Earthquakes
Section 1 How and Where Earthquakes Happen
There are over 3,000,000 earthquakes each year. That
means that there is about 1 every 10 seconds.
Earthquakes are nature’s most destructive forces. They
are the shaking of the earth's crust caused by the
release of energy. The plates move on convection
currents in the mantle - stress is built up, must be
released, a shaking of the earth happens. A minor
cause of earthquakes is volcanoes.
Elastic rebound theory
1.
2.
3.
4.
Stress between two moving plates increase.
Plates bend with the stress.
Stress becomes too great – the plates snap or slip past each other.
Plates will go back to the original form, only in different positions.
The focus is the point source of an earthquake; the place where the
earthquake happened. Even thought the depth of the focus
varies, most continental quakes are classified as shallow; about
70km below, more than 70km is intermediate; 300-650 deep.
The ones that cause the most surface damage are the shallow ones
since the energy is closer to the surface.
The Epicenter is the point directly above the focus on the surface.
Seismic waves
Seismic waves are the earthquakes energies being transmitted through the
earth. They travel in all directions from the focus. They look like when
you drop a pebble into a pond.
There are two types of waves:
Body Waves: travel through the body of the medium (ground)
Surface Waves: are on the surface rather than inside the earth
They travel through the interior of the earth. The rate that a wave travels
depends on what they are traveling through. They bend depending on
the density of the layer, similar to the refraction of light waves. Body
waves are the first arriving tremors and aftershocks.
1.
P-Waves
- Primary waves; first waves
- Compressional waves (like sound); they squeeze and stretch
- Travel through all substances (gas, liquid, solids)
2.
S-Waves
- Sheer waves; secondary
- Moves at right angles; perpendicular (up and down)
- Cannot move through liquids (molten material) or gases below 2900km
- S waves are about 60% the rate of P waves
They move similar to water waves and travel just under the
surface. When P & S waves hit the surface sometimes areas
very far away from the epicenter will be affected. These
waves cause considerable damage because of their low
frequency, long duration and large amplitude.
1. Love Waves (L waves)
Move side to side perpendicular to the waves travel
direction. Looks like a snake traveling (horizontal shearing)
Speed approx 90% that of S waves
2. Rayleigh Waves
Move more slowly; 70% slower then S waves
Move matter elliptically (like pond ripples)
Also called ground roll,
Seismic waves are useful tools for scientists to explore. The type of material
the waves travel through affects the speed and direction of their
movement. By studying the speeds and directions of the waves, scientists
can learn more about what makes up the interior of the Earth.
Earth’s Internal Layers
3 Compositional layers
Crust
Mantle
Core
5 Mechanical
Lithosphere
Asthenosphere
Mesosphere (mantle)
Outer Core
Inner Core
The shadow zone is an area of the earth that receives no seismic
waves; because of the refraction of the waves going through
different densities of materials.
Refraction - the bending of waves because of density differences
Studying Earthquakes gave us the inferred layers of the earth. S
waves can’t go through liquid, so we concluded a layer must be
liquid since no S waves would occur in the shadow zone.
Convergent Oceanic Environments

When either an oceanic and oceanic plate collides one will subduct. An
earthquake will occur. The same is true when an oceanic plate collides
with a continental plate, an earthquake will also occur.
Divergent Oceanic Environments

The Mid-Ocean Ridge has earthquakes occurring all of the time because
the plates are pulling away from each other.
Continental Environments

When two continental plates converge, diverge or move horizontally
away from each other, they can build up pressure which will cause
quakes to happen and even mountains to be built.
This is an area with numerous, closely spaced faults. They form at
plate boundaries.
Earthquakes Away From Plate Boundaries
Not all quakes result from movement along plate boundaries. In 1811812 a series of quakes occurred near New Madrid, Missouri. The
waves were so strong that they were felt as far away is the Eastern
Coast and New England.
http://www.ldeo.columbia.edu/LCSN/recenteqs/Quakes/quakes0.html
Seismology – Is the study of the earthquakes and the seismic
waves
Recording Earthquakes
Seismograph
The instrument used to measure seismic waves
Seismogram
The record that is made of the waves
Since P waves are the fastest moving waves, they are the first to
appear on the seismograph. The S waves would be the
second to arrive and the last would be the surface waves.
Locating the Earthquake
To determine the distance to an epicenter, scientists analyze the arrival
times of the P and S waves. The longer the lag time between them,
the further away the actual earthquake occurred.
Time Travel graph
It shows the relationship between the arrival times of the P & S
waves and the distance to the epicenter.
How to locate the epicenter of an earthquake: You need three
seismographic stations to record the earthquake so you can
determine through triangulation.
Locating an earthquake epicenter if I know the difference in arrival
times of P & S waves I can determine the distance to the epicenter
Scientists study the amount of energy release and the change that they
caused. This is the study of the magnitude and intensity of an
earthquake.
Magnitude
It is the measure of the strength of earthquake. It is determined by
measuring the amount of ground motion caused by the earthquake.
Richter scale
It measures the ground motion from an Earthquake. It is used for
smaller earthquakes now.
Movement Magnitude
It measures the strength based on the size of the area of the fault that
moves, the average distance that the fault block moves and the rigidity of
the rocks in the fault zone. The larger the number, the stronger the
earthquake is.
Intensity
Intensity is the measure of the effect of an earthquake.
Modified Mercalli Scale:
Expresses the intensity in Roman numeral from I to XI and provides a
description of the effects of each Earthquake Intensity.
Most injuries from earthquakes are a result from
the collapse of buildings and other structures
along with other objects crashing about. Other
dangers can be explosions, floods, landslides
and fires.
A tsunami is huge ocean waves that are formed y submarine
earthquakes, volcanoes or landslides. The speed depends on
depth of water; at 4500 meters (usual ocean dept) can travel
750km/hr. At the shore it slows down and reaches enormous
height.
Foundation failure will happen due to the up and down
and side to side motion of the ground.
Liquefaction:
The ground will become liquefied due to the friction and
moisture content of the ground. This will make the
ground unstable for foundations, weak and unsafe.
After Shocks:
They are smaller earthquakes that occur after larger ones. They
can cause further damage to areas like fires from damaged
gas lines and power outages.
Earthquakes can happen anywhere. Some places will get more of them and
some areas more destructive one because of their proximity to
boundaries. By following safety measures some damages may not be as
bad.
Before an Earthquake
Make sure you have a plan to meet family members after the disaster. Learn
how and where to report damage. Make sure you have supplies and
flashlights in an area that you can get to in case of an emergency.
During the Earthquake
Stay Calm.
Take cover in a strong building under something sturdy. If you are outside,
move into an open area. Stay away from electrical wires and gas and
pipe lines. Don’t move in an area that is prone to have them. Make sure
you build structures that can withstand them. If you are in a car, stop the
car in an area that is not by buildings or on bridges.
After an Earthquake
Watch out for electrical problems or wires lying around. Don’t go back into
your home unless you are given an all clear to do so. Make sure that your
area is safe and free from aftershocks.
Scientists have studied earthquakes for a long time trying to predict when
they will occur. There is no reliable way to forecast them. Today
scientists are looking for small changes in the earth’s crust to see if they
can predict an earthquake.
Seismic Gaps
A seismic gap is an area along a fault that experienced only a few earthquakes
recently, but where strong ones have occurred in the past.
Foreshocks
Foreshocks are little earthquakes that precede an earthquake by a few
seconds. Not a reliable source for earthquake prediction.
Changes in Rocks
Scientists look for stress and strain in rocks. They look for magnetic changes,
natural gas seepage and other related cracks in the rocks in an area.
Reliability of Earthquake Forecasts
Very unreliable, they can only continue to study them.
1906 -
San Francisco 8.3
1964 -
Alaska, unrecorded because of the severity,
anywhere from an 8.5 to a 9.0
800 km of fault affected, vibrations continued for 18 months,
Over 10,000 aftershocks
1960 -
Chilean earthquake, 9.5
1995 –
Kobe Japan, 7.2 one of the worst in Japan’s history, killing 6,433 people and causing more
than $100 billion in damages.
2004 -
Sri Lanka 9.0-magnitude earthquake
between the Indonesian island of Sumatra and the Indian Andaman Islands
2008 -
China earthquake, 7.9 devastated the country just before the Olympics
http://earthquake.usgs.gov/eqcenter/eqarchives/year/byyear.php
Description
Richter
Magnitudes
ess than 2.0
Micro
0-2.9
Minor
0-3.9
Minor
0-4.9
Light
0-5.9
Moderate
0-6.9
Strong
0-7.9
Major
0-8.9
Great
0-9.9
Great
Earthquake Effects
Frequency of Occurrence
Micro earthquakes, not felt. About 8,000 per day
Generally not felt, but
About 1,000 per day
recorded.
Often felt, but rarely causes
49,000 per year (est.)
damage.
Noticeable shaking of indoor
6,200 per year (est.)
items, rattling noises.
Significant damage unlikely.
Can cause major damage to
poorly constructed buildings
over small regions. At most 800 per year
slight damage to welldesigned buildings.
Can be destructive in areas
120 per year
up to about 100 miles
across in populated areas.
Can cause serious damage
18 per year
over larger areas.
Can cause serious damage
1 per year
in areas several hundred
miles across.
Devastating in areas several
1 per 20 years
thousand miles across.