Week 4, When the Earth Shakes in Canada... Flashcards
Extensional Regime
Two plates are being pulled apart. Normal faulting.
Mega Thrust Earthquakes
Inter-plate
Shear interface between subducting and overriding plates
M8-9+
Pleoseismic Evidence of a megathrust earthquake
- Coastal marsh soil profiles
- Drowned coastal forests
- Seabed turbidite layers
- Japanese tsunami records
- First Nations oral records
Coastal Marshes
- Peat: deposited when marsh sits above high tide
- Mud: Deposited at low tide
- Sand: brought in by tsunami
Drowned coastal forests
Ghost forests.
Abrupt subsidence in event of earthquake drowns trees in saltwater. Can use dendrochronology to study tree rings to determine date of tree death, thus giving us an idea of when a megathrust earthquake might have occurred.
Seabed turbidite layers
Mud interlayered with sand.
Japanese tsunami records
A tsunami occurs in Japan with no earthquake. Tsunami would take 9 hours to get to Japan, the only earthquake to fit this data is the megathrust that occured in the Cascadia subduction zone. Very accurate data.
First Nations oral records
Spoke of a tsunami where the Pachena Bay people were lost.
Megathrust cycle
Interseismic Period:
- Period between earthquakes.
- Takes place over years.
- We experience all of the effects of evidence seen in the paleoseismic record.
- The toe of North America is dragged down, causing upwelling in Vancouver.
Coseismic Period:
- Happens in a matter of minutes.
- Toe of North America plate jumps up.
Seismology
Study of Earthquakes
Seismometer
Instrument that detects vibrations of earth to measure earthquakes.
Seismograph
Recording instrument to record what is detected from seismometer.
Wave Properties, Function of Position:
- Amplitude (A): Maximum value
- Wavelength: Distance of one wave cycle
- Velocity (V): Speed at which wave moves
Wave Properties, Function of Time:
- Period (T): Measured in seconds of one cycle
- Frequency: Distance travelled per time
Body Waves
Propogate through interior of earth.
Compression
Shear
Surface Waves
Propogate along surface of earth.
Rayleigh
Love
Compressional Waves
- Body waves
- P Waves
- Particle motion is same direction as direction of propogation.
- Can propogate through any material, liquids, solids, and gases.
Waves do not transport material, only
energy.
Shear Waves
- Body waves
- S waves
- Particle motion is perpendicular to direction of propogation. Increases with density.
- No size change, only distortion.
- Slower than P wave.
- Can only exist in solids, not liquids or gases.
Rayleigh Waves
- Particle motion is retrograde elliptic, opposite the direction of propogation at surface. Ground roll like a boat at sea.
- Lr Wave
- Long wavelength
- Only exists in solids, not liquids or gases.
- Slow
Love Waves
- Particle motion perpendicular to propogation direction.
- Lq Waves, faster than Rayleigh waves, smaller amplitude.
- Damaging to buildings built to withstand vertical stress. Not horizontal motion.
- Can only exist in solids, not liquids or gases.
- Destructive because of shaking direction.
Earthquake Magnitude
Objective energy measurement, an attempt to measure earthquake energy release.
Richter Scale
Frequency reduces as magnitude increases.
Earthquake Intensity
Used to asses local earthquake effects
What people feel and how they experience the shaking in the area.
Modified Mercalli Intensity Scale
Depends on:
- Magnitude
- Distance to hypocentre
- Shaking duration
- Local soil/rock type
Modified Mercalli Intensity Scale
12pt scale based on standardized observtions
1-5 not too bad, 6+ escalates quickly.
Iso-seismal map
Tells us about spacial intensity of an earthquake.
Richter Scale
Logarithmic, increases by 10.
Ex. M4-M5= 10x shaking
M4-M6= 100x shaking
M4-M7= 1000x shaking
