Download Earthquakes - Chapter 10

Earthquakes
Last time we considered faults.
Today we discuss the consequence of a fault’s movement
1.
2.
3.
4.
Slinky
Beaker, Wet Sand, Weight
Ball Point Pen
A seismogram
What is an earthquake?

An earthquake is the vibration of Earth
produced by the rapid release of energy
 Rock subjected to elastic deformation snaps
back and/or breaks
 Energy radiates in all directions from the
break’s location, AKA source, the “focus”
 Energy moves like waves as moving rocks
push on their neighbors
 Seismographs record the event
Anatomy of Earthquakes

Earthquakes and faults
 Earthquakes are associated with faults
 Motion along faults can be explained by plate
tectonics
Causes of earthquakes
Sudden release of accumulated strain
energy
 Creation of fracture zones at faults by
rupturing rocks
 Creation of new faults by rupturing rocks
 Shifting of rocks at preexisting faults
 Deformed rock straightens back to original
shape, but offset from matching beds on
the other side of the fault.

Elastic rebound 1
 Mechanism for Earthquakes
– Rocks on sides of fault are deformed by
tectonic forces
– Rocks bend and store elastic energy
– Frictional resistance holding the rocks
together is overcome by tectonic forces
Elastic rebound 2
 Earthquake mechanism
– Slip starts at the weakest point (the focus)
– Earthquakes occur as the deformed rock
“springs back” to its original shape (elastic
rebound)
– The motion moves neighboring rocks
– And so on.
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Aftershocks
The change in stress that follows
a main shock creates smaller
earthquakes called aftershocks
The aftershocks
“illuminate” the fault
that ruptured in the main
shock
Red dots show location of
aftershocks formed by 3
earthquakes in Missouri
and Tennessee in 1811/1812
Symbols: Reelfoot Fault is a
Reverse Fault, teeth point to
the upthrown side, so to the
hanging wall. Cottonwood
Grove a Transform Fault,
note the arrows pointing in
the directions of movement
Normal Fault Quake - Nevada
Reverse Fault Quake - Japan
Divergent
HW Down
HW Up
Convergent
Transform
Strike Slip Fault Quake - California
San Andreas: An active
earthquake zone

San Andreas is the most studied strike-slip
(transform) fault system in the world
 Displacement occurs along discrete segments 100
to 200 kilometers long
 Most segments slip every 100-200 years
producing large earthquakes
 However, some portions exhibit slow,
gradual displacement known as fault creep.
The rocks have low strength minerals that
cannot store strain, they just crumble and
smear continuously as the plates move.
Fence offset by the 1906 San
Francisco earthquake
Landscape Shifting, Wallace Creek
San Andres Fault
San Andreas Fault
Earthquake Hazards
Four major hazards occur during
earthquakes
 One is well known: the collapse of
buildings crushes people
 Three more are less well known

– Fire
– Liquifaction
– Tsunami
Fires caused by 1906 San Francisco Earthquake
Gas mains break, fires shaken out of furnaces and fireplaces. Water mains break.
Debris blocks streets. Fire Fighters cannot drive to the fire.
Liquefaction
Crystals from dredge muds are
arranged like pick-up-sticks.
Pressure waves from the
earthquake force the crystals
apart. Now your house is being
supported by water.
Makes “quick clay”
Before we consider Tsunami

We need some background in seismology
Seismology
Seismometers - instruments that
record seismic waves
Formerly: Recorded the movement
of Earth in relation to a stationary
mass on a rotating drum or
magnetic tape
Today: use motion sensors similar
to those in your smart phone
A seismograph designed to
record vertical ground motion
The heavy mass doesn’t move much
The drum moves
Lateral Movement Detector
In reality, copper wire coils move around magnets, generating current which is recorded.
 Types
of seismic waves
Surface waves
–Complex motion, great destruction
–High amplitude and low velocity
–Longest periods (interval between
crests)
–Termed long, or L waves
Two Types of Surface Waves
Most of the destruction
Larger amplitude than body waves

Types of seismic waves (continued)
 Body waves
– Travel through Earth’s interior
– Two types based on mode of travel
– Primary (P) waves
 Push-pull motion
 Travel thru solids, liquids & gases
– Secondary (S) waves
 Moves at right angles to their
direction of travel
 Travels only through solids
P and S waves
Smaller amplitude than surface (L) waves, but faster, P arrives first
Locating the source of
earthquakes
Focus - the place within Earth where
earthquake waves originate
Epicenter – location on the surface directly
above the focus
 Epicenter is located using the difference in
velocities of P and S waves
Earthquake focus and
epicenter
Delay between P and S arrivals gives distance to epicenter
Note how much bigger the surface waves (aka L waves) are.
Body Waves
P to S delay
Graph to find distance to epicenter
Average S wave speed is
3800 km in 12 minutes
Average P wave speed is
3800 km in 7 minutes
Locating the epicenter of an earthquake
 Three seismographs from different
observatories needed to locate an epicenter
 Each station determines the time interval
between the arrival of the first P wave and
the first S wave at their location
 A travel-time graph then determines each
station’s distance to the epicenter
Locating Earthquake Epicenter
 Locating
the epicenter of an earthquake
A circle with radius equal to distance to
the epicenter is drawn around each
station
The point where all three circles
intersect is the earthquake epicenter
Epicenter located using three seismographs
Earthquake Belts
95% of energy released by earthquakes
originates in narrow zones that wind
around the Earth
These zones mark of edges of
tectonic plates
Locations
of
earthquakes
80% of seismic
energy around
from
1980
to
1990
Pacific Rim
Broad bands are subduction zone earthquakes, narrow are MOR
Depths of Earthquakes
Earthquakes originate at depths ranging from 5 to
nearly 700 kilometers
Definite patterns exist
– Shallow focus occur between mid-ocean ridges
– Deep earthquakes occur in Pacific landward of oceanic
trenches
– Central continent (intraplate) earthquakes are of
various causes. Some causes still uncertain.
Devastating earthquakes occur less than 60
kilometers because cold rock is more elastic, and
transmits waves better than warmer rocks below
Earthquake Depth and Plate Tectonic Setting
Weakest are the
divergent zone
earthquakes
Strongest Here
Strongest, with worst Subduction Zones discovered by Benioff
Tsunamis, at entrance to
subduction zones
Earthquakes in subduction
zones
Recent example, 9.0 Christmas 2004 Earthquake and Tsunami, Sumatra
Earthquakes at Divergent
Boundaries - Iceland
A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR
Measuring the size of
earthquakes
 Two
measurements describe the size of an
earthquake
 Intensity – a measure of earthquake shaking
at a given location based on amount of
damage to buildings.
 Magnitude – estimates the amount of energy
released by the earthquake.
Intensity scales
 Modified Mercalli Intensity Scale was
developed using California buildings as its
standard
 Drawback is that destruction may not be
true measure of earthquakes actual energy
Earthquake destruction
 Amount
of structural damage depends
on
Intensity and duration of vibrations
Nature of the material upon which the
structure rests (hard rock good, soft
bad)
Design of the structure
Magnitude scales
 Richter magnitude - concept introduced by
Charles Richter in 1935
 Richter scale
–Based on amplitude of largest seismic
wave recorded
–LOG10 SCALE
Each unit of Richter magnitude
corresponds to 10X increase in wave
amplitude and 32X increase in Energy
Magnitude scales
 Moment magnitude was developed because
Richter magnitude does not closely estimate
the size of very large earthquakes
–Derived from the amount of displacement
that occurs along a fault and the area of
the fault that slips
Tsunamis, or seismic sea waves
Incorrectly called “tidal waves”
Result from “push” of underwater fault
or undersea landslide
In open ocean wave height is < 1 meter
In shallow coast water wave can be > 30
meters (more than about 98 feet)
Very destructive
Formation of a tsunami
3. Deep water
Wave 1 meter
above surface,
very fast
2. Wave as deep as water
1.Water pushed up
SNAP
4. In shallows wave rears up,
and slows down.
The Hawaiian Islands vulnerable.
Honolulu officials know exactly how
long it takes a Tsunami to reach
them from anywhere
Tsunami 1960, Hilo
Hawaii
Tsunami Model, Japan
Earthquake
Tsunami
Model,
Alaska Quake
Earthquake prediction
 Long-range
forecasts
 Calculates probability of a certain
magnitude earthquake occurring over a
given time period
 Short-range
predictions
 Ongoing research, presently not much
success
Long Term Predictions
Seismic Gaps
Long Term Predictions
 Strain
Energy - accumulates uniformly
- release irregularly
 Some locked by friction “Seismic
gaps”
–Prime candidates for major
earthquake
 Some release energy continuouslycreep
–No major earthquakes there
Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE
Seismic Gap along Himalayas
USGS web page on October 8 2005 magnitude 7.6 in Pakistan
Can earthquakes be predicted?
Short Term, Not very well

Short-range predictions
 Goal: provide warning location & magnitude
within a narrow time frame
 Research on precursors due breaking prior
slip. Breaking of rock lets the fault slip
 Breaks called fracture zones. The thickness
of the fracture zone is proportional to the
length of the fault.
 Breaking causes volume increase (dilation)
and uplift in the rocks.
 Dilatancy causes many measurable changes
Short-Term Earthquake Prediction
Dilatancy of Highly Stressed Rocks
BOX OF
ROCKS
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Investigating Earth’s Interior
Earthquakes help us understand Earth’s Interior
Structure. We use:
 Speed changes in different materials
due changes rigidity, density, elasticity
 Reflections from layers with different properties
 Attenuation of Shear Waves in fluids
 Direction changes (Refraction)

Result: 3 Major Layers of Earth
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Shallow Components of Earth
!
Seismic-wave velocities are faster in the upper mantle
Mohorovičić discontinuity
Croatian seismologist Andrija Mohorovičić
Velocity increases w depth, waves bend back to surface.
Waves that travel via mantle arrive sooner at far destinations
Wave Velocities
Crust slow
Mohorovičić discontinuity
Upper Mantle Fast
Asthenosphere
Slow
Lower Mantle Fast
Mineralogy of Earth’s Layers
This slide for graduate
students.
http://pubs.usgs.gov/gip/interior/
The S-Wave Shadow Zone
Since Shear (S) waves
cannot travel through
liquids, the liquid
outer core casts a
large shadow for S
waves covering
everything past 103
degrees away from
the source.
The P-Wave Shadow Zone
Discovery of the solid
inner core
P-waves pass ing through
the liquid outer core bend,
leaving a low intensity
shadow zone 103 to 143
degrees away from the
source, here shown as the
north pole
HOWEVER, P-waves
traveling straight through
the center continue, and
because speeds in the
solid inner core are faster,
they arrive sooner than
expected if the core was
all liquid.
Inge Lehmann
Behavior of waves through center reveals Earth’s Interior
End of Earthquakes