Earthquakes and Tsunamis Flashcards
What are the major fault types?
- Normal fault
- Reverse fault
- Thrust fault
- Left lateral strike-slip fault
- Right lateral strike-slip fault
Normal fault
Hanging wall is down relative to the footwall
Reverse fault
Hanging wall is up relative to foot wall at a high angle
Thrust fault
Hanging wall is up relative to footwall at a low angle
Faults occur because directed stress builds up in rocks:
- Compression
- Tension
- Shear
Motion and usual location of compression
Squeezing, convergent plate boundaries
Motion and usual location of tension
Stretching, divergent plate boundaries
Motion of shear
Slipping along a plane parallel to the stress
The accumulation of stress in a rock produces:
Strain or deformation
Strain takes place in 3 stages:
- Elastic deformation
- Ductile deformation
- Brittle deformation
What is elastic deformation?
Fully reversible (stretching the spring a little and it springs back). When stress is proportional to strain (Hooke’s Law of Elasticity)
What is ductile deformation?
Irreversible (stretching the spring too much and it won’t go back to its original shape). Folding, change in volume of rock.
What is brittle deformation?
Fracture (stretching the spring so much it breaks). Rock breaks.
What are faults?
Planar fractures in rocks, that vary in size and scale, where the rocks on either side have moved.
Earthquakes are caused by:
movement along faults
Elastic Rebound Theory
Friction exists along the fault plane, so the rock on either side of the fault resist movement
- Stress added
- Rocks deform, store energy
- Rocks break, release energy (earthquake)
- Rocks rebound to undeformed shape
Most earthquake foci occur at depths <15km in the crust. Why don’t earthquakes occur in the mantle?
The mantle behaves like ductile plastic because it can flow, increased depth = increased heat and pressure, results in plastic characteristics
Where do the deepest earthquakes occur?
Convergent boundaries and subduction zones, in addition to shallow/intermediate as well
Shallow, intermediate, and deep earthquakes can happen at subduction zones because of
the push and pull of subduction zones = earthquakes
Transform and divergent boundaries are predominantly associated with
shallow earthquakes
How do we detect earthquakes?
Seismic waves (vibrations) are conducted through the Earth, which allows for detection and triangulation
Body waves are
P-waves and S-waves that are generated at the focus and travel through the interior of the Earth
What do body waves generate when they arrive at the Earth’s surface?
Surface waves
P-wave Properties
Body wave
Relative speed: fastest out of all the waves
Passes through/along: solids and liquids
Type of motion: compression-extension/pushes and pulls
S-wave Properties
Body wave
Relative speed: 2nd fastest
Passes through/along: solids only
Type of motion: transverse side-to-side wave, shear
Love Wave Properties
Surface wave
Relative speed: 3rd fastest
Passes through/along: the Earth’s surface
Type of motion: side to side
Rayleigh Wave Properties
Surface wave
Relative speed: slowest
Passes through/along: the Earth’s surface
Type of motion: retrograde elliptical that moves in the opposite direction of the waves -> up and down, forward and back
Liquids do not resist changes in shape because
they can flow, which results in absorption of S-waves rather transmission
Surface waves tend to have the greatest:
amplitude in near-surface layers of sediment and are the most destructive of the earthquake waves
Seismographs all over the world record the arrival of
earthquake waves
How do seismographs work?
- Needle/fix mass doesn’t move
- When the ground moves, the base moves causing the frame to move, and the fix mass then records seismic activity and amplitude
- They tend to be installed where there is not much “background noise”
- Seismograph stations are located all over the world
- Tons of stations allows for more accurate triangulation of the epicenter
Clusters of seismic activity recorded:
P-waves first, then S-waves, and then surface waves (love then Rayleigh technically lol)
Earthquakes attenuate as they
travel Earth (amplitude of the waves is diminished)
If the earth was completely homogeneous, waves would
travel along essentially straight paths
The interior of the Earth is compositionally layers and the density of rocks generally:
increases with depth, particularly in the mantle
Changes in density cause the waves to:
bend or refract as they travel through Earth
- For P-waves, this produces a shadow zone where no P-waves are detected
S-waves cannot travel through liquids, therefore:
they cannot travel through the outer core
- There is also the solid inner core which bends P-waves
Modified Mercalli Scale
Relies on describing and rating effects on humans and infrastructure (intensity)
- Subjective to individuals
Richter Scale
Measures magnitude (amount of energy released) - Each level increase indicates 10x the amount of shaking and 33x the amount of energy released
Moment Magnitude (Mw)
Mw = (2/3)logM0 - 10.7 M0 = seismic moment, proportional to average slip on the fault x ruptured areas on the fault x rigidity of the faulted rock
Can we predict earthquakes?
No, can predict where, but not when, based on historical data
Animal behaviour can predict earthquakes, but not reliably
What other hazards are associated with earthquakes?
- Displacement along faults can cause changes in ground level (vertical displacement)
- Earthquakes can trigger landslides. Debris can block the flow of rivers, leading to flooding
- Fires can be caused by ruptured gas lines, fallen power lines, etc.
- Shaking can cause saturated sands to undergo liquefaction (all pore spaces are filled with water, and with shaking, the particles “float” like quicksand)
- Tsunamis
Earthquake damage
Geological setting can influence the amount of ground motion:
- Example of Mexico City earthquake: extreme damage because the city is constructed on lake bed deposits
- Despite moderate magnitude (7.1), shaking amplified by weak sediments
- Velocity went down, amplitude went up = more shaking -> extreme damage
Hazard maps
- Generated based on near-surface conditions, sediment type
- Red areas are more prone to weakening during shaking
- Have a high liquefaction potential, could lead to collapse of structures
Associated hazards: liquefaction
- Intense shaking can cause saturated sediment to lose strength and behave more like a fluid
- Can cause the ground to subside, fracture
- Fluidized sediment can flow upward along fractures
- Causes ground surface to settle unevenly, leading to widespread damage
Triggered landslides
- Earthquakes and landslides are closely linked natural hazards
- Ground motion and shaking causes rock or sediment to rapidly move downslope = slope failure
- Landslides can occur on continents or in submarine areas, where they can also cause tsunamis
Associated hazards: tsunami
A tsunami is a series of waves that result from the displacement of water
Can be triggered by:
- Large magnitude seismic event that displaces seafloor (up or down)
- Collapse of a volcano
- Submarine landslides (also potentially triggered by seismicity)
- Asteroid impact
Tsunami Formation Steps
- Earthquake rupture in seafloor pushes water upwards, starting the tsunami
- Tsunami moves rapidly in deep ocean, reaching speeds of up to 950km/hr with wave height <1m
- As the tsunami nears land, it slows to about 45km/hr but is squeezed upward, increasing in height
- Tsunami heads inland, destroying all in its path (trough of the wave may arrive first, exposing seafloor)
Earthquake Induced Tsunamis
- Wave height in the open ocean is ~1m and the distance between crests is ~200km, which would be unnoticed in the open ocean
- As the wave approaches land, water depth and velocity decrease, while wave amplitude (height) increases
- As it approaches shore, it transforms into a turbulent surge that can consist of a series of waves
- In some cases, the trough arrives first, causing water to recede
- Run-up is the maximum horizontal and vertical distance travelled on land
Associated hazards: triggered submarine landslides, tsunamis
Factors that contribute to triggering potential:
- Rapid sediment deposition leads to unstable slopes (unconsolidated sediment is more susceptible)
- High fluid pressure weakens sediment
- Plunging river discharge that forms turbidity current at the seafloor
What regions are at risk?
Coasts, especially those next to vertical fault displacements
Mechanisms for induced earthquakes all change
the stress state in the earth
Humans can induce earthquakes by:
- Injecting liquid waste deep underground
- Pumping gas or fluid from underground
- Hydraulic fracturing (fracking)
- Building a dam and flooding a valley
- Nuclear detonations underground
