids)
– The inner core and outer core boundary is at a
depth of 5,200 km, here P waves increase in speed
again (because inner core is solid like the
lithosphere)
How the seismic waves move through
Earth’s Interior: Terms that describe
layers of Earth in reference to earthquakes
• Moho – boundary where dense mantle rock meet less
dense crust rock. (32km under continents, 5-10km oceans)
• Transition Zone –middle mantle at depth of 400-670km
(separates upper mantle from denser lower mantle)
• Shadow Zone -the wide belt on other side of Earth
opposite the focus where seismograph stations do not
receive waves. (because waves are bent/refracted as they
pass through layers/materials of different densities).
Transition Zone
Terms that
describe layers of
Earth in reference
to earthquakes
Moho
Shadow Zone
(shaded area)
Label which is S wave & which
is P wave.
P wave
(Compression)
S wave
(Shear)
Draw how S & P waves would travel through layers, also
label the Moho, Transition Zone and Shadow Zone:
1st Remember P waves travel through: Any material
S waves travel through: Solid ONLY
Draw how S & P waves would travel through layers,
also label the Moho, Transition Zone and Shadow
Zone:
Focus
Locating and Measuring Earthquakes
Geologists use a seismograph to detect and
record waves of an earthquake.
Richter Scale assigns a single number to
quantify the energy contained in an
earthquake. (It’s the scale the measures the
magnitude of the earthquake.
Seismograph
Seismograph: A device that records
earthquake waves.
Seismogram: The “picture” drawn by a
seismograph.
• Which type of wave would reach the seismograph
first (because it travels faster than others)?
P Wave (it travels faster and through all materials)
• Label which is the P wave and which is the S wave
on the seismograph to below and to the right.
P wave
S wave
The Richter Scale measures the magnitude of
an earthquake. It is a base-10 logarithmic
scale based on the waves measured by a
seismograph.
Example: An earthquake that measures 5.0 on
the Richter scale has a shaking amplitude 10
times larger.
To locate the epicenter of an
earthquake scientist must know
the distances from at least three
(3) different stations
How far is the epicenter of an earthquake
from a seismic station, if the difference
between the arrival time of the P and the S
wave is 5 minutes?
Draw the circles showing distances from each
seismograph station to the epicenter of the earthquake to
determine where the epicenter actually occurred.
Epicenter of this earthquake is?
(Reno – 200 miles, Chicago – 2,000 miles, Phoenix – 700 miles)
200
600
1000 (miles)
Hazard and Damage Caused by
Earthquakes:
• Most buildings can withstand fairly violent up-anddown shaking
• Few buildings can survive side-to side shaking (so
many collapse)
• Liquefication is when loose soil temporarily takes
on some of the properties of liquid
– building on top of this loose soil that undergoes
liquification or settling may cause collapse in buildings
Hazard and Damage Caused by
Earthquakes:
• A large Earthquake can be followed by a series of
smaller earthquakes called aftershocks. Aftershocks
can still cause damage and be fairly strong and their
frequency diminishes quickly over time.
• When large earthquakes occur they may rupture
(break) gas lines, which could cause fires.
• Underwater earthquakes and landslides can sometimes
cause huge ocean waves called tsunamis.
– Indonesia, Sri Lanka, Thailand (Indian Ocean)
killed over 200,000 people on December 26, 2004.
Preventing Earthquake Damage –
What can we do??
• Implement and enforce good engineering practices
and building codes so that buildings can withstand
a certain amount of shaking (especially in
earthquake prone areas)
• Try to predict when, where, and at what
magnitude an earthquake will occur. (By using
data from past, and plotting earthquake foci along
a fault to see where the fault has not moved, thus
being where stress is building).