Transcript Earthquakes Lecture

Earthquakes
Earth Science
What is an Earthquake?
A
vibration of the Earth produced by a rapid
energy release
 The point within the earth where an earthquake
starts is called the focus
 Energy
travels from the focus in all directions- think of
a stone being tossed into a pond
 Stone sets waves of motion in water just as seismic
waves move through earth
The point on the surface of the Earth above the focus is
called the epicenter

The Epicenter
The epicenter is the
location directly above the
focus
Earthquakes over a 10 yr. period

Pacific Ring of fire
Faults
 Faults
are fractures in the Earth where plate
movement has occurred
 There are three types of forces that create faults.
 Compression
- squeezes
 Tension - stretches
 Shear – causes slippage
 Each
of these three forces can create a different
kind of fault.
Earthquake causes
 Elastic
 Rock
Rebound hypothesis
is bending causing elastic stress
 Eventually the rock will slip and break at the weakest
point (the focus)
 The vibrations we feel occur as the rock returns to its
original shape
Types of faults
 Normal
Faults- caused by tension forces
 Reverse Faults- compression
 Strike-Slip Faults- shear forces
Normal Faults
Tension causes rocks to be
pulled apart.
 Rock above the fault
surface moves downward
in relation to rock below
the fault surface.
 Can help to form
mountains such as the
Sierra Nevada Mountain
Range

Reverse Fault
Compression causes rock
to break.
 Rocks above the fault
surface are forced up and
over the rocks below the
faults.
 Can form mountains such
as the Rockies.

Strike-Slip Fault
Two of Earth’s plates
move past each other due
to shear forces.
 There is very little vertical
movement
 Ex: San Andreas Fault

Earthquake Info
Seismic waves – waves
that move through the
Earth’s crust
 Seismic waves travel
outward from the focus of
an earthquake
 Surface Waves- these
travel along Earth’s outer
layer


Body waves- travel
through Earth’s interior
Primary Waves (P) –
waves that travel in the
same direction the wave is
moving
 Secondary Waves (S) –
move through the Earth
causing particles to move at
right angles to the direction
of the wave.

Wave Facts
Surface waves (S) are the slowest waves, primary (P Waves) are
the fastest. P Waves can only travel through solids.
 P waves travel 1.7 times faster than S waves


Surface waves cause the most destruction during an
earthquake.
Measuring Earthquakes
 Seismologists
use seismographs to measure
earthquakes.
 The more energy that an earthquake releases, the
greater its magnitude.
 One of the largest earthquakes occurred in Haiti
very recently and killed over 200,000 people
Finding the Epicenter- Distance




Remember P Waves travel faster
than S waves
The greater the distance between
the P wave and S wave the
farther the source
1) find the time interval between
the first P wave and first S wave
using the seismogram
2) use the graph to find distance
from the seismogram
Finding the Epicenter- Direction
 Can
use 3 or more
seismograms to do this
 Create a circle using
direction found on
graph
 Where circles intersect
is the exact location of
the epicenter
Richter

Earthquakes are ranked on
the Richter Scale –
measures amount of
energy released
1-10
 For each increase of 1.0,
amplitude of the largest
surface wave is 10 times
greater
 About 32 times as much
energy is released

Tsunami
 Means
“Harbor Wave”
 If an earthquake happens underwater, it can create
a huge wave.
 This wave travels to the surface
 After it reaches the surface, it can spread out for
thousands of kilometers in every direction
 As the wave approaches the shore, it can form a
towering crest that can reach 30m high.
Earthquake Safety
Be prepared – know if an
area has had earthquakes
in the past or if it is near a
pate boundary.
 Make your home
earthquake safe

Take heavy objects down
from high shelves
 Hot water heaters and gas
appliances should be
securely in place
 During earthquakes, stay
away from windows or
anything that could fall on
you.

Living on a Fault

Seismic Safe structures
Resistant to vibrations
 Buildings with bases of alternating rubber and steel can
withstand quakes of up to 8.3 on the Richter scale without major
damage

Buildings and highways can be made so they will bend but
not break
 Seismic safe structures are more expensive to build than
regular structures.

More Earthquake Damage
Anchorage, Alaska 1964 – Magnitude 9.2
Oakland, CA 1989
Northridge, CA 1994 – 62 killed
Former hospital in Mexico City 1985 –
Magnitude 8.1 with 7.5 aftershock 18 hours
later
Turkey 1999