Learning Objectives
After reading, studying,
and discussing the chapter, students should be able to:
·
The cause of earthquakes.
·
Types of seismic waves and their
propagation.
·
How an earthquake epicenter is
located.
·
Worldwide distribution of
earthquake epicenters.
·
How the magnitude of an
earthquake is determined.
·
Other destructive forces that
can be triggered by an earthquake.
·
The status of earthquake
prediction.
·
Earth's interior structure and
composition.
·
The types of folds and faults
and how they form.
·
The relation between mountain
building and plate tectonics.
Chapter Outline___________________________________________________________________
I. Earthquakes
A. General
features
1.
Vibration of Earth by release of energy
2. Associated with movements
along faults
a.
Explained by plate tectonics theory
b.
Mechanism explained by H. Reid
1.
Early 1900s
2. Rocks "spring back"
(elastic rebound)
c.
Fault creep
3. Often
preceded by foreshocks
4. Often
followed by aftershocks
B. Earthquake
waves
1. Study of
is called seismology
2. Recording instrument
(seismograph) records a trace, called a seismogram
3. Types of
earthquake waves
a.
Surface waves
1.
Complex motion
2.
Slowest velocity of all waves
b.
Body waves
1.
Primary (P) waves
a. Push-pull (compressional)
b. Travel through
1. Solids
2. Liquids
3. Gases
c. Greatest velocity of all waves
2.
Secondary (S) waves
a. "Shake"
(shear)
b.
Travels only through solids
c. Slower velocity than P waves
C. Locating an
earthquake
1. Focus is within Earth, where
waves originate
2.
Epicenter
a. On
surface, directly above focus
b. Found using difference
in arrival times of P and S recordings
c.
Three station recordings used to find
3. Earthquake zones are
correlated with plate boundaries
a.
Circum-Pacific belt
b.
Oceanic ridge system
D. Earthquake
magnitude
1.
Introduced by Charles Richter, 1935
2. Richter
scale
a.
Magnitude
1.
Amplitude of largest wave
2.
Large quakes near magnitude 8.6
3.
Less than 2.0 usually not felt
4. Each unit of
magnitude increase equals about a 30-fold energy increase
E. Earthquake
destruction
1. Factors
that determine destruction
a.
Magnitude of earthquake
b.
Proximity to population
2.
Destruction from
a. Ground shaking can cause
liquefaction where the saturated material turns to fluid
b.
Tsunamis, or seismic sea waves
c.
Fire
d.
Landslides and ground subsidence
F. Prediction
1. Short-range–no reliable
method yet devised
2.
Long-range
a.
Premise is earthquakes are repetitive
b.
Region given probability of a quake
II. Earth's interior
A. Knowledge from
study of P and S waves
1. Travel
times through Earth vary
2. S waves
travel only through solids
B. Structure
1. Crust
a.
Thin outer layer
b. Varies in thickness
1.
70 km in some mountain regions
2. Less than 5 km in
oceanic regions
c. Two
parts
1. Continental
crust–less dense, granitic rocks
2. Oceanic
crust–basaltic composition
d.
Lithosphere
1. Crust and uppermost
mantle (100 km thick)
2.
Cool, rigid, solid
e. Mohorovicic
discontinuity (Moho separates crust from mantle)
2. Mantle
a.
Below crust
b.
2885 km thick
c. Composition similar to
peridotite
d.
Asthenosphere
1.
Upper mantle
2.
Between 100 km and 350 km
3.
Hot, weak rock
4.
Easily deformed
5.
Up to 10% is molten
6.
Key to plate movement
3. Outer
core
a.
Below mantle
b.
2270 km thick
c. Mobile liquid (S waves do not travel through)
d.
Mainly iron and nickel composition
e.
Related to Earth's magnetic field
4. Inner core
a. 1216
km radius
b. Solid
c. Iron
and nickel composition
d. High
density
III. Geologic structures
A. Two basic types
1. Folds
2. Faults
B. Folds
1. Rocks bent
into a series of waves
2. Types of
common folds
a. Anticline–upfolded, or
arched, rock layers
b.
Syncline–downfolds, or troughs
3. Folds can
be
a.
Symmetrical
b.
Asymmetrical
c.
Overturned
4. Most from compressional
stresses that shorten and thicken the crust
5. Other
types
a. Domes
1.
Circular, or slightly elongated
2.
Upwarped displacement of rocks
3.
Oldest rocks in core
b.
Basins
1.
Circular, or slightly elongated
2.
Downwarping of rocks
3.
Youngest rocks in core
C. Faults
1. Fractures
in crust with displacement
2. Types of
faults
a.
Dip-slip faults
1. Move along tilt (dip)
of fault
2.
Types
a. Normal faults
(tensional forces)
b. Reverse faults
(compressional forces)
c. Thrust faults
b.
Strike-slip faults
1.
Movement along trend, or strike
2.
Transform faults
a. Large
b. Often with plate boundaries
3. Pulling of
crust apart can form
a.
Graben
1.
Downdropped block
2.
Bounded by normal faults
3. Can produce an
elongated valley (e.g. Great Rift Valley, Africa)
b.
Horsts
1.
Relatively uplifted blocks
2.
Flanking grabens
IV. Mountain building
A. Processes that
uplift mountains
B. Major mountain
systems
1. Related to
plate tectonics
2. Most form
along convergent plates
C. Associated with
1. Convergent
boundaries
a.
Oceanic-continental crust
1.
Andean-type mountains
2.
Subduction zone forms
3.
Deformation of continent margin
4.
Volcanic arc forms
5.
Accretionary wedge
6.
Examples
a. Sierra Nevada Range
b. California's Coast Ranges
b. Where
continental crusts converge
1.
e.g.. India-Eurasian plate collision
a. Himalaya Mountains
b. Tibetan Highlands
2.
Others
a. Alps
b. Appalachians
2.
Continental accretion
a. 3rd
mechanism of mountain building
b. Small
crustal fragments
1.
Collide with, and
2.
Accrete to continental margins
c.
Terranes –accreted crustal blocks
d. Occurred along Pacific
Coast
