Transcript 19.1 Earthquakes

19.1 Earthquakes
Forces Within Earth
• Most earthquakes are the result of
movement of the Earth’s crust produced
by plate tectonics
• At the boundary between two plates,
stress often builds up
• Stress is the total force acting on crustal
rocks per unit area
• When stress > strength of rocks,
earthquake is felt
• 3 Types:
– Compression: tries to shorten a material
– Tension: tries to lengthen a material
– Shear: stress that causes a material to
twist
Types of Stress
Deformation
• The deformation of a material in response
to stress is called strain
• Elastic Deformation: when stress is low, it
is caused when a material is compressed,
bent or stretched; when the stress returns
to zero, the original rock shape is
maintained
• Plastic Deformation: more stress than
elastic deformation, this type of strain
produces permanent deformation
Faults
• A fracture or system of fractures along which Earth
moves
– Reverse faults form as a result compression.
• This causes rock on one side of a reverse fault to be pushed up
relative to the other side.
– Normal fault forms from the pulling apart of rock and
stretches the crust.
– Strike-slip faults are caused by horizontal shear. The
movement at a strike-slip fault is mainly horizontal and in
opposite directions, similar to the way cars move in
opposite directions on either side of a freeway.
Earthquake Waves
• The vibrations of the ground produced during an
earthquake are called seismic waves.
• Every earthquake generates three types of
seismic waves: primary waves, secondary
waves, and surface waves.
• Focus: the point IN earth where earthquake
waves originate
• Epicenter: the point ON Earth’s surface directly
above the focus
Earthquake Waves
3 Types of Seismic Waves:
1) Primary Waves: (P-waves, or Pressure waves)
similar to pushing a slinky in the direction in
which it is stretched
2) Secondary Waves: (S-waves) similar to holding
a jump rope in one hand and raising and
lowering it
3) Surface Waves (Love and Rayleigh): the most
destructive, cause the ground to move sideways
and up and down like an ocean wave
Because they happen INSIDE the earth, P-waves
and S-waves are also called body waves
How do we know all this about
the interior of the earth?
Determining what’s inside the earth
• Why does light change angles when you
look at a fish tank from various sides?
• Light waves change as they pass through
different mediums (media?)
Earth’s internal structure
In the same way, seismic waves
change speed and direction when at
the boundaries between different
materials.
Determining Earth’s Interior
• Because the speed of seismic waves depends
on temperature and composition, it is possible
to use seismic waves to determine the
composition of the earth.
• When an earthquake happens, seismometers
all over the earth receive the resulting waves at
different times.
Earth’s internal structure
• As P-waves and S-waves initially travel
through the mantle, they follow fairly direct
paths.
• When P-waves strike the core, they are
refracted, which means they bend.
Earth’s internal structure
• the back-and-forth motion of S-waves
does not travel through liquid.
• However the longitudinal motion of Pwaves DOES go through liquids
Earth’s internal structure
• The data collected for the paths and travel times
of the waves inside Earth led to the current
understanding that Earth has an outer core that
is liquid and an inner core that is solid.
Earth’s Composition
• Seismic waves change their speed as they
encounter boundaries between zones of
different materials.
• In general, the speed of seismic waves
decreases as temperature increases.