11/5/2015
What Is an Earthquake?
What Is an Earthquake?
GEOL 110: GEOLOGY
Chapter 11:
EARTHQUAKES AND
EARTHQUAKE
HAZARDS
What is an Earthquake?
Occurs by the sudden and rapid movement of
one block of rock sliding past another, causing
the ground to shake.
Why does it occur?
• Build up of stress causes
sudden slippage
• Most earthquakes occur
along preexisting faults
Lisa James/Register
This presentation is not for public distribution and only intended for educational and reference use within the associated course.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
What Is an Earthquake?
What Is an Earthquake?
Points of an Earthquake
• Focus/Hypocenter: location of slippage (generally along a
fault)
• Epicenter: location at surface
• Seismic Waves: stored-up energy originating from
hypocenter.
• Waves radiate outward in all directions
• Seismic energy dissipates with distance
What causes an
Earthquake?
• Stress builds up in rocks on both
sides of a fault until they break/slip
• Large landslides, meteorites, and
volcanic eruptions produce weak
earthquakes
How does stress build up?
• Movement of plates bend rocks
• Friction keeps rock in place until too much stress
• Rocks have been moved to a new location
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Faults and Earthquakes
What Is an Earthquake?
Aftershocks and Foreshocks
• Foreshocks precede the main
shock; smaller
Plate Boundaries, Faults and Earthquakes
• Plate boundaries are the source of most large
earthquakes
• Earthquakes can occur on new and pre-existing
faults
– Can precede several day, weeks,
months
• Aftershocks follow the main shock;
smaller
– Due to fault surface adjusting to the
displacement caused by the main shock
– Fewer and fewer as time passes
– Aftershocks can cause damage to
already weakened structures
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3 types of Boundaries?
earthquakes.usgs.gov
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Faults and Earthquakes
Types of Faults that cause
earthquakes
• Normal fault: associated with divergent
boundaries; doesn’t cause many earthquakes
• Reverse and Thrust fault: associated with
convergent boundaries;
– Megathrust fault: boundary between
subducting and overlying plate; causes
very large earthquakes.
– Displace water and cause tsunami
• Strike-slip fault: associated with transform
boundaries
Major Fault Types
Divergent: Footwall
moves up
Convergent:
Footwall moves
down
B. Reverse Fault =
convergent
B. Megathrust Fault =
convergent
C. Strike-slip Fault =
transform
geology.about.com
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Faults and Earthquakes
Faults and Earthquakes
San Andreas Fault
2 Large Earthquakes on
San Andreas Fault
San Andreas is the most studied fault system in the world
• Slow, gradual displacement known as fault creep
• Other segments regularly slip, producing small earthquakes
• Other segments remain stuck and store elastic energy for a few
hundred years before they break loose, resulting in a major earthquake
Faults and Earthquakes
Fault Rupture and Travel
• Most faults are locked except for brief,
abrupt movements
• Faults do not slip all at once
• Initial slip begins at focus and
travels along the fault surface
• Slippage adds strain to adjacent
sections triggering more slippage
• Slippage mainly travels in one
direction (Napa EQ traveled North)
• Fault slip is the amount of
displacement on the fault surface
Oct. 17,1989
Loma Prieta, M7.1
April 18, 1906 Great SF
Earthquake, M7.8;
Epicenter off shore ~2miles
Creep along Calaveras Fault
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© 2014 Pearson Education, Inc.
A. Normal Fault =
divergent
USGS/2015
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Epicenter in Santa Cruz Mtns
Displacement of Structures Along a Fault
Fault Slip: amount of displacement
1906 earthquake
displacement 8.5 ft.
C.Roughley 2014
At least 46 cm of right-lateral displacement
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Seismology: The Study of Earthquake Waves
What is seismology?
What is seismology?
Japan
India
Mexico
Seismology: The Study of Earthquake Waves
Mongolia, China
© 2014 Pearson Education, Inc.
• The study of earthquake
waves
• Earliest studies of
earthquake waves date
back almost 2000 years to
the Chinese
• The Chinese Seismoscope
Invented 132 AD by
inventor, Imperial Historian,
and Royal Astronomer
Zhang Heng
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Seismology: The Study of Earthquake Waves
Instruments that Record Earthquakes
• Seismographs / seismometers record the
movement of Earth in relation to a stationary mass on
a rotating drum or magnetic tape
• Seismograms are the records produced by
seismographs
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Seismic Waves
What are Seismic Waves?
• Seismic Waves: waves of energy that travel through earth’s
layers
• Wave: energy moves from one place to another and matter
doesn’t move from place to another (basic).
• Eg: ocean waves carry energy to the shore, but water doesn’t
build up at shoreline
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Seismic Waves
Different types of energy travel in waves:
• Soundwaves carry noise through the air to
our ears
• Light, heat, radar carry energy waves as
electricity and magnetism.
• Gamma Rays produced by radioactivity.
• Radio waves carry radio and tv signals
Seismology: The Study of Earthquake Waves
Seismic Waves: waves of energy that travel
through earth’s layers
Types of seismic waves:
• Body waves: travel through earth’s interior
• Surface waves: travel in the rock layers just
below Earth’s surface
www.studyblue.com
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Seismology: The Study of Earthquake Waves
Body Waves
Seismology: The Study of Earthquake Waves
Body Waves
• P-Waves (primary waves):
compressional waves; push/pull;
stretch rocks parallel to wave
direction; travel through solid,
liquid, gas
• S-Waves (secondary
waves/shear waves): move
particles perpendicular to wave
direction; changes shape of
material; travel through solids
only! (change shape; liquids not
rigid therefore cannot shear or
change shape)
P-wave
S-wave changes shape of
material
http://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gif
http://www.geo.mtu.edu/UPSeis/images/S-wave_animation.gif
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Seismology: The Study of Earthquake Waves
Seismology: The Study of Earthquake Waves
Surface Waves
Surface Waves
Travel through the crust
• Raleigh waves: rolls along the
ground like an ocean wave;
ground moves up and down
• Love waves: moves the
ground side to side
• Largest amplitude on
seismogram; most damage
http://www.geo.mtu.edu/UPSeis/images/Rayleigh_anim
ation.gif
http://www.geo.mtu.edu/UPSeis/images/Love_animatio
n.gif
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Body Waves vs. Surface Waves
P-waves are faster than S-waves and arrive first.
Typical Seismogram
Waves on a seismogram
P-wave speed:
Water: 1450 m/s
Granite: 5000 m/s
S-wave speed:
~60% of P-wave
S-waves do not
travel through
outer core
(liquid)
Surface waves are slowest ; last to arrive
at a recording station; cause most property
damage
Surface wave speed: ~90% of S-wave
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© 2014 Pearson Education, Inc.
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Typical Seismogram
How to find the epicenter:
1. Amount of time between the P and S wave on a seismogram can
tell how far away the earthquake was from that location.
2. Draw a circle on a map around the seismograph station; radius is
the distance to the earthquake; earthquake lies somewhere on the
circle.
3. Point of intersection is location of epicenter.
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Determining the Size of Earthquakes
Intensity scales (shaking)
• 1902, Guiseppe Mercalli developed intensity scale based
on CA buildings
• Modified Mercalli Intensity scale based on damage to
structures and where the earthquake is felt
• Drawback: destruction may not be a true measure of the
earthquake’s actual severity
Determining the Size of Earthquakes
How do we determine the size of an
earthquake?
2 ways to describe:
• Intensity: a measure of the degree of
earthquake shaking at a given locale based
on the amount of damage
• Magnitude: an estimate of the amount of
energy released at the source of the
earthquake
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Seismic Intensity Map
DYFI? Intensity Scale:
South Napa
Earthquake, Aug 24
Mw 6.0
7.0 miles deep
http://earthquake.usgs.gov/earthquakes/map/
www.studyblue.com
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Determining the Size of Earthquakes
Magnitude scales (energy released)
Richter magnitude ML
• Introduced by Charles Richter in 1935;
seismologist and geophysicist; worked at
CalTech with Beno Gutenberg
• The Richter scale is ratio of amplitude of
the largest seismic wave to a minor
amplitude
• 1-10 relate to energy earthquake releases;
each level indicates ~32-fold increase in
energy released.
• Not accurate for large earthquakes
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Determining the Size of Earthquakes
Magnitude scales (energy released)
Moment magnitude Mw
• Developed in the 1970s to replace
Richter magnitude scale. Today’s
standard.
• Calculated by determining the average
amount of slip on the fault, area of the
fault, and strength of the fault rock.
• Each unit increases ~32-fold increase
in energy released
• Used to update Richter magnitude
earthquakes
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Annual Occurrence of Earthquakes with
Various Magnitudes
Comparison between the Richter and Moment
Magnitude Scales
Earthquake
Richter Scale
Moment Magnitude
New Madrid, MO, 1812
8.7
8.1
San Francisco, CA 1906
8.3
7.7
Prince William, AK 1964
8.4
9.2
Northridge, CA,1994
6.4
6.7
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Determining the Size of Earthquakes
Earthquake size example:
What is the intensity? What is the magnitude?
Earthquake Destruction
Damage from Earthquakes
Dependent on:
• Magnitude of the earthquake
• Proximity to the epicenter
http://earthquake.usgs.gov/earthquakes/map/
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Earthquake Destruction
Damage from Seismic Vibrations
The amount of damage to human-made structures depends on:
• Intensity
• Duration (Loma Prieta 15 seconds; 1963 Alaska 3-4minutes)
• Geology
• Building materials and construction practices/codes of the region
Earthquake Destruction
Damage from Seismic Vibrations
• Amplification of seismic waves
• Soft sediments amplify seismic waves more than solid bedrock
• Liquefaction
•
https://www.youtube.com/watch?v=TzlodnjPAuc
2010 Haiti Earthquake
Mw 7.0
Depth: 8.1miles
Intensity: X (extreme)
52 aftershocks Mw 4.5 or greater
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Photo Marco Dormino/ The United Nations United Nations Development
Programme - originally posted to Flickr as Haiti Earthquake
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Earthquake Destruction
Earthquake Destruction
• Landslides and Ground
Subsidence
• Ground shaking causes
loose sediments on a slope
to slump
• Fire
• Can start when gas and
electrical lines are
destroyed by an
earthquake
• Broken water lines make
fire control problematic
Tsunami
Series of large ocean waves
• Most are generated by displacement from a megathrust fault
• In open water, the wave amplitude is less than 1 meter and the
wavelength can be larger than 700 meters
• Close to shore, the water “piles up” and some tsunamis can
exceed 30 meters in height
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Tsunami Generated by Displacement of the
Ocean Floor
Japan Tsunami
The city of Taro is famous because the
city was most enthusiastic to protect
the city from the Tsunami in Japan.
They built the seawall whose height is
10m, probably highest in Japan.
The Tsunami of 2011 was higher than
this seawall and destroyed most of
downtown area.
Tsunami generated from the 2011 Tohoku earthquake (9.0Mw)
•40 meters high and traveled up to 6 mi inland
•Affected not only Japan but also the west coast of North America
K.Haichi 2011
Tsunami is expected to come to here.
K.Haichi 2010
Sign reads “Tsunami of 1933 came to here”.
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Earthquake Destruction
Tsunami warning system
• Observations in the Pacific
Ocean.
• A series of deep-water
buoys in the Pacific Ocean
detect energy released by
earthquakes
• Tidal gauges measure sea
level rise and fall
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K.Haichi 2011
After 2011 Tsunami
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Earthquake Belts and Plate Tectonics
Circum-Pacific Belt
• 25,000 miles of subduction boundaries along the CircumPacific Belt
• Most earthquakes occur along megathrust faults of
convergent plate boundaries
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Earthquake Belts and Plate Tectonics
Can Earthquakes Be Predicted?
Alpine-Himalayan Belt
• Tectonic activity is attributed to the collision of the
African and Indian Plates with the Eurasian Plate
• Divergent plate boundaries are associated with frequent,
but weak seismic activity
Can earthquakes be predicted?
Short-Range Predictions
• The goal is to provide a warning of the location and
magnitude of a large earthquake within a narrow time
frame
• Research has concentrated on monitoring possible
precursors of major earthquakes:
• Monitor changes in ground elevation
• Measure strain in the rocks
• Measure changes in groundwater level
• Frequency of foreshocks
Currently, no reliable methods exist for making short-range
earthquake predictions
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Can Earthquakes Be Predicted?
Can Earthquakes Be Predicted?
Early Warning Systems
Long-Range Forecasts
• Berkeley early warning system
• Japan early warning system
• Give the probability of
earthquakes of a certain
magnitude occurring on a
time scale of 30 to 100 years
(or more)
• Useful guide for building
codes. Eg: Building the
Trans-Alaskan Pipeline over
the Denali Fault
– Over 4200 seismometers
– 2 record P-wave and give warning
– Few seconds to a minute
http://blogs.agu.org/landslideblog/2015/10/28/af
ghanistan-earthquake-1/
Seismo.berkeley.edu
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Can Earthquakes Be Predicted?
Long-Range Forecasts
• Seismic gaps: tectonically
quiet zones along a fault
where strain is currently
building up
• The stored strain will be
released in a future
earthquake
© 2014 Pearson Education, Inc.
What Is an Earthquake?
Earthquakes and Faults
exercise
• Afghanistan earthquake
•> 360 deaths (Pakistan) and
~2,000 injured.
•http://blogs.agu.org/landslideblog/2015/10/28/afgha
nistan-earthquake-1/
• Paleoseismology is the
study of prehistoric
earthquakes
• Trench across a fault zone,
look for evidence of ancient
faulting (mud volcanoes and
offset sedimentary strata)
http://earthquake.usgs.gov/earthquakes/map/
This presentation is not for public distribution and only intended for educational and reference use within the associated course.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
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