Geological Hazards Exam 2 Flashcards
Earthquakes
Physical response to the release of built up stresses in the lithosphere.
Strength of the rock is exceeded –> rapid release of elastically stored energy (seismic waves and heat)
Release of plate tension
Too much stress is released and an earthquake happens because the plates slip.
How is an earthquake located?
Seismologists use the difference in arrival time between the P and S waves to calculate the distance between the source and the seismograph. Data is used from multiple seismographs. Circles are drawn around each seismograph. The earthquake originated in the intersection between the circles.
Seismograph
Instrument that measures motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.
Mercalli Scale
Measures shaking intensity
Uses California buildings as standards
Maps intensity by destruction of buildings
Richter Scale
Based on the amplitude of the largest seismic wave
Only useful for shallow earthquakes over short distances
Log based scale
Now walled Local Magnitude (M1)
Moment Magnitude
Measures the strain energy released
Uses total energy released
More difficult to calculate (can still be one with a seismograph)
Variables: strength of the rock, area of surface, average amount of slip.
Moment = XAD
X = 32 GPA (crust) or 75 Gpa (mantle)
A = Area (LW)
D = Average displacement along rupture surface
How is one earthquake different from another? What can very?
Different types of faults (strike-slip), zones (subduction zone), boundaries (convergent boundaries), and locations can create different types of earthquakes.
Dip-slip faults
3 categories: Normal Faults, Reverse Faults, Thrust Faults.
2 broad categories of faults
Dip-slip faults
Strike-slip faults
Extension (earthquakes)
Dip-slip faults
Same amount of rock takes up more space
Basin or range
Form along divergent boundaries
Compression (earthquakes)
Dip-slip faults
Rock smashed together so it takes up less space
Himalayan Mountains
Normal Fault
Dip-slip fault
Can result in extension or compression
Form along divergent and convergent boundaries
Reverse Fault
Dip-slip fault
Rocks squishing together
Angle up to 60 degrees
Form along convergent boundaries
Thrust Fault
Dip-slip fault
Between 10 and 30 degree angle
Created at convergent boundaries and subduction zones
Strike-slip Fault
Usually large earthquakes
Types: right lateral, left lateral
Form along transform boundaries
Divergent Boundaries
Plates moving away from each other or spreading
Shallow earthquakes usually happen at these boundaries.
Convergent Boundaries
Plates moving towards each other
Transform Boundaries
Plates sliding past each other
Main hazards associated with earthquakes (5)
Shaking Liquefaction Landslides Tsunami Damage to infrastructure
Hypocenter / focus
Means “below the center”
The position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture.
Occurs at the focal depth below the epicenter.
Epicenter
The point on the Earth’s surface that is directly above the hypocenter or focus and where the fault begins to rupture.
The point where an earthquake originates.
P-Wave
Pressure wave Primary wave Compression and Contraction Moves quickly Travels
S-Wave
Shear wave
Verticle motion
Moves slower
Travels –>
L-Wave
Love wave
Moves from side to side
Moves slower
Travels –>
Rayliegh Wave
Move like ocean waves
Slow
Moves particles vertically and horizontally
Nomogram
A graphical calculating devise, a two-dimensional diagram designed to allow the approximate graphical computation of a function.
Tension
Stress which stretches rocks in two opposite directions. The rocks become longer in a lateral direction and thinner in a vertical direction. Results in jointing in rocks. Rare. Found in divergent boundaries.
Contraction Theory
Earth was a molten blob –> surface cooled and contracted and caused some parts of the Earth’s crust to buckle upwards forming mountain ranges and some downwards forming valleys and ocean basins (raisins)
Shear Stress
Experienced at transform boundaries where two plates are sliding past each other.
Shearing faults are transform faults in the ocean and strike-slip faults on continents.
Elastic Deformation
A sufficient load is applied to a crust layer causing it to change shape. A temporary shape change that is self-reversing after the force is removed so that the object returns to its original shape, is called elastic deformation. A change in shape of a material at low stress that is recoverable after the stress is removed. Stretching of bonds but the atoms do not slip past each other.
Plastic Deformation
When the stress is sufficient enough to permanently deform the crust. Involves the breaking of a limited number of atomic bonds by the movement of dislocations. Atoms slip past one another at a much lower stress level.
Fracture
Horizontal line between active transform faults.
Strike-slip activity occurs here.
Tensor Diagrams
Visual depiction of multilinear functions or tensors.
Liquefaction
Process by which laden sediment is disturbed, allowing the movement of pore water upward.
Destabilizes ground
Water-saturated granular layer of earth will compact leaving the water nowhere to go but up through sediment layer and to the surface.
Similar to lateral spread.
Elastic Rebound Theory
Applying load until plates deform –> plates finally slip and release energy
Fence going across the two plate surfaces is going to bend when plates are deforming
After the plates slip, they will go back to having no energy.
Brittle Deformation
Locally developed mesoscopic deformations such as faulting or jointing. In short, breaking the rock. It is also possible to deform rocks without breaking them (plastic deformation)
Brittle strain is a type of permanent strain
Permanent ductal strain (ductal deformation) is like squishing a ball of Play Doe.
Strain
A change in material shape and/or volume caused by stress.
2 Types: temporary and permanent
Temporary Strain
Can apply a load
Deforms but the deformation isn’t permanent
Stick bends but doesn’t break in response to the load.
Permanent Strain
Too much load
Broken stick
Stress
Force per unit area
S = F/A
Pressure exerted on rocks inside the earth create earthquakes
Where do earthquakes occur?
Along plate boundaries
Along fault zones
Atomic Decay
Heat is due to the decay of radiogenic isotopes via multiple decay mechanism.
Energy for earthquakes
Convection zones are driven by the heat produced by atomic decay.
Fault Scarp
Forms when a fault has broken the surface of the earth.
The new surface exposed is the fault scarp itself.
Geodesy
GPS stations set up to record relative plate motions.
Calculate changes in plate direction and rate in that direction.
San Francisco 1906 earthquake
5:12 am Magnitude: 7.8 Was not fully understood until plate tectonics (~50-60 yrs later) Lead by a foreshock Mercalii Index of VII-IX Shaking 40-60 seconds Caused fire in SF which was more destructive than the shaking. 700-800 dead (underestimate) Actual = 3-4x more dead
Foreshock
An earthquake that initiates a larger earthquake.
How to mitigate earthquake damage
Anticipate hazards (geological maps of areas, historical evidence, model hazards). Enact codes (building codes, quality construction elsewhere). Educate!
Great Alaska 1964 earthquake
2nd most powerful earthquake ever recorded
Rupture length unknown
Magnitude: 9.2
$311 million in damage
Maximum uplift: 11.5m (~33ft)
Lateral spread intensified this earthquake
Death toll: 128 (113 from tsunami, 15 from earthquake)
33’ uplift
Mercalii Scale: III-IV
Associated with tsunami
Lateral Spread
Form of mass wasting
Water laden sediment under thick sediment
Extends rock
How is a tsunami generated?
Earthquakes (dip-slip faults)
Landslides (terrestrial or underwater)
Volcanic eruptions (pyroclastic flows, violent undersea eruptions)
Bolide impact (direct impact by an asteroid or comet)
Tsunami Height (open water)
Measured from crest to trough Long wavelength Low amplitude Varying period (series of waves that come in at different intervals) Speed: 500-950kph
Tsunami Height (near land)
Measured from crest to trough Progressively shortening wavelength Amplitude increases Varying period Speed reduces
Warning System (tsunami)
Warnings are issued (large earthquakes or volcanic eruptions)
Local areas are checked to see if there is displacement of water.
Open ocean tsunami are monitored by a buoy system.
DART (Deep-ocean Assessment and Reporting of Tsunamis) largest and most robust network in the world
Prediction models
Tsunami characteristics
Wave velocities: 950kph
Wavelength: 200km
Wave moves on shore and keeps going
Tsunami public misconceptions
Can occur in any body of water
False sense of security because of length of time between waves
Ignorance about what it means when shoreline moves very far out
Not a “tidal wave” because they don’t have anything to do with tides.
That they can outrun a tsunami
There are multiple waves, not just one.
First wave is not always the largest
Tsunami waves are similar to…
Rayleigh waves because of the way they move particles.
Tsunami
Literally means “harbor wave”
A great sea wave produced by submarine earth movement or volcanic eruption.
Inundation (tsunami)
An overflow; a flood; a rising and spreading of water over grounds.
Run-up (tsunami)
Distance water travels in-land.
Usually measured in km.
Seiche (tsunami)
A standing wave in an enclosed or partially enclosed body of water. Have been observed on lakes, reservoirs, swimming pools, bays, harbors and seas. Means “to sway back and forth.” Extremely long wavelengths. The effect is caused by resonances in a body of water that has been disturbed by one or more of a number of factors. The key requirement is that the body of water be partially constrained to allow formation of standing waves.
Most Model (tsunami)
= Method of Splitting Tsunami) Model
Has preset models
Takes real data from DART and refines predictive models.
Estimates run-up distance and issues warnings.
Destruction of Tsunami
Water has a lot of mass therefore the force of a tsunami can be quite extreme.
Debris becomes a battering ram.
False sense of security because of length between waves.
Mitigating Tsunamis
Sea walls
Evacuation
High ground
Largest wave ever recorded
1958 Lituya Bay, Alaska, 1720 feet (524 meters)
Boxing Day Tsunami 2004
230,000 deaths Fault rupture length: 1600 km Fault type: megathrust Magnitude: 9.1 Total movement: 15m Tsunami: up to 30m Bad because: unexpected, no warning system, many did not know what to do, total devastation.
The Great Tohoku Tsunami 2011
18,000 deaths Fault type: megathrust Fault rupture length: 480 km Magnitude: 9.0 Total movement: 2.4 m Tsunami: up to 40.5 m Inundation distance: 10km
Japan Tsunami
Japan is the most proactive nation in the world with regard to natural disaster preparedness.
The tsunami was larger than anticipated and overtook barriers.
Far grater inundation than expected.
Collateral damage (e.g., landslides, fires)
Difficult terrain
Puerto Rico Tsunami 1918
6 m tsunami
Affected Caribbean
Earthquake magnitude: 7.3
116 people killed, 100 people missing
Grand Banks Tsunami 1929
No height recorded Affected North America and Europe Earthquake magnitude: 7.4 Submarine landslide 26 people died
Lisbon Tsunami 1755
6-12 m Tsunami
Affected Portugal, Spain, North Africa, and the Caribbean
Earthquake Moment Magnitude Index: XI
Killed: 60,000-100,000
Mass Wasting
Down slope movement of rock, regolith, and soil under the direct influence of gravity.
Gravity-driven down-slope failure of material.
Triggers of mass wasting
Saturation of material
Oversteepening/undercutting slopes
Adding weight to the slope
Removal of vegetation
Ground movement from earthquakes
Rock type
A trigger is not the sole cause of the event but the last of many causes.
Gradients over 15 degrees
Clay-rich soil or shale instead of bedrock
Unusual water increase from heavy rainstorms
Loss of vegetation
Change in supporting material through erosion
What holds the landscape together so there isn’t MORE mass wasting events?
Vegetation
Friction
Grain friction is “cohesion”
Role of water on loose material (mass wasting)
Small amounts of water increases cohesion
Large amounts of water decreases cohesion
Role of water on solid rock (mass wasting)
Water reduces the strength of the rock regardless of amount (faults, joints, in pore space)
Acts like a lubricant
Types of mass wasting (8)
Debris flow Earth flow Translational landslide Rock fall Rock avalanche Lateral spread Rotational landslide (slump) Creep
Debris flow (type of mass wasting)
Water rich
Very fast
A lot of material transported with it
Very powerful
Earth flow (type of mass wasting)
Little bit less water and a little bit slower than debris flow.
Translational and rotational landslides
Transported down-slope on some slip angle.
Rock fall (type of mass wasting)
Dry event
Failure depends on how steep the slope is
Less than 100 cubic meters worth of material driven down-slope.
Rock avalanche (type of mass wasting)
More than 100 cubic meters worth of material driven downslope
Completely dry
Lateral Spread (type of mass wasting)
Tunagin Heights earthquake resulted in a lateral spread
Rotational Landslide (slump)
Sliding Surface ends up looking like a spoon Water accumulates --> downslope failure Initially more mass up top --> trying to reach equilibrium by sliding mass down and trying to even it out. Crown develops above the head scarp
Humocky (mass wasting)
Rubble produced by the mass wasting event
Creep (mass wasting)
Happens very slowly
Ground slope failing over a long period of time
Very bad for buildings because they start to crack and the foundation starts to fail.
Top portion of soil profile (surface of the earth) starts to fail.
Relatively dry
Solifluction (mass wasting)
Relatively wet
Happens in arctic environments
Warmer layer (tundra frozen stuff melts into water making it move) above the permafrost layer (permanently frozen ground) –> downslope failure –> creates solifluction lobes (works like tractor treads running over themselves).
Features of old mass wasting events
Tilted trees Pistol-butt trees Hummocky or lumpy bumpy land Talus slopes Old debris flow valleys
Talus slopes (mass wasting)
Talus means broken rock
Broken rock falls off face such as a cliff or mountain.
Constantly building talus cones by rock breaking, slipping and falling down making a cone of broken rock.
Old debris flow valleys (mass wasting)
Death valley
Series of drainages called illuvial fans
Fans get overridden by new deposits (old debris flows get cut by younger debris flows)
Cohesion
Grain friction
Porosity
Volume of open space in a sediment.
Permeability
The ability of a sediment to transport fluid through the sediment.
Angle of Repose (mass wasting)
The steepest angle a loose material (soil) can attain and stay stable.
Changes due to external factors.
Montmorillonite or “Quick Clays”
Readily absorb water and expands the sheet layers as a result.
Destabilizes slope and creates a mud slide
Water downpours on it –> clay expands –> slope fails –> landslide
Mitigation of Landslides
Geotextile fabric (waterproof) Rock bolts Netting Shotcrete Dewatering (perforated pipes) Increasing load at toe Vegetation
How do earthquakes occur?
Stress
Strain
Brittle deformation (faults)
Hanging wall
Part of the fault which is moving
Footwall
Part of the fault not moving
What color shirt was Whitmore wearing on review day?
Grey
Scarp
Forms when the fault has broken the surface of the earth.
