Download Chapter 11 -2 Study Guide

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Chapter 11-2 Features of Earthquake Study Guide Ms. Grady
What is a wave?
 Rhythmic (regular/pulsing/vibration) movement that carries
energy though matter and space.
Remember that seismic waves are responsible for most damage caused
by earthquakes. The surface of the ground can ripple just like waves do
in the water. Seismic waves produce shock waves through the Earth.
ripple
Origin of Seismic Waves
You have already learned that rocks move past each other along faults which
create stress at points where the rocks’ (irregular/not normal) surfaces catch
each other.
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The stress builds up and then the rocks break (elastic limit is exceeded or
pushed as far as it can take). Then energy is released (let go).
This energy is let go in the form of a seismic wave.
Focus = the point where this energy is released or first let go of occurs
(happens).
Focus (plural is foci)
 Most faults occur (happen) between the surface and a depth of 70
kilometers.
 The point beneath the surface where the rocks break and move is called
the focus. The focus =the underground origin (where it begins) of an
earthquake.
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Epicenter (EH pih sent ur)
 Directly above the focus on the Earth’s surface is the epicenter.
 Earthquakes waves reach the epicenter first.
 During an earthquake, the most violent shaking is found at the epicenter.
What are seismic (size mhk) Waves?
What are the characteristics of the different types of waves?
Seismic Waves
 The waves of energy generated (produced/made) by an earthquake are
called seismic waves.
 There are 3 main types of seismic waves:
1. Primary
2. Secondary
3. Surface
When earthquakes happen, all 3 different types of seismic waves are produced
and are created at the same time. However, each wave acts or behaves
differently.
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Primary Waves cause particles in rocks to move back and forth parallel to
the direction of the wave (see picture of the coiled spring). Then they
compress and stretch the rocks.
 Seismic waves that travel the fastest are P waves (primary waves).
 P waves travel through solids, liquids, and gases.
 Primary waves move at different speeds depending on the density of
the material through which they are moving.
 As P waves move deeper in the Earth, they move faster.
 P waves are push-pull waves (back and forth).
If you squeeze one end of a coiled
spring and then let it go, you cause
it to compress and then stretch as
the waves travel through the
spring.
Particles in rocks also compress
and stretch apart spreading primary
waves through the rock.
P WAVES example
Secondary Waves (S-waves)
S waves move through Earth by causing rock particles (up and down) to move
at right angles to the direction of the wave.
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 Seismic waves that do not travel through the Earth as fast as P waves
are called Secondary Waves or S – Waves.
 S waves travel through solids.
-Not liquids or gases
Surface Waves or L waves
 L waves originate (start) on the Earth’s surface at the
epicenter. They move along the surface the way waves travel in the
ocean (up and down and side to side).
 Surface Waves cause most of the destruction from earthquakes.
 Surface waves cause rock particles to move in a backward rolling
motion and a side to side swaying motion.
- Many buildings are unable to take intense shaking because they are
made with stiff (not bending) materials. Surface waves cause
different parts of a building to move in different directions which
causes the building to fall apart.
Review
 P waves (primary waves) are the fastest.
 Secondary waves are slower.
 Surface waves are the slowest.
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Seismograph
 Invented in 1893 by John Milne.
 A seismograph detects (discovers/finds) and measure seismic waves.
 A weight attached to a spring remains nearly still even when the Earth
moves.
-A pen attached to the weight records any movement on a roll of paper
on a constantly rotating drum.
-The drum moves with the Earth and affects the line.
 There are two types of seismographs that measure Earth’s movement;
1. Vertical 2. Horizontal
Seismologists
 Seismologists study earthquakes.
 Seismologists can determine the strength of an earthquake by the
height of the way line recorded on the paper.
 The seismograph record of waves is called a seismogram.
Seismograph Stations
 Each type of wave reaches a seismograph station at a different time
based on its speed.
 Primary waves arrive first.
 Secondary waves travel slower and arrive later.
 The difference in arrival time is used to estimate (approximation) the
distance from the station to the epicenter.
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Locating an Epicenter
 Scientists need readings from 3 or more stations to determine the
location of an epicenter.
 A circle is drawn around each station. The radius of each circle is = to
the station’s distance from the epicenter. The point of intersection is
the location of the epicenter.
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What do we think the interior of the Earth is like?
 We have never been there. It’s too hot and there is way too much
pressure there.
Information from Earthquakes
 Much of the information scientists have about the Earth’s interior has
come not only from complex (complicated) instruments but also from
earthquakes.
Interior of the Earth
 Seismic waves change speed as they move through the Earth’s layers.
 Seismic waves speed up when they pass through the bottom of the
crust and enter the mantle.
 This boundary is known as the Moho.
Vocabulary - Moho = the boundary between the crust and the mantle.
 The boundary is named for the scientist, Andrija Mohorovicic, who
discovered this information.
 It is called the Mohorovicic discontinuity or Moho.
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Seismic Speed in the Core
 The core is divided into two layers based on how they travel through it.
 Primary waves slow down and secondary waves do not travel through
the liquid outer core.
 Primary waves speed up again when they reach the solid inner core.
The
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Inner Core
The solid inner layer is called the inner core.
The temperature reaches 5,000 degrees C,
This temperature is at which iron and nickel usually melt.
The extreme pressure pushes the particles so tightly together that
they remain solid.
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Outer Core
 Surrounding the inner core is the second layer of the Earth, called the
outer core.
 The outer core begins about 2900 km below the Earth’s surface and s
2250 km thick. The temperature range is from 2,200 to 5,000 degrees
C.
 The heat makes the iron and nickel molten.
Seismic Speed in the Mantle
 The mantle is divided into layers based on changes in seismic wave
speed. Both P and S waves slow down again when they reach the
asthenosphere.
 They speed up when they move through a more dense part of the
mantle.
Mantle
 mantle = The layer of the Earth directly above the outer core.
 The mantle reaches a depth of about 2,900 km below the surface.
 About 88% of the volume of the Earth and 68% of the mass are in the
mantle.
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Composition (structure) of the Mantle
 After studying rocks from volcanoes and the ocean floor, scientists
think the mantle is made of elements; silicon, oxygen, iron, and
magnesium.
 It has less silicon and aluminum and more iron and magnesium then
the crust.
 The density of the mantle increases with depth.
 The high temperature and pressure in the mantle allow the solid rock
to flow slowly, which changes its shape.
 This is the property of plasticity (being shaped and formed).