What are some of the important points of plate tectonics?
Plate boundaries are the main cause of volcanoes
Ocean to continental convergence
Continental volcanic arc (chain) forms on continental plate
Ocean to ocean convergence
Volcanic island arc forms on overriding oceanic plate
Many Mountain ranges and geologic structures are created due to the interaction of plates
Klamaths
Blues
Main cause of Earthquakes
Divergent
Shallow, often small quakes
Subduction
All depths and magnitudes of quakes
Cont-Cont Collision
Shallow to intermediate quakes, small to large
Transform boundaries
Shallow small to large quakes
Shallow < 100 km, inter 100-300 km, deep 300-600 km
How does the depth of an earthquake relate to damage?
As plates move faults are created due to stresses
Faults
Fracture in lithosphere created due to stress
Wide variety of sizes from tiny to large
Stress build up causes movement along faults which cause earthquakes
Different types of stresses create different types of faults
Fault types
Faults represent regions of fractured rock
Occur when stress is placed on rocks
Tensional (pull apart)
Compressional
Shear
Fault morphology
Head wall
Foot wall
Normal fault
Created by tensional forces
Head wall moves down relative to foot wall
Reverse Fault
Created by compressional forces
Head wall moves up relative to foot wall
Thrust Fault
Similar to reverse fault but angle of fault is low
Strike slip fault
Motion is similar to transform boundary
Shearing motion or shear stress
2 types…
Right lateral
Left lateral
20th century
It was determined that earthquakes occurred after years of stresses built up in the crust.
The stress is released along faults causing an earthquake
This concept is known as the Elastic Rebound Theory
Elastic Rebound Theory
Stress Builds up in Rocks
Rocks deform
Stress reaches point which exceeds rock’s strength and rock breaks
Rock “springs back” to pre-earthquake stress level
When energy is “released” it moves out in all directions.
Registered as P waves, S waves and surface waves (more in a bit)
Some Vocabulary
Focus or Hypocenter
Place inside of earth where fault moves
Epicenter
Location on surface above focus
Foreshock
Aftershock
20th Century
Early 1900 Mohorovicic used velocity of seismic waves to help explain the earth’s internal structure
First accurate seismic instruments developed
Seismic Waves
Represent energy released by fault movement
2 basic types of seismic waves
Body waves
Surface waves
Body Waves
Travel through the “body” of the earth
Reflected, refracted or absorbed at each mechanical and chemical boundary
Changes velocity at each mechanical and chemical boundary
2 types of body waves
P-waves
S-waves
P-waves
Primary or pressure waves
Move by compressing material
No rotational motion
Similar to sound waves in air
Travels through solids and liquids
Velocity is ~6.8 km/sec (<25km depth)
S-waves
Secondary or shear waves
Moves in a vertical motion and causes shearing and rotation of material (no
compression)
Only travels through solid material
Velocity is ~ 3.8 km/sec (<25km depth)
Surface waves
Moves along the surface or near the surface of the earth
Does not penetrate interior of the earth
Larger in amplitude and longer in duration than body waves
2 basic types
Rayleigh waves
Love waves
Rayleigh Waves
Cause ground roll
Moves as a vertical ellipse (vertical and horizontal motion)
Love Waves
Created by multiple reflections of s-waves
Motion is like a snake
Seismographs are used to record waves
1st to arrive is P-wave
2nd to arrive is S-wave
Last are surface waves
How is the epicenter of an earthquake calculated?
Draw radius from 1st seismic station equaling distance from epicenter
Draw radius from 2nd seismic station equaling distance from epicenter. Two possible
locations now for quake.
Third seismic station will define where quake is
Richter Magnitude
A measure of energy released during an earthquake (table16.3)
Theoretically a scale between 1-10
Log based 10 with regards to wave amplitude
mag 5 à mag 6 wave amplitude increases by 10
mag 5 à mag 7 wave amplitude increase by 100
Energy released is 32x
mag 5 à mag 6 increase energy released by 32x
Mag 5 à mag 7 increase energy released by 32 x 32= 1024x
Problems with the Richter Scale?
Moment Magnitude scale
Uses the seismic moment
Area of fault that has move
Average displacement of fault
Type of material involved (shear modulus)
More accurate analysis of energy released
WHY?
How do the two scales compare
Location
Richter
Moment
Loma
Prieta
7.0
7.2
Northridge 6.4 6.7
Mercalli Intensity
Forecasting/prediction?
How accurate are scientist at predicting earthquakes?
Long term vs short term
Hazards
Liquifaction
Loose sand and water compact with shaking
Often occurs in “fill”
Large quake not required
Creates Blow outs or sand boils
Clay capped sand
Landslides
Triggered by all sized earthquakes
Big problem in NW
Nisqually quake?
Minimal slides due to dry year
Shaking and Ground Motion
Cracks and Fissures
Ground Off set
Buildings
Why is damage so much more
extensive in 3rd world countries than here in
Un-reinforced masonry vs wood framed
To withstand an earthquake you want a building
Flexible
To move a one unit
To stay on its foundation
Wood
Flexible: bends before it breaks
Framing holds building together as a complete unit
May be damaged by won’t fall down
Un-reinforced masonry/brick
VERY brittle
Fractures cause broken pieces to move as individuals
Retrofitting:
Masonry
Add steel framing
E-quake bolts
Install external buttress
Politics involved
Very expensive
Historic buildings, schools, museums
Tear down and build new is cheaper
Many cities are retrofitting
Starbucks in
1998 8.5 million on x-frame bracing
2001 Nisqualli quake
Olympia Capitol building
Just finished repairing/retrofitting
What can be done to help prevent collapse?
High Rise Buildings:
Base isolation structures
Rollers (ball bearings)
Shocks (spring type systems)
Rubber/steel pads
Bridges:
Many bridges have poor attachment at footing
VERY expensive to fix
2 bridges in CA à $50 million
Oregon Bridges:
How many bridges are there?
Current retrofit and repair
Budget?
After Loma Prieta and Northridge quake
Began examination of bridges
Learned from CA earthquakes
2003 legislature
$1.3 billion was set aside for work on 365 state bridges in the next decade.
Tsunamis
NOT A TIDAL WAVE!!
Created by the displacement of water
Earthquakes, volcanoes, landslides
How does a wave work?
Start by looking at regular ocean waves
Ocean waves
Wave length, Wave height and motion on open ocean.
Primarily get swells…no breakers
Waves near the shore
Waves break as wave base interacts with the floor. Slows base while top continues at original speedà wave falls over
How are tsunami waves different
Much longer wavelengths
Long period
Normal wave has a period of 5-15 seconds
Tusnami wave has a period of 7 min to 1 hour
Tsunamis travel up to 500 mph depending on water depth
Tsunamis have no expression on open ocean
Size of wave at shore depends on
Seafloor configuration (
Distance from quake
Crescent City 21 ft wave
San Fran 3 ft wave
Wave size won’t indicate damage
How are Tsunamis monitored
Tsunami warning system
26 countries participate
Monitors
Sensors measure change in water pressure as waves pass over them
Indonesian Tsunami
Do all earthquakes cause tsunamis?
Why was the tsunami in
Where else did the tsunami travel?