Review
Plate tectonics
Surface of the earth is divided into plates
Plates move around and interact with each other in several ways
Divergent Plate Boundary
2 plates moving away from each other
Occurs primarily in the ocean crust
Creates the Mid-ocean Ridge system
Largest mountain range on the surface of the earth
Volcanic mountains that CREATE the ocean crust
Primarily associated with oceans but there are some places where they can be found on the continents
Baja
Convergent Plate Boundaries
Plates come together
Different types of plates will interact in different ways
3 different convergent plate boundaries based on the types of crusts that are interacting with each other
What are the three types?
Continental to oceanic convergence
Subduction occurs
Ocean plate is forced under continental
Volcanic chain forms on continental plate
Oceanic to oceanic convergence
Older oceanic crust subducts
Older crust is denser due to being colder (farther from where it was created)
Creates Island Arc volcanoes
Continental to continental
Very large mountains form from this process.
EX//
Often starts as a ocean to ocean subduction
Transform Plate Boundaries
Plates slide by each other
Often associated with divergent boundaries
Mechanisms
Slab pull
Heat convection cells
Radioactive decay
Ridge push
How do Plate boundaries relate to earthquakes?
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
As plates move other fault structures 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
Faults cause earthquakes to happen away from plate boundaries
As stress builds in crust due to plate motion, faults away from a plate boundary can be come active.
Middle
of the
Basin and Range
New
1811-1812
3 earthquakes occurred from Dec-Feb
Magnitudes 8.1, 7.8, 8.0
Still seismically active today
Recent quakes in this region
In 2003 USGS forcast:
7-10% chance of mag 8 in next 50 years
25-40% chance of mag 6 or greater in next 50 yrs
1975-1995
August 1886 mag 7.0
What is an earthquake?
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
Some Vocabulary
Focus or Hypocenter
Place inside of earth where fault moves
Epicenter
Location on surface above focus
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 and refracted at each mechanical and compositional boundaries
Changes velocity at each mechanical and compositional 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 (nocompression)
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
Generated best by shallow quakes
Does not penetrate interior of the earth
Larger in amplitude and longer in duration than body waves
2 basic types
Rayleigh waves
Love waves
Seismographs are used to record waves. Record created is a seismogram
1st to arrive is P-wave
2nd to arrive is S-wave
Last are surface waves
How is the epicenter of an earthquake calculated?
Richter Magnitude
A measure of energy released during an earthquake
A scale between 1-10
Often people think that a magnitude 4 is 10x greater than a mag 5…sort of but not quite…
Log based with regards to wave amplitude not energy
mag 5 à mag 6 s-wave amp 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
Moment Magnitude Scale
Also used to calculate energy released
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
How do the two scales compare
Location Richter Moment
Loma
Prieta 7.0 7.2
Northridge 6.4 6.7
Mercalli Intensity
Can an
earthquake have a small Richter magnitude and an high Mercalli rating?
What type of quake (deep or shallow) would have a higher Mercalli
rating and why?
Earthquake Hazards
What are some of the things that happen during an earthquake that can cause damage?
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
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…
To be 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
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?
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 recurrence 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
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?
Earthquakes in the NW…Do we get them?
What type of plate boundaries do we have in the Pac NW?
Would these cause quakes?
In 1987 Cascadia subduction zone declared major threat
Potential to produce quake of mag 9.5
Evidence for large quakes in the Pacific NW
Submerged coastline due to subsidence
Ancient Tsunami deposits
Underwater landslide deposits
Inland crustal faults and quakes
Created by tectonic interactions
Hazards due to quakes in the NW
Based on what we talked about what could be some of the hazards the NW would face if a great quake occurred?