 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

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
( ( ( (
Frictional resistance holding the rocks together
Hands Demo
) ) ) ) ) ) )


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 is long line, dip is short line
Note the angle of dip given 45 o
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
Normal Fault Quake - Nevada
Strike Slip Fault Quake - California

 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

Demo: Liquifaction


Records the movement of
Earth in relation to a stationary mass on a rotating drum or magnetic tape

The heavy mass doesn’t move much
The drum moves

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

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

Earthquakes at Divergent
Boundaries - Iceland
Crust pulling apart – normal faults

 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
–
LOG
10
SCALE
Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in Energy


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

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
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

2005

Short-Term Earthquake Prediction
Dilatancy of Highly Stressed Rocks
45


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)

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