Science Starter- Answer the above!
WHAT IS AN EARTHQUAKE
AND HOW DO WE MEASURE
THEM?
Chapter 8, Section 8.1 & 8.2
Looking Back
What is an Earthquake
 How is it measured
 Seismograms/Seismograph
 Faults and Fault Types
 Richter Scale and Moment
magnitude

Let’s Review

List the three different seismic waves:
 Surface

What is a fault?
A

fracture in Earth where movement has occurred.
What is the source of an earthquake called?
 The

Waves, P Waves, and S Waves
focus
What order to the three basic types of seismic
waves reach a seismograph in?
P
Waves, S Waves, Surface Waves
Looking Ahead
Tomorrow we are going to start
Chapter 9 (Plate Tectonics)
 Vocabulary Quiz Chapter 8 on
October 31st!!
 Test Chapter 7 – 9 on November 4th!

Today’s Plan


Finish Sections 8.2 & 8.3
Bill Nye episode on Earthquakes
Destruction from Earthquakes

The Good Friday Alaskan Earthquake in 1964 was
one of the most violent earthquakes to jar North
America in the 20th century
 Moment
Magnitude of 9.2
 Lasted 3-4 minutes
 Killed 131 people
 Thousands homeless
 Affected major ports and towns
 Luckily the schools and businesses were closed so the
damages were not higher!
Good Friday Quake Damage
Good Friday Quake Damage
Seismic Vibrations


The 1964 Alaskan Earthquake gave geologists new
insights into the role of ground shaking as a
destructive force.
The damage to buildings and other structures from
earthquake waves depends on several factors:
 Intensity
of vibrations
 Duration of vibrations
 Nature of material on which the structure is built
 Design of the structure
Building Design


All multistory buildings in Anchorage, Alaska, were
damaged by the vibrations of the earthquake.
However the more flexible wood buildings like
homes were less damaged.
 Steel

also withstands vibrations.
Engineers have learned that unreinforced stone or
brick buildings are the most serious safety threats
during earthquakes!
 Why
do you think that is?
Building Design
Brick
Steel
Liquefaction



Loosely consolidated sediments are saturated with
water.
When an earthquake shakes these sediments a
process called liquefaction can occur.
Stable soil turns into a liquid that is not able to
support buildings or other structures.
 May
settle and collapse
 Underground storage tanks may float toward the
surface.
Liquefaction in action

http://www.youtube.com/watch?v=u8hfCN6k3YE
Liquefaction in Action

http://www.youtube.com/watch?v=j0sLyJpfTE8
Tsunamis

Most death’s associated
with the 1964 Earthquake
were caused by seismic
sea waves (tsunamis).




Indonesia 2004
Japan 2011
Tsunami means “harbor
wave”
Often called tidal waves

Incorrect because these
waves are not caused by
tides.
Causes of Tsunamis



A tsunami triggered by an earthquake occurs where
a slab of the ocean floor is displaced vertically
along the fault.
Can also occur when the vibration of a quake sets
an underwater landslide in motion.
Other causes (no common):
 From
land landslides
 Meteor Strikes
Causes of Tsunamis


Once formed, a tsunami resembles ripples (like
when you throw a pebble into a pond).
Tsunami travels across the ocean at speeds of 500
to 960 kilometers per hour (310-600mph)
 Tsunamis
in the ocean often pass without notice because
they are usually less than 1 m high and the distance
between wave crests is anywhere from 100 to 700 km
apart.
 As wave enters shallow coastal water, the waves slow
down and water PILES UP to heights that sometimes are
higher than 30m (100ft).
Tsunami Formation
Tsunami Crests in the Ocean (Japan 2011)

http://www.youtube.com/watch?v=rUED6HeGJ7w
Tsunami Damage (Japan 2011)

http://www.youtube.com/watch?v=J2hUwFo6Vpc
Tsunami Comparison

Please write down
some observations you
notice about the
BEFORE DAMAGE and
AFTER DAMAGE of the
Tsunami in Japan
2011.
Here
Tsunami Warning System




The destruction from a large tsunami in the Hawaiian Islands led to
the creation of a tsunami warning system for the coastal areas of
the Pacific.
Large earthquakes are reported to the Tsunami Warning Center in
Honolulu from seismic stations around the Pacific.
Scientists use water levels in tidal gauges to determine whether or
not a tsunami has formed.
Within an hour of the reports, a warning is issued.



Usually enough time to evacuate all but the area closest to the epicenter.
Sometimes the epicenter is too close to land to do much.
Only one or two destructive tsunamis are generated worldwide each
year.

Only 1 tsunami every 10 years causes major damage and loss of life.
Other Dangers of Earthquakes


Vibrations from earthquakes cause other dangers!
Landslides:
 With
many earthquakes, the greatest damage to
structures is from landslides and ground subsidence (or
the sinking of the ground) due to vibrations.
 Violent shaking causes rocks and soil to fall on slopes
 Can cause large sections of ground to collapse, liquefy,
or subside.
 Can
cause foundations to collapse
 Rupture gas and water pipelines
Landslides in China
Fire



The 1906 San Francisco
earthquake reminds us of
the major threat of fire.
City contained mostly
wooden structures and
brick buildings.
Greatest destruction was
caused by fires that
started when gas and
electrical lines were cut.


Water lines also broken by
earthquake.
Fire couldn’t be stopped
Predicting Earthquakes

Short Range Prediction:


Goal of short-term predictions is to provide an early warning of
the location and magnitude of a large earthquake.
Researchers monitor possible precursors (things that precede and
may warn of a future earthquake).







Uplift of land
Subsidence of land
Strain in rocks near active faults
Water levels and pressures in wells
Radon gas emissions from fractures
Small changes in electromagnetic properties of rocks
But so far, short-range predictions of earthquakes have not
been successful!
Predicting Earthquakes

Long Range Forecasts:

Give the probability of a certain magnitude earthquake
occurring within 30-100+ years.


Based on the idea that earthquakes are repetitive or cyclical.





Important for updating building codes.
As soon as one is over, forces start building up again.
Scientists study historical records of earthquakes to see if there
are any patterns of recurrence.
Study seismic gaps: an area along a fault where there has not
been any earthquake activity for a long period of time.
Only LIMITED success in predicting earthquakes with longterm forecasts.
Scientists do not yet understand enough about HOW and
WHERE earthquakes will occur to make accurate longterm predictions.
INTERNAL STRUCTURE OF
EARTH
Earth’s Interior

Interior of Earth does not lie very far beneath our
feet but we cannot reach it.
 Deepest
well has only been drilled 12km into Earth’s
crust.


So how do we know what the inside of Earth looks
like?
Most knowledge of the interior comes from the study
of earthquake waves that travel through Earth!
Layers defined by Composition

If Earth were made out of the same materials throughout,
seismic waves would spread through it in a straight line at a
constant speed.




This is not the case!
Seismic waves reaching seismographs located father from
an earthquake travel at faster average speeds than those
recorded at locations closer to the epicenter.
General increase in speed with depth is due to increased
pressure, which changes elastic properties of deeply buried
rock.
The paths of seismic waves are refracted, or bent, as they
travel.

This is because of earth’s interior!
Layers defined by Composition

Earth’s interior consists
of three major zones
defined by its chemical
composition:
 Crust
 Mantle
 Core
Layers defined by Composition - CRUST

The crust, the thin, rocky outer layer of Earth, is
divided into:
 Oceanic
Crust
 7km
thick, made of the igneous rocks basalt and gabbro.
 Younger (180 million years or less)
 Density of 3.0g/cm3
 Continental
 8-75km
Crust
thick, average thickness is 40km
 Made of many rock types, but mostly granodiorite.
 Density of 2.7g/cm3
 Some parts are over 4 BILLION years old!
Layers defined by Composition - Mantle

Over 82% of Earth’s
volume is contained in
the mantle.


Solid, rocky shell that
extends to a depth of
2890 km.
Boundary between crust
and mantle represents a
change in chemical
composition.

Dominant rock in upper:
peridotite (3.4g/cm3)
Layers defined by Composition - Core


Sphere composed of
an iron-nickel alloy.
Extreme pressures so
the density is almost
13g/cm3
 13
times heavier than
water.
Layers defined by Physical Properties


Earth’s interior has a gradual increase in temperature, pressure, and
density with depth.
When a substance is heated, its chemical bonds weaken and its
mechanical strength is reduced.





If temperature exceeds melting point, the material’s bonds break and
melting begins.
But temperature is not the only thing that determines if something
melts!
Pressure also increases with depth and increases rock strength.
Depending on the physical environment (temp and pressure) a
material can act like a brittle solid, a putty, or a liquid.
Earth can be divided into layers based on physical properties:

Lithosphere, Asthenosphere, Outer Core, and Inner Core
Layers defined by Physical Properties –
Lithosphere and Asthenosphere

Earth’s outermost layer consists of the crust and the
UPPERMOST mantle and forms a relatively cool, rigid
shell called the lithosphere.


Averages about 100km in thickness.
Beneath the lithosphere is a soft, weaker layer known
as the asthenosphere.
Temperatures/pressure conditions that result in a small
amount of melting, but not a lot.
 Close enough to melting temperatures that they deform
easily. (Ex: hot wav is weaker than cold wax)


Lithosphere and Asthenosphere both part of upper
mantle.
Asthenosphere and Lithosphere
Layers defined by Physical Properties –
Lower Mantle

From a depth of about 660km down to near the
base of the mantle lies a more rigid layer called
the lower mantle.
 Despite
their strength, rocks of lower mantle are very
hot and still capable of gradual flow.

The bottom few hundred km of mantle (laying on
top of hot core) contains softer, more flowing rock
like that of the asthenosphere.
Layers defined by Physical Properties –
Inner and Outer Core


The core is composed mostly of an iron-nickel alloy is
divided into two regions with different physical
properties.
Outer core:
Liquid layer 2260km thick
 Flow of metallic iron in this layer generate Earth’s magnetic
field!



Without the magnetic field, our atmosphere would burn off!
Inner Core:
Sphere having radius of 1220 km.
 High temperature, but pressure is so high that the material is
compressed into a solid state.

Discovering Earth’s Layers

1909 – Croatian seismologist (Andija Mohovoricic)
presented evidence for layering within the Earth.



Studied seismic records and found that the velocity of seismic
waves increases abruptly below about 50km of depth.
This boundary separates the crust from the mantle and is
called the Mohorovicic discontinuity (Moho for short).
Another boundary was found between the mantle and outer
core.



Seismic waves from even small Earthquakes can travel around the
world.
Observed that p-waves were bent around the liquid outer core
beyond about 100 degrees away from an earthquake.
Outer core also causes p waves that ravel through the core to
arrive several minutes later than expected.

This region where bent P waves arrive bent is called the shadow zone.
Layers of Earth and Seismic Waves
Discovering Earth’s Layers




The bent wave paths can be explained if the core is
composed of material that is different from the
overlying mantle.
P waves bend around the core in a way similar to
waves being bent around the corner of a building.
Outer core does not stop P waves in the shadow
zone, but bends them.
S waves CANNOT travel through the outer core so
geologists concluded that this area is liquid.
Layers of Earth and Seismic Waves
Discovering Earth’s Composition


Early seismic data and drilling technology indicate
that the continental crust is mostly made of lighter
granitic rocks.
Until the late 1960s, scientists had only seismic
evidence they could use to determine the
composition of oceanic crust.
 Recovery
of ocean-floor samples was made possible
with the development of deep-sea drilling technology
 The crust of the ocean floor has a basaltic composition
Discovering Earth’s Composition

Composition of rocks of the mantle and core is
known from more indirect data:
 Lava
that reaches earth’s surface comes from partially
melted asthenosphere within the mantle sometimes.

Meteorites that hit earth provide with hints about
Earth’s composition.
 Assumed
to be made of the same material from which
Earth was formed.
 Composition
ranges fom metallic meteorites made of iron
and nickel to stony meteorites composed of dense rock
similar to peridotite.
Discovering Earth’s Composition

Earth’s crust contains a smaller amount of iron than
do meteorites so geologists believe that the dense
iron and other dense metals sank toward Earth’s
center during the planet’s formation.
 Lighter
substances may have floated to the surface to
create the less-dense crust.


Earth’s core is thought to be mainly dense iron and
nickel, similar to metallic meteorites.
Surrounding mantle is thought to be composed of
rocks similar to stony meteorites.
Let’s Review

What is the composition of Earth’s Core?
 Iron

and Nickel Alloy
What is the composition of Earth’s mantle?
 Peridotite

What destructive events can be triggered by an
earthquake?
 Landslides,

tsunamis, and fires
What is a seismic gap?
 An
area along a fault that has not had any earthquake
activity for a long period of time.