Chapter 8: Earthquakes
8.1 What is an Earthquake?
8.2 Measuring Earthquakes
8.3 Destruction From Earthquakes
8.4 Earth’s Layered Structure
Fault Types
Earthquake
Earthquake
An earthquake is a vibration of Earth produced by the rapid
release of energy.
An earthquake is a vibration of Earth produced by the rapid
release of energy.
Earthquakes are often caused by slippage along a break in
Earth’s crust called faults.
Earthquakes are often caused by slippage along a break in
Earth’s crust called faults.
Seismograph
Normal Fault
Reverse Fault
Reverse Fault - Pushes
together
Normal Fault - Pulls apart
Strike Slip fault - slides
side to side
Faults
Napa Quake 2014
Earthquake Risk in Sacramento
Fault Types
Normal Fault: extension, hanging wall goes down
Reverse fault: compression, hanging wall goes up
San Andreas Fault at
Carrizo Plain
Strike Slip Fault
Focus and Epicenter
Focus - the point inside the Earth where the earthquake
starts.
Epicenter - location on Earth’s surface directly over the focus.
Cause of Earthquake - Elastic Rebound Theory
Draw it!
Most Earthquakes are produced by the rapid release of
elastic energy stored in rock that has been subjected to great
forces.
On your whiteboard, draw the focus, epicenter, and a
sentence explaining the Elastic Rebound Theory. Show me
Rocks bend under applied pressure
When the strength of the rock is exceeded, it suddenly
breaks
Energy released as the vibration of an earthquake.
Aftershocks and Foreshocks
Measuring Earthquakes
Aftershocks - smaller earthquakes that occur after the main
quake. Can destroy already weakened structures.
Seismograph - instrument that records earthquake waves.
Seismogram - written record on the seismograph
Foreshocks - small earthquakes that occur before the main
quake
Earthquake Waves
Surface Waves ( L and R Waves)
move up and down as well as side to side.
Cause the most destruction.
Only travel on the surface of the Earth
Body Waves
Can travel through the Earth’s Interior.
Two types
Primary (P Waves) - pull and compress rock in the
direction of the wave motion.
Secondary (S Waves) - vibrate the Earth perpendicular to
the direction of wave motion. Cannot travel through Gases
and Liquids
Your turn!
On your whiteboard, list the 2 types of surface waves and 2
types of body waves and draw a diagram showing how each
moves.
Which wave travels fastest? Which can go through anything
in the earth?
Locating an Earthquake
Locating an Earthquake
Use the travel time between
the P and S waves.
Once the distance to the epicenter is known, trace a circle on
a map with that radius.
From this time, determine
the distance from a travel
Time graph as we
constructed last time.
You will need information from at least 3 stations to
triangulate the location.
Earthquake Zones
Measuring Earthquakes
Most earthquakes occur in narrow zones around the world.
Richter Scale - outdated, logarithmic scale. An increase in 1
on the scale equals an earthquake 10 times more powerful.
Circum-Pacific Belt
Mediterranean Belt
Ring of Fire
Moment magnitude - most widely used measurement,
measures actual energy released by Earthquake.
Modified Mercalli Scale - Scale based on the damage done,
uses Roman numerals from I to XII.
Your turn!
Begin Day 2
Destruction From Earthquakes
Answer on your whiteboard:
Napa Quake 2014
What are 3 different scales we use to measure earthquakes?
Earthquake Risk in Sacramento
The damage to buildings from earthquakes depends on
several factors
Volcano experiment
Fault Types
Tsunamis
Normal Fault: extension, hanging wall goes down
Reverse fault: compression, hanging wall goes up
1)
Building Design
Brick and unreinforced stone most dangerous
Wood structures are better
Reinforced steel is best
Intensity and duration of the vibrations
Nature of the material on which the structure is built
Design of the structure
2)
Liquefaction
Soils that are saturated with water, turn to pudding during the
shaking.
Buildings sink
Underground tanks and pipes can float to the surface
3)
Tsunamis
Triggered by an earthquake
Occurs when a slab of ocean floor is displaced vertically
along a fault or during an undersea landslide.
Sends out a wave travelling up to 900 km/hr roughly 1 meter
high
As wave approaches shore, it slows down and gets higher.
4)
Tsunami Warning System
5)
Warning center based in Honolulu Hawaii
Sends warning all around the Pacific during an earthquake if
a Tsunami is detected.
6)
Other Dangers
Landslides, rockfalls, and ground subsidence
Breaking of gas and water lines
Fires
Aftershocks
Predicting Earthquakes
7)
Earth Layers Defined by Composition
No reliable way to predict Earthquakes
Crust
We can give a probability of an Earthquake occurring based
upon history and current ground measurements.
Mantle
Core
8
Oceanic Crust
-About 7 km thick
made mostly of basaltic rock
density about 3.0 g/cm3
about 180 million years old or
less.
9
Continental Crust
10
8 to 75 km thick, average is
about 40 km
Made of granitic rock
Density about 2.7 g/cm3
Some rocks over 4 billion
years old
11
Mantle
82% of Earth’s volume
Semi-solid rocky shell
Extends to depth of 2890 km
Made mostly of peridotite
Density of 3.4 g/cm3
12
Core
13
Made of Iron-Nickel alloy
Density about 13 g/cm3
Notice the layers get denser as
they go down
Layers Defined by Physical Properties
14
Lithosphere - about 100 km thick, solid, rocky
Asthenosphere- rock is soft and near melting point to 660 km
Lower Mantle- More rigid layer (660 to 2890 km)
Outer Core- Liquid (2260 km thick), molten iron creates
magnetic field
Inner Core- 1220 km thick. Solid layer due to pressure
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16
Day 2
Discovering Earth’s Layers
17
In 1909, Andrija Mohorovicic showed seismic waves increase
in speed at about 50 km below surface.
Called Moho discontinuity.
Only P waves can travel through core
P waves bent at boundary of core and mantle
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19
Discovering Earth’s Composition
20
Seismic data and drilling showed crust made of Granitic rock
Ocean drilling samples show Sea floor made of basalt
Lava samples show mineral content deep into the mantle
similar to rocky meteorites
Core thought to be similar to metallic meteorites.