L6: Earthquakes Flashcards
What is an earthquake?
Earthquake: A shaking or vibration of the ground
How does an earthquake occur/what happens?
Notes!
Define slip
The distance of displacement
The more ______ = the bigger the ______
- stress/pressure
- Deformation
Earthquakes are ______. What does this mean?
- Cyclical
- Recurring bc the force behind the plate movement are always present
List the 3 types of faults
- Normal faults
- Thrust faults
- Strike-slip
Normal faults
- due to tensile stress. From tensional forces; pulling things apart.
- One thing slides down relative together
- Slides down
Thrust faults
- due to compressive stress
- Compression; mountain building
- Slides up
Strike-slip
- faults due to shearing stress
- Side to side
What are secondary effects of earthquakes?
- Fires
- Landslides
- Liquefication
- Tsunamis
Define focus
- The point on a fault at which the first movement or break occurs during an earthquake
- Where the rupture happens; not at one specific point in the rock can be a large region. We look at a focal point. Elastic energy is gone as soon as it ruptures
Define epicenter
The point on the Earth’s surface directly above the focus; can mark where the earthquake was
What are seismic waves?
When the blocks slip suddenly at the time of the earthquake, intense vibrations called seismic waves travel outwards from the focus much as waves ripple outward from the spot a stone was dropped in a still pond.
Elastic rebound
Rocks go back to original undeformed state
Richter Magnitude
- The damage an earthquake causes is typically measured as the Richter magnitude –the amount of ground displacement or shaking it produces at the epicentre. The scale is logarithmic, so that means that an earthquake of magnitude 5 causes ten times as much ground movement as one of magnitude 4
- Based on the amount of energy that’s given off after an earthquake
- Low earthquakes bc lots of interfaces of rocks
What are the families of seismic waves
- Body waves
- Surface waves
Body waves
- Travel outward in all directions from the focus and have the capacity to travel through earths interior
- P and S waves
List the types of seismic waves.
- P waves
- S waves
- Surface waves
What is seismology?
The study of the propagation of mechanical energy; released by earthquakes through the Earth
P waves
- known as compressional or primary waves that move parallel to direction of movement. - - They move faster through solids (6-8 km/s) than liquids.
- About how the elements are jostled
- Expands and extracts. The fastest of all seismic waves
- 2x as fast as an S wave
- Can compress liquid; can travel through everything
- Moving through the ground
- P-waves travel through the crust at speeds between 6 km/s (continent) to 7 km/s (ocean crust)
- Below the crust, the velocity increases even further to 8 km/s
S waves
- known as shear or secondary waves. They move perpendicular to direction of movement. - They only travel through solids (4-5 km/s).
- Moves up and down
- Do not move liquid or air; a solid is needed
- Useful for seeing where water, oil, and gas are below us
- Moving through the ground
Surface waves
- confined to Earth’s surface. They are the slowest moving but the most damaging.
- Cause all the damage; problematic
What do we know about earths interior?
- Look at diagrams in slides
- How do we know theres a liquid outer core? - No S waves
- Must be something partially molten hence no S W
- We know theres a solid inner core by the P waves bc P waves can move thru liquid and solid
- Light sound and seismic waves reflect and refract
How can we tell how deep the crust is?
- We can tell how deep the crust is by examining P waves
- P-waves travel through the crust at speeds between 6 km/s (continent) to 7 km/s (ocean crust)
- Below the crust, the velocity increases even further to 8 km/s.
- This increase marks a sharp boundary between crustal rocks and underlying mantle known as the Mohorovicic discontinuity (or Moho). The faster speeds are a consequence of denser materials
Earthquakes at shallow depths
- Occur at spreading ridges
- associated with divergent boundaries and transform faults, as well as on continents due to crustal movements
Earthquakes at deep depths
- Occur along subduction zones
- at subduction zones they occur because the brittle lithosphere is forced deep into the asthenosphere.
Strong earthquakes
subduction occurring; deep subduction zones are problematic whereas near surface ones are not
Why doesn’t Alberta have EQ’s?
in a stable plate not near an active margin unless somewhere below us that we don’t know about
Why are there some unusual Canadian EQ’s?
Some earthquakes are unusual in that they are not associated with plate margins. They often occur on old faults that were once part of ancient plate boundaries, while others may arise as a result of isostatic rebound
Ground motion
major effect of ground motion includes the offset of rocks that can break power lines, pipelines, buildings, roads, bridges and other structures
What are some strict codes enforced to help minimize effects of ground motion?
- permit excess slack in power line and pipeline construction
- designing earthquake- resistant buildings
- building only on solid rock.
Ground failure
Landslides arise when structures are built on hill locations with unstable slopes
Liquefaction
occurs when water saturated soil is shaken, thereby reducing the friction between soil particles that give the soil strength, causing the soil to become like quicksand. Basically creates cements; water displaces from the solid
Tsunamis
- Tsunamis are seismic sea waves. When an undersea or near-shore earthquake occurs, the sudden movement of the seafloor creates waves that travel through the ocean as broad swells on the surface. However, when the tsunamis approach land, they develop into large breaking waves as the waves touch the shallow ocean floor. Tsunamis can travel quickly, with speeds in excess of 1000 km/hr.
- Essentially displacement under the ocean
