GEOLOGY LECTURE OUTLINE –
Earthquakes and Faulting – (Ch 11)
I. Lecture Content
Introduction – Earthquakes in Our Own Backyard
Fundamentals of Earthquakes and Faults – Definitions
Earthquakes and Plate Tectonics
Elastic Rebound Theory
Fundamentals of Seismic Waves
Locating Earthquake Epicenters
Measuring the Size of Earthquakes
Earthquakes as Natural Hazards
Prologue - Living with Earthquakes in So-Cal
II. Living with Earthquakes in
A. Southern California is one Huge Active Fault Zone
· Earthquakes occur along numerous active faults
· Map of
--- Web page: http://quake.wr.usgs.gov/recenteqs/latest.htm
· The San Andreas Fault Zone – a Transform Plate Boundary
ü San Jacinto Fault
ü Ellsinore Fault
ü Newport-Inglewood Fault
ü Continental Borderland Faults
ü Regional “Blind Thrust” Faults
B. Location, Frequency, and Size of Recent Quakes
San Jacinto Fault
Continental Borderland Faults
Regional “Blind Thrust” Faults; Sylmar; Northridge
C. Personal Experiences of Earthquakes
D. Understanding Earthquake Hazards
III. Fundamentals of Earthquakes and Faults
A. Earthquake defined:
· The sudden release of energy, usually along a fault,
which produces “seismic” vibrations within Earth.
1. Earthquakes vary in several ways.
Nature of ground motion
2. The record of an earthquake (a seismogram) is made by a seismic wave-measuring device (a seismograph).
(see Fig. 9.5)
3. The point where an earthquake’s energy is released is known as the focus. (see Fig. 9.6)
4. The point on the Earth’s surface directly over the earthquake’s focus is called the epicenter. (see Fig. 9.6)
5. Monitoring, measuring, and predicting earthquakes is the job of the seismologist.
B. Fault defined: A plane of weakness, within the upper crust, where displacement occurs, usually in the form of earthquake.
1. The nature of fault displacement (rupture) varies from
minute creep (mm’s) to massive offsets (10’s of m’s).
2. Faults, like volcanoes, are classified as either, active
(frequent rupture) or inactive/extinct (no ruptures).
3. Faults are also classified by the geometry of the
C. Seismology defined:
· The scientific study of earthquakes (and earthquake-related processes and mechanisms).
1. Includes the monitoring and measuring of both seismic waves (earthquakes), and fault motions (rupture).
2. Most seismologists live in Earthquake Country.
IV. Earthquakes, Fault Zones and Plate Tectonics
A. Global Distribution of Earthquakes
1. Vast majority of quakes occur along plate boundaries.
2. 80% of all earthquakes occur in the circum-Pacific belt.
3. 15% of quakes within the Mediterranean-Asiatic belt.
4. Remaining 5% occur along ocean spreading ridges and
within plate interiors.
5. About 150,000 earthquakes (strong enough to be felt)
are recorded by seismographs worldwide each year.
Another 900,000 tiny (micro) earthquakes also occur.
B. Concentration of Earthquakes at Plate Boundaries
1. Most major quakes occur in four tectonic regions
· Subduction zones - Along dipping "Benioff zones"
ü Downgoing slab surface is a giant thrust fault
· Continental Collision zones - Major thrust/fold belts
· Transform boundaries - Giant strike-slip fault systems
· Oceanic spreading ridges - Rift Valleys
2. Average earthquake focal depth varies for each region
· Subduction zones - shallow to deep focus
· Continental collision zones - shallow to intermediate
· Transform and Divergent boundaries - shallow focus
C. Interactive Maps of Recent Earthquakes on the Web
1. Recent Quakes in
2. Recent Quakes in The
3. Recent Quakes from around The World
4. Shake Maps of the
D. Interactive Maps of Historic Earthquakes on the Web
1. Historical Quakes of
2. Historical Quakes of the World
3. World Quakes Greater than 7.0
V. The Elastic Rebound Theory – The “Best” Explanation
A. Elastic Rebound Theory Explained:
· Fault (rocks) ruptures when pressure (binding stress) accumulates in the rocks on either side of the fault to a level that exceeds the rocks’ strength.
· Prior to rupture, the rocks slowly deform (bend) as they build up stored energy.
· Stored energy from the accumulated stress is released as the ruptured rock snaps back to their original position.
B. Illustration of the ERT - (see Figure 9.3)
VI. Fundamentals of Seismic Waves
A. Three Forms of Energy Released from Earthquakes
1. Heat Energy - caused by friction in the fault zone
2. Body Waves - Travel through the solid Earth in a manner
somewhat like sound waves. There are two types of
body waves; they are named P- and S-waves.
3. Surface Waves - Travel only along the ground surface of
the Earth in a manner similar to water waves. There
are two types of surface waves; they are named R-
and L- waves.
4. Each type of seismic wave has a unique behavior in
the way it moves (propagates) through the ground.
B. Body Waves - Two Types: P-waves and S-waves
1. P-waves - short for primary waves (see Figs. 9.12 & 9.14)
· The fastest seismic wave
· Can travel through solids, liquids, and gases
· Compressional, or push-pull
· Are similar to sound waves
2. S-waves - short for secondary waves
· Slower than P-waves - (see Figs. 9.12 & 9.14)
· Can only travel through solids
· Shear waves; material moves side-to-side
3. Velocity of body waves varies with type of material.
· Faster through denser, more elastic rocks
· Slower through less dense, more plastic rocks
· Wave velocity increases with depth, as a rule
· P-wave velocities are always faster than S-waves
for any given material
· S-waves cannot travel through liquids or gases
· P-waves arrive at seismic stations before S-waves
4. Illustration of Body Waves (see Figs. 9.12)
C. Surface Waves - Two Types: R-waves and L-waves
- short for
· Slower of than the L-waves
· Wave motion along surface similar to a rolling
ocean wave = back-and-forth, up-and-down
· The sensation of a R-wave is like that on a boat
2. L-waves - short for Love waves
· Faster than the
· Similar in form to a S-wave
· Wave motion along surface similar to a slithering
snake = undulating, side-to-side
· Most destructive of the seismic waves
3. Surface waves are most destructive in loose/wet material
4. Illustration of Surface Waves (see Fig. 9.13)
VII. Earthquake Epicenters - Finding its Location
A. Locating an Earthquake's Epicenter
· An earthquake's epicenter is located using seismic data
gathered from several seismic stations
· The fact that P-waves travel faster the S-waves means
that with increasing distance from the epicenter the
difference in arrival time between the P- and S-waves
will progressively increase. (see Fig. 9.14)
· Since average speeds for P- and S-waves are well known
for any specific distance from their source (focus), the
difference in arrival time between the P- and S-waves
can be used to calculate the distance to the epicenter.
· Seismologists have assembled time-distance graphs for
all P-S time intervals for calculating the distance to the
epicenter for each seismic station. (see Fig. 9.15)
B. Calculating an Earthquake's Epicenter
1. Measure the arrival time for both, the very first P-wave
arrival and the very first S-wave arrival.
2. Calculate the P-S time interval.
3. Using the time-distance graph, a circle with a radius
equal to the distance from the epicenter is drawn
around each seismic station. All potential epicenters
are located on the circle. (see Fig. 9.16)
4. It takes data from three differently located seismic
stations to calculate the actual earthquake epicenter.
That means you have to do steps 1 through 3 for at
least three stations (see Fig. 9.16)
VIII. Earthquake Size - Measuring its Intensity and Magnitude
A. Measuring an Earthquake's Size (Strength)
· Size (strength) of an earthquake is measured two ways.
1. Intensity - based on kinds of damage done
2. Magnitude - based on amount of energy released
B. Measuring an earthquake's intensity is done by
qualitatively assessing the quake experience and
1. Physical damage to buildings and structures
2. Physical sensations felt by people and animals
3. Behaviors of people and animals
Ø The most widely used intensity scale is the
Modified Mercalli Intensity Scale (see Table 9.2)
Ø Modified Mercalli Intensity Maps (Fig. 9.17 & 9.18)
C. Measuring an earthquake's magnitude is done by
calculating the total amount of energy released by the
quake. This is a quantitative measurement.
1. Magnitude is based on measuring the amplitude of
of the largest seismic wave as recorded by a
1. The most accurate method for determining the magnitude,
In terms of energy released, is the moment magnitude.
2. The most widely used magnitude scale is the
Richter Magnitude Scale (see Fig. 9.19).
3. Richter's Magnitude Scale is open-ended and starts
at one (1). Each jump in whole number represents a
30-fold increase in energy released.
4. Each jump in whole number represents a 10-fold
increase in wave amplitude.
5. Calculating the magnitude on the Richter Scale is done
by taking the measured maximum amplitude of the
largest seismic wave and charting it against the
measured P-S time interval (represented in the P-S
time-distance graph). (see Fig. 9.19)
6. The line drawn between the two scales interests the
centrally-located magnitude scale at the appropriate value.
7. The largest recorded earthquake on Earth has been 8.6 M
D. Create and Analyze Your Own Virtual Earthquake
1. This website has an interactive program that allows you
to choose a region (several possible locations) where you
can create a virtual earthquake.
2. You are then given three seismograms from three
different seismic stations in the region, which you will
use to calculate both, the epicenter and magnitude of
the virtual earthquake.
IX. Earthquake Hazards
A. Ground Shaking
1. The type of ground (subsurface) beneath a location is a
critical factor on how much ground shaking occurs,
for a given distance from the epicenter.
2. Structural failure and collapse
· Homes and Buildings
· Bridges and Dams
3. Ground failure
4. Falling/Flying objects
5. Getting knocked off your feet or thrown against something
1. Broken gas mains
2. Damaged electrical wire
3. Wooden structures
1. Razing and flooding of coastal communities
D. Volcanic Eruption
1. Earthquake-triggering of an active volcano
2. Volcanic eruption hazards
X. Major Earthquake Preparation and Survival - 3 Steps
A. Always be prepared for an earthquake
· Know how to make your home earthquake-ready
· Have an earthquake survival kit at home.
B. Know what to do when an earthquake occurs
· While at home
· While traveling
C. Know what to do after an earthquake occurs
· While at home
· While traveling
D. Quake Preparation and Survival Info on the Web
XI. Earthquake Prediction
A. Compiling Historical Earthquake Data of Active Faults
1. Statistical analyses of earthquakes recorded by
seismographs for specific faults and fault systems
2. Paleo-rupture histories of active faults done by trenching
across known fault surfaces.
· Use stratigraphic principles to decipher a sediment pile
that is laced with buried rupture surfaces
3. The above combined information can be used to predict
a fault's frequency of rupture, including how big.
B. Monitoring Active Fault Zones
1. Real-time monitoring on/around active faults
using a variety of instruments
· GPS/Laser sights
· Water well sensors
· Electrical sensors
Potential 6.9 Magnitude Quake on the
1. An earthquake planned 6.9 M scenario created for the RCF
using a computer-generated simulation
· Perceived shaking
· Potential damage
· Peak acceleration
· Peak velocity
· Instrumental intensity
2. Earthquake scenario map of
XII. Earthquake Resources on the Internet
A. General Information and Activities
B. Historical Earthquake Archives
XIII. Earthquake and Fault Vocabulary
Internet Image Glossary: http://earthquake.usgs.gov/image_glossary/
Elastic rebound theory