Download Earthquakes - Chapter 10

What is an earthquake?

An earthquake is the vibration of Earth
produced by the rapid release of energy
 Energy radiates in all directions from its
source, the focus
 Energy moves like waves
 Seismographs record the event
Slinky, Rubber Band SEISMOGRAM
Beaker, Wet Sand, Weight
Cardboard Fault models
Chewing Gum
Wood meter stick or plastic ruler
pencil
Anatomy of Earthquakes
Earthquakes are associated with faults
Earthquakes are caused by
sudden release of
accumulated strain energy
along Faults
Rocks on sides
of fault are
deformed by
tectonic forces
Rocks bend
and store
(
elastic energy
(((
Hands
Frictional
resistance
holding the
rocks together
is overcome by
Demo forces
tectonic
(((
))))
)))
 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.
– DEMO – elastic rebound w/ ruler
Relationship
Between
Stress and
Strain
Demo: Rubber Band
Strain can be a change in shape (a deformation) due to an applied stress
Relationship
Between
Stress and
Strain at low
Temps and
Pressure or
Sudden Stress
Demo: Pencil
Relationship
Between
Stress and
Strain under
High Temps
or Pressure
Demo: gum
Strike and Dip
Strike is long line, dip is short line
Note the angle of dip given 45o
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
Vertical
Movement
along Dip-Slip
Faults
Divergent
Convergent
Horizontal Movement Along
Strike-Slip Fault
Reverse Fault Quake - Japan
DEMO – Types of faults
Strike Slip Fault Quake - California
Normal Fault Quake - Nevada
Fence offset by the 1906 San
Francisco earthquake

San Andreas is the most studied transform fault
system in the world

discrete segments 100 to 200 kilometers long

slip every 100-200 years producing
large earthquakes
 Some portions exhibit slow, gradual displacement
known as fault creep
Fires caused by 1906 San Francisco Earthquake
Gas mains break, fires shaken out of furnaces. Water mains break, cannot
fight fires. Debris in streets, Fire department cannot reach fires.
Landscape Shifting, Wallace Creek
San Andreas Fault, a Transform Margin
Liquefaction
Demo: Liquifaction
Seismology
Seismometers - instruments that
record seismic waves
Records the movement of
Earth in relation to a stationary
mass on a rotating drum or
magnetic tape
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.
Seismic Waves 1: Surface waves
–Complex motion, great destruction
–High amplitude and low velocity
–Longest periods (interval between crests)
–Termed long, or L 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
Demo: P and S waves
Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
Earthquake focus and
epicenter
Note how much bigger the surface waves are
Graph to find distance to epicenter
Locating Earthquake Epicenter
Epicenter located using three seismographs
95% of energy released by earthquakes originates
in narrow zones that wind around the Earth
These zones mark of edges of tectonic plates
Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates
Earthquake Depth and Plate Tectonic Setting
Subduction Zones discovered by Benioff
Earthquake in subduction
zones
Earthquakes at Divergent
Boundaries - Iceland
Crust pulling apart – normal faults
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
 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 severity
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
Destructive waves called “tidal waves”
Result from “push” of underwater fault
or undersea landslide
In open ocean height is > 1 meter
In shallow coast water wave can be > 30
meters
Very destructive
Formation of a tsunami
Tsunamis are actually huge, extending from
the fault on the sea floor up to the surface, but
they don’t stick up more than a meter or so in
the deep ocean. However, when they reach
shallow water they must rear up and slow
down. Discussion: Kinetic vs. potential energy
Honolulu officials know exactly how
long it takes a Tsunami to reach
them from anywhere
Tsunami 1960, Hilo
Hawaii
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
Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE
Seismic Gap along Himalayas
2005
Short-Term Earthquake Prediction
Dilatancy of Highly Stressed Rocks
45
Investigating Earth’s Interior
Seismology helps 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)

Investigating Earth’s Interior
47
Surface Components magnified
!
Seismic-wave velocities are faster in the upper mantle
Velocity increases w depth, waves bend back to surface.
Waves that travel via mantle arrive sooner at far destinations
Mohorovičić discontinuity
Wave Velocities
Upper Mantle Fast
Asthenosphere
Slow
Lower Mantle Fast
The S-Wave Shadow Zone
http://en.wikipedia.org/wiki/Richard_Dixon_Oldham
Since Shear (S) waves
cannot travel through
liquids, the liquid
outer core casts a
larger shadow for S
waves covering
everything past 103
degrees away from
the source.
The P-Wave Shadow Zone
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
P-waves 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 reveal Earth’s Interior