Download Chapter 8

Chapter 8
Earthquakes and the Earth’s Interior
8.1 What In an Earthquake?
• Key Concepts
1. What is a Fault?
2. What is the cause of earthquakes?
8.1 What In an Earthquake?
• Earthquake
– The vibration of Earth produced by the rapid
release of energy within the Lithosphere.
• Cause of Earthquakes
– Slippage along a break in the lithosphere “Fault”
• Fault – fractures in the Earth where movement has
occurred.
8.1 What In an Earthquake?
• Focus
– The place where an earthquake starts
• Energy travels in all directions from the focus in the
form of “seismic waves”
– Seismic waves travel much like wave ripples on a pond.
• Epicenter
– Location on the surface directly above the focus.
8.1 What Is an Earthquake?
8.1 What Is an Earthquake?
• Faults and Changes to Earth’s Surface
– Land along a fault can shift up to tens of meters in
a single earthquake.
• Over time this can
– Push up coastlines, mountains, and plateaus.
– Land movements can be:
• Vertical (uplift) – can form “Fault Scarps”
• Horizontal (offset)
8.1 What Is an Earthquake?
8.1 What Is an Earthquake?
8.1 What Is an Earthquake?
• San Andreas Fault
– One of most studied faults in the world.
– Extends 1300 km through California into the
Pacific ocean.
– Has produce many large earthquakes
• ex: 1906 San Francisco earthquake moved the land 4.7
meters.
8.1 What Is an Earthquake?
• Cause of Earthquakes
– Deformation of Rocks
• Forces inside the Earth slowly change the shape (bend)
rocks and deform them.
– Elastic Rebound
• Tendency for the deformed rock along a fault to spring
back after an earthquake.
8.1 What Is an Earthquake?
• Elastic Rebound Hypothesis
– Most earthquakes are produced by the rapid
release of energy stored in rock that has been
subjected to great forces. When the strength of
the rock is exceeded, it suddenly breaks, releasing
some of its stored energy as seismic waves.
8.1 What Is an Earthquake?
8.1 What Is an Earthquake?
• Aftershocks and Foreshocks
– Not all energy is released by one single large
earthquake.
• Foreshocks – small earthquakes produced before a
larger quake.
• Aftershock – smaller earthquakes which are produce
after (minutes – weeks) a large quake.
8.1 What Is an Earthquake?
Key concepts
1. What is a Fault?
•
Faults are fractures in the Earth along which
movement has occurred.
2. What is the cause of earthquakes?
• According to the Elastic Rebound Hypothesis,
most earthquakes are produced by the rapid
release of energy stored in rock that has been
subjected to great forces. When the strength of
the rock is exceeded, it suddenly breaks, releasing
some of its stored energy as seismic waves.
8.2 Measuring Earthquakes
• Key Concepts
1.
2.
3.
4.
What are the 2 categories of seismic waves?
How are seismic waves recorded?
How is the size of an earthquake measured?
How is an earthquake epicenter located?
8.2 Measuring Earthquakes
• Seismic Waves
– 2 Main Types
• Surface waves – travel along Earth’s surface.
• Body Waves – travel through Earth’s interior.
8.2 Measuring Earthquakes
• Body Waves
– 2 Types
• P Waves (push waves)
– Compressional Type waves
– Travel faster than S Waves
– Travel through both liquids and solids
• S Waves (shake waves)
– Transverse waves: energy travels at right angles to matter.
– Slower than P waves
– Only travel through solids.
8.2 Measuring Earthquakes
• Surface Waves
– Caused when Body Waves reach the surface.
– Travel more slowly than Body Waves.
– Larger than Body Waves.
– Travel up and down as well as side to side.
– Most destructive Seismic Waves.
8.2 Measuring Earthquakes
• Recording Seismic Waves
– Seismograph: Instrument used to record seismic
waves.
• Greek roots
– Seismo: Shake
– Graph: Write
8.2 Measuring Earthquakes
8.2 Measuring Earthquakes
• Seismogram
– Produces a time record of motion during an
earthquake.
• Stronger the quake the larger the waves on the
seismogram.
• Shows both P and S waves.
8.2 Measuring Earthquakes
8.2 Measuring Earthquakes
• Measuring Earthquakes
– Magnitude (M) is a measure of the size of seismic
waves.
– 3 Scales used to measure earthquakes
1. Richter Scale – Quake magnitude
2. Moment Magnitude – Quake magnitude
3. Modified Mercalli Scale – Quake intensity
8.2 Measuring Earthquakes
• Richter Scale
– Familiar but outdated
– Based on height of largest P or S wave
– Increase of 1 = ten fold increase
• Ex: M 5.0 is ten times greater than M4.0
– Usually only useful for shallow quakes (>500 km)
due to waves getting weaker the farther they
travel
– Not used by most Scientists
8.2 Measuring Earthquakes
• Moment Magnitude
– Used by most Scientists
• Estimates energy released by earthquake
– Derived from the amount of displacement at the
fault.
• Calculate with
–
–
–
–
Seismographic data
Movement along fault
Area of surface break
Strength of rock
8.2 Measuring Earthquakes
8.2 Measuring Earthquakes
• Modified Mercalli Scale
– Measure of earthquake’s effects at different
locations
– 12 Step scale express in Roman Numerals
• I Barely Felt – XII Total Destruction
– Same earthquake can have different Mercalli Scale
ratings at different locations.
8.2 Measuring Earthquakes
• Locating an Earthquake
– Differences in the travel time of P & S waves is
used to determine the distance from the epicenter
– 3 Seismograph locations are then used with a
globe to pin point earthquake location.
8.2 Measuring Earthquakes
8.2 Measuring Earthquakes
8.2 Measuring Earthquakes
• Key Concepts
1. What are the 2 categories of seismic waves?
•
Earthquakes produce 2 main types of seismic wave – body
waves and surface waves.
2. How are seismic waves recorded?
•
Scientists have developed an instrument to record seismic
waves – the seismograph
3. How is the size of an earthquake measured?
•
The Richter Scale & Moment Magnitude Scale measure an
earthquakes magnitude. The Modified Mercalli Scale
measures quake intensity.
4. How is an earthquake epicenter located?
•
A time travel graph, data from seismographs made at 3
locations, and a globe can be used to determine the epicenter.
8.3 Earthquake Hazards
Key Concepts
1. What are the major hazards produced by an
earthquake?
2. How can earthquake damage be reduced?
8.3 Earthquake Hazards
• Causes of Earthquake damage
– Types of Earthquake related Hazards
1.
2.
3.
4.
Seismic shaking
Liquefaction
Landslides and mudslides
Tsunamis
8.3 Earthquake Hazards
• Seismic Shaking
– Ground vibrations caused by seismic waves
• Can jolt and twist structures
1.
2.
Unreinforced concrete, block, or brick structures can fail
Wood structures may stay in tact but can be jolted off their
foundations
– Worst damage occurs near epicenter or on loose
soil or filled land
8.3 Earthquake Hazards
8.3 Earthquake Hazards
• Liquefaction
– The liquefaction of once solid soil due to shaking.
– Results of Liquefaction:
1. Buildings, bridges, dam, etc… to fail.
2. Underground storage tanks and sewer line can come
to the surface
8.3 Earthquake Hazards
8.3 Earthquake Hazards
• Landslides & Mudflows
– Mass movements of soil which can cover large
areas.
– Usually occur in areas with steep slopes and loose
soil.
• Landslides – loose sediments and rock.
• Mudflows – soil with high water content
8.3 Earthquake Hazards
8.3 Earthquake Hazards
• Tsunamis
– Wave formed when the ocean floor shifts
suddenly
– Earthquake pushes up slab of ocean floor along a
fault line.
– Cause little damage at sea
– Cause major damage when the wave hits land.
8.3 Earthquake Hazards
8.3 Earthquake Hazards
• Reducing Earthquake Damage
– Ways to reduce earthquake damage and loss of
life.
1. Determine earthquake risk

Seismic gap: area along a fault with no quake activity for a
long period of time
2. Build earthquake resistant structures
3. Follow earthquake safety precautions
4. Tsunami warning and evacuation
8.3 Earthquake Hazards
Key Concepts
1. What are the major hazards produced by an
earthquake?
•
Earthquake related hazards include: seismic
shaking, liquefaction, landslides & mudflows, and
tsunamis
2. How can earthquake damage be reduced?
•
Earthquake damage and loss of life can re reduced
by determining the earthquake risk for an area,
building earthquake resistant buildings &
structures, and following earthquake safety
precautions
8.4 Earth’s Layered Structure
Key Concepts
1. What are the Earth’s layers based on
composition?
2. What are Earth’s layers based on physical
properties?
3. How did scientist determine Earth’s structure
and composition?
8.4 Earth’s Layered Structure
Layers of Earth
 Crust
 Mantle
 Core
8.4 Earth’s Layered Structure
Layers Defined by Composition
 Crust
•
Thin rocky outer layer (8 – 75km)
–
–
Continental Crust
» Mostly granitic
» Less dense
» Old (up to 4 billion years)
Oceanic Crust
» Mostly basaltic
» Less dense (2.7 g/cm3 average)
» Younger (180 million years)
8.4 Earth’s Layered Structure
Layers Defined by Composition
 Mantle
•
82 % of Earth’s volume
–
Upper Mantle mostly Peridotite (3.4 g/cm3)
8.4 Earth’s Layered Structure
Layers Defined by Composition
 Core
•
Sphere composed mostly of Iron-Nickel Alloy (13
g/cm3)
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
• Earth can be divided into layers based on
physical properties into 5 layers
1.
2.
3.
4.
5.
Lithosphere
Asthenosphere
Lower Mantle
Outer Core
Inner Core
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
Lithosphere
– Earth’s outer most layer
• Composed of crust and upper most mantle
• Rigid
• 100 km+ thick
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
Asthenosphere
– Lies just under lithosphere
• Softer and weaker than lithosphere
• Rocks are hot (near their melting point) which makes
then easy to deform (change).
• Lower Lithosphere and Asthenosphere are
both part of the upper mantle.
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
Lower Mantle
– Lies at a depth of 660 km to base of mantle
– More rigid than upper mantle
– Hot
– Rocks capable of gradual flow
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
Outer Core
– Liquid mostly iron-nickel alloy
– 2260 km thick
– Generates earth it’s magnetic field
8.4 Earth’s Layered Structure
Layers Defined by Physical Properties
Inner Core
– Sphere with radius of 1220km
– Solid and compressed (very high pressure)
8.4 Earth’s Layered Structure
• Discovering Earth’s Layers
– During the 20th century, studies of the paths of P
& S waves through the Earth helped scientists
identify the boundaries of the Earth’s layers and
determine that the outer core is liquid.
8.4 Earth’s Layered Structure
• Andrija Mohorovicic
– 1st person to use seismic
waves to describe Earth’s
interior.
• Mohorovicic’s
Discontinuity – “Moho”
8.4 Earth’s Layered Structure
– P Waves
• Travel trough liquids & solids, and travel fast.
– S Waves
• Only travel through solids, and travel slow
– Reflection & Refraction of waves
– Reflection: waves bounce off
» Can show a “S Wave Shadow”
– Refraction: wave course is changed (bent)
» Can Show a “P Wave Shadow”
8.4 Earth’s Layered Structure
• Discovering Earth’s Composition
– To determine the composition of Earth’s layers,
scientists studied seismic data, rock samples from
the crust and mantle, meteorites, and high
pressure experiments on Earth materials.
• Direct data – Drilling, etc…
• Indirect data – Speed of waves through different
materials, Comparison of meteorites to other Earth
material etc…
8.4 Earth’s Layered Structure
Key Concepts
1. What are the Earth’s layers based on composition?
•
2.
Earth’s interior consists of 3 major layers defined by their chemical
composition – crust, mantle, & core.
What are Earth’s layers based on physical properties?
•
3.
Earth can be divided into layers based on physical properties –
Lithosphere, Asthenosphere, Mantle, Outer Core, & Inner Core.
How did scientist determine Earth’s structure and composition?
•
•
During the 20th century, studies of the paths of P & S waves through
Earth helped scientists establish the boundaries of Earth’s layers &
determine that the outer core is liquid.
To determine the composition of Earth’s layers, scientists studied
seismic data, rock samples obtained by drilling, samples of lava
rock, and the materials that make up meteorites.