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EARTHQUAKES
YEAR 7
SICHUAN
EARTHQUAKE
CHINA 2008
CHINESE
INSTRUMENT FOR
EARTHQUAKES
1. FORCES IN EARTH’S CRUST
 An earthquake is the shaking and
trembling that results from the
movement of rock in the crust.
 The movement of plates creates
powerful forces that squeeze or pull the
rock in the crust.
 These forces are stress.
 Stress change the shape and the
volume of rock.

Types of stress
1.
2.
3.
Shearing: stress that pushes a mass
of rock in 2 opposite directions
Tension: pulls on the crust,
stretching rock so that it becomes
thinner in the middle.
Compression: squeezes rock until
it folds or breaks
• Any change in the volume or shape of
Earth’s crust is called deformation.
A fault is a break in the crust where
slabs of crust slip past each other.
 Faults usually occur along plate
boundaries, where the forces of
plate motion compress, pull or shear
the crust so much that the crust
breaks.


There are 3 types: normal (tension)
reverse (compression)

strike-slip (shearing),
Hanging wall and Foot wall
STRIKE-SLIP FAULT
REVERSE FAULT
NORMAL
FAULT
The forces of plate movement can
change Earth’s surface
Anticline
 Syncline
 Fault-block
mountains
 Plateau

Fault-block mountains
Plateau :A large area of flat land
elevated high above sea level.
2. MEASURING EARTHQUAKES
Most earthquakes begin in the
lithosphere.
 The focus is the point beneath Earth’s
surface where rock that is under stress
breaks, triggering an earthquake.
 The point on the surface directly above
the focus is called the epicenter.




An earthquake produces vibrations
called waves.
These waves carry energy in all
directions.
Seismic waves are vibrations that travel
through Earth carrying the energy
released during an Earthquake (from the
focus, through Earth’s interior and
across the surface)

There are 3 types of seismic waves:
1. Primary waves (P): compress and expand
like an accordion. They cause building to
contract and expand. Travel through solids
and liquids.
2. Secondary waves (S): vibrate from side to
side as well as up and down. They shake
the ground back and forth. Only travel
through solids.
3. Surface waves: when P and S waves
reaches the surface, some of them become
surface waves. It produce severe ground
movements.
SURFACE WAVES

A seismograph records the ground
movements caused by seismic waves as
they move through the Earth.

The zigzag pattern of lines used to represent an
earthquake is called a seismogram. It is
produced by a seismograph.


Geologists use seismic waves to locate an
earthquakes epicenter.
Seismic waves travel at different speeds.



P waves arrive first followed by S waves
To tell how far the epicenter is from the
seismograph, scientists measure the difference
between the arrival times of P and S waves.
Then they draw at least 3 circles using data
from different seismograph set up at stations
all over the world. The point where the 3
circles intersect is the location of the epicenter.
Seismic wave arrival times
5
4.5
Difference
in arrival
time of P
and S
waves
(min)
4
3.5
3
2.5
2
1.5
1
0.5
0
0
1000
2000
3000
4000
Distance to Epicenter (km)
5000


Magnitude is a measurement of earthquake
strength based on seismic waves and movement
along faults.
There are 3 scales:
1.
2.
3.
The Mercalli scale: The level of damage at a given
place
The Richter scale: Measured by a seismograph
The Moment Magnitude scale:


Use by geologist today
Rating system that estimates the total energy
released by an earthquake


Below 5 consider small or cause little damage
Above 5 produce great destruction
4. EARTHQUAKE SAFETY
Geologist can determine earthquake
risk by locating where faults are
active and where past earthquakes
have occurred.
 Even with data from many sources,
geologists can’t predict when and
where a quake will strike.

Causes of earthquake damage
include
1. Shaking
2. Liquefaction
3. Aftershocks
4. Tsunamis.

SHAKING

Shaking by seismic waves can trigger
landslides or avalanches.

It can damage or destroy buildings
and bridges, topple utility poles, and
fracture gas and water mains.
LIQUEFACTION
 Occurs when an earthquake turns loose, soft
soil into liquid mud
AFTERSHOCKS

earthquake that occurs after a larger
earthquake in the same area
TSUNAMIS
When an earthquake
jolts the ocean floor,
plate movement
causes the ocean floor
to rise slightly and
push water out of its
way. If its strong
enough, the water
displaced by the quake
forms large waves.

EARTHQUAKE SAFETY
 Best way to protect yourself is to drop, cover
and hold.
 Indoors: Avoid windows, mirrors, wall
hangings…
 Outdoors: move to an open area such as a
playground. Avoid vehicles, power lines,
trees and buildings. Sit down to avoid being
thrown down.
SAFER BUILDINGS
New buildings must be made stronger
and more flexible. Older buildings need
to be modified.
 Base isolated building rests on rubber
pads or springs to absorb energy from
the quake.
 Making utilities safer (flexible joints and
valves on pipes)

Earthquake
Response of Baseisolated Building
Earthquake Response
of Conventional FixedBase Building