Download Chapter 4: Earthquakes

Chapter 4: Earthquakes
4.1 Forces in Earth’s Crust
Stress
• Stress is a force that acts on rock to change its
shape or volume.
• Energy caused by stress is stored in rocks until
it changes shape or breaks.
Stress
• Tension: stress force that pulls on the crust
and thins rocks in the middle; occurs when
plates pull apart.
• Compression: stress force that squeezes rock
until it folds or bends; occurs when plate
comes together.
• Shearing: stress that pushes a mass of rock in
opposite directions; occurs when plates slide
past each other.
Normal Faults
• formed from rocks being pulled apart at
divergent boundaries
• one block, called the hanging wall, sits over
the fault and the other block, called the
footwall, sits beneath the fault.
• The hanging wall slides down and the footwall
moves up.
Reverse Faults
• formed from rocks being pushed together at
convergent boundaries
• The hanging wall moves up and the footwall
moves down.
Strike-slip Faults
• The rocks on either side of the fault slip past
each other at a transform boundary.
• There is little up and down movement.
Folding Earth’s Crust
• Folds are bends in rock that form from
compressions shortening and thickening the
crust.
– Upward fold into an arch – anticline
– Downward fold into a V shape – syncline
Forming Mountains
• Compression from convergent plates colliding
can build mountain ranges.
• Fault block mountains form from tension in
Earth’s crust and faulting.
– If there are two normal faults near each other, the
land between could eventually drop down
creating a valley.
Plateaus
• Plateau: large area of flat land elevated high
above sea level.
• Forces push up large, flat blocks of rock.
Chapter 4: Earthquakes
4.2 Earthquakes and Seismic Waves
Seismic Waves
• Earthquake: shaking and trembling that
results from movement of rock beneath
Earth’s surface.
• Plate movements and stress in Earth’s crust
can lead to earthquakes, releasing great
amounts of energy.
Seismic Waves
• Seismic waves: vibrations that are similar to
sound waves; travel through Earth carrying
energy from earthquakes.
• The focus of an earthquake is the area
beneath Earth’s surface where the rocks are
shifted (action that causes Earthquake).
• The epicenter is the point on Earth’s surface
above the focus.
Seismic Waves
P waves (primary waves)
• compress and expand the ground
• first waves/fastest moving
• travel through solid, liquid, and gas
Seismic Waves
S waves (secondary waves)
• travel at half the speed of P waves and arrive
second
• travel through solids only
• strong enough to shake structures
Seismic Waves
Surface waves
• move most slowly
• produce severe ground movements; can cause
ground to roll or buildings to shake back and
forth.
Measuring Earthquakes
• Seismograph: instrument that records and
measures the seismic waves.
Measuring Earthquakes
Modified Mercalli Scale
• Rates the amount of shaking
• Rated by observations; useful in areas where
there aren’t instruments available.
Measuring Earthquakes
Richter Scale
• Finds the magnitude; a number assigned to an
earthquake based on size.
• Depend on the size of the waves recorded by a
seismograph.
Earthquake Magnitude
• Bill Nye!
Measuring Earthquakes
Moment Magnitude Scale
• Rates the total energy an earthquake releases.
• Geologists use data from seismographs and
other sources.
Comparing Magnitudes
• A 1-point increase on a scale represents an
increase of 32-times more energy.
• The higher an earthquake is rated, the more
energy it has and the more damage it can do.
Locating an Earthquake
• You need seismogram info from 3 places:
– Find the difference of arrival times
between the P and the S waves.
– The difference is used to find the distance
to the epicenter – the longer it takes, the
farther away the epicenter.
– A circle is drawn around each point
(location). The point where they all cross is
the epicenter.
Chapter 4: Earthquakes
4.3 Monitoring Earthquakes
Earthquake Risk
• Largely depends on how close a given location
is to a plate boundary.
• In N. America, there are plate boundaries in
California, Washington, and Alaska.
• Around the world, the Ring of Fire contains
boundaries where several volcanoes and
earthquakes occur.