Download Notes-Earthquakes

Earthquakes
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- Shaking or vibration of the ground
-An earthquake is the result of a sudden release of energy in the Earth's
crust that creates seismic waves
- Rocks undergoing deformation break suddenly along a fault line
- Earthquakes affect more than 35 countries, can have high death tolls (i.e.
750,000 people in ’76 in China), primary feature is ground shaking,
secondary phenomena (aftershocks) are fire, landslides, ground subsidence,
snow and ice avalanches and floods
Where are earthquakes found?
- The earth’s surface is composed of a number of mobile “tectonic plates”
which are in constant motion
- Most earthquakes are found at plate margins
Plate Tectonics
-The constant movement of the plates is referred to as plate tectonics
- There are three main types of plate boundaries:
Divergent (Gentle earthquakes, no recorded deaths due to these quakes)
Convergent
Transform (Earthquakes do not occur at the transform plate boundaries)
Divergent Margins
- At the divergent margins two tectonic plates are in the process of being
created
- Magma is injected into a crack in the earth’s crust (magma is created by
decompression melting of the earth’s mantle), then it cools and becomes
new crust
- The middle of the Atlantic Ocean is a divergent margin which is being torn,
or rifted, apart…the two plates are separating continuously at a rate of
several cm/yr
- The earthquakes that occur along the Atlantic are gentle earthquakes
Convergent Margins
- Instead of two plates being created, they are being consumed…
- Here an oceanic plate slides beneath a continental plate, since the former
is denser
- Geologists refer to this process as subduction
- Large, destructive earthquakes occur here
- If two continental plates collide, they do not undergo subduction, because
they are too buoyant
- Instead, intense compression occurs
- Large, destructive earthquakes also are generated in this situation
- When Two continents collide, the ocean between them closes and they
keep colliding and form a mountain chain
Transform Margins
- The third type of plate margin is called a transform boundary
- Here, plates are neither created nor destroyed, they simply slide by one
another
- Causes gentle earthquakes but no volcanoes
- No creation or destruction of plates, they just slide by one another
- No subduction zone, no rifting
- Horizontal movement but the fault between the two plates is vertical
Faults Associated With Earthquakes
- Faults are planes of weakness along which the Earth has been broken and
where there is constant movement
- Movements on a fault can be either slow (ductile deformation) or fast
(brittle fracture)
- When a fault behaves in a brittle manner and breaks, earthquakes are
generated
Three Types Of Dominantly Vertical Faults
- A normal fault is the result of tensional forces (e.g., rifting)
-Reverse and thrust faults are the result of horizontal compression
-If the angle is less than 45 degrees, the fault is called a thrust fault
-Blind thrust faults are the most dangerous because we can’t see them at
the surface (faults are hidden underground)
Faults Whose Movement Is Dominantly Horizontal
- These faults are termed strike-slip faults
- They are a small-scale version of transform plate tectonic margins
- They are termed left-lateral (sinistral) or right-lateral (dextral)
according to their movement
-Two plates grinding together
-Fault is vertical but the movement of the plates is horizontal
Elastic Rebound Theory
Theory that seismologists use to explain (in terms of energy) how
earthquakes happen in nature
-Earthquakes are releases of energy
-Compressive forces result in a deformation of the rock which causes energy
to be stored in the rock
-The amount of energy stored becomes too high to there has to be a rupture
in the rock in order to create more surface area and release the energy
stored
-When we exceed the elastic limit the rock ruptures and the energy has to
be released at the surface
-After the release of energy the rocks bounce back to their original shape
-Rocks that are below 300 km below the surface are too deep to generate
earthquakes
Violent versus gentle earthquakes
Why are some earthquakes much more violent than others?
-The killer earthquakes happen to be in the ring of fire where the subduction
zones are (reverse faults and blind thrust faults occur here because there’s
more compression)
-The more gentle earthquakes occur at the divergent boundaries (normal
faults occur here)
-The worst earthquakes occur at subduction zones because we compress the
rock, under compression the rock stores A LOT of energy and it takes much
longer for them to overcome the elastic limit therefore it takes time for the
rock to break
-When the rocks at subduction zones finally reach the elastic limit there is
way too much energy and this creates incredibly violent earthquakes
-Under tension (divergent boundaries) the rocks tend to be very weak so
they exceed the elastic limit very fast and they don’t have enough time to
store a large amount of energy so the earthquakes on divergent margins are
not particularly violent (magnitude 2-3)
Sizes of earthquakes
The size of an earthquake is the main factor in its destructiveness
Two ways to estimate size:
- Richter magnitudes
- Mercalli Index
Richter Magnitudes
- Richter magnitude measures the movement of the earthquake and the
energy released during vibration
- The Richter magnitude measures the maximum amplitude of ground
shaking (vibrational energy)
- It is a logarithmic scale
- 1 Richter unit difference is x 10 for ground motion and x 33 for energy
- Globally, small earthquakes are more frequent than large:
~800,000/yr for events of magnitude 2.0-3.4, while an event of magnitude
8 occurs once every 5-10 years
The modified Mercalli intensity scale
- Based on observations/what people have felt when the earthquake
occurred
- Qualitative approach as opposed to a quantitative one
- Magnitudes do not necessarily describe the destructiveness of an
earthquake…the earthquake may be close (more destructive) or distant
(less destructive) from a population center…and the event may be shallow
(more destructive) or deep (less destructive)
- The modified Mercalli intensity scale is used to assign a measure of
destructiveness to an earthquake (degree of damage caused)
- It is qualitative and based upon observed effects on people and
damage to buildings
- The scale ranges from Mercalli I: very weak, not felt by people to
Mercalli XII: total destruction
Estimating The Epicenter Of An Earthquake
- This requires data from at least three seismic stations
- Time difference between P wave and S wave is used to determine
epicenter
Mitigating earthquakes:
- Seismic hazard maps and risk maps help to properly site and construct
buildings
Where to build your dream or trophy house - and where not to
build:
- Avoid unstable soils and unconsolidated materials...
- Avoid mountainous terrain prone to landslides…
- Above all, avoid active faults !
Appropriate building codes which can withstand earthquake
damage:
-
Bedrock foundations best
Avoid asymmetrical buildings
Bolt house firmly to foundations
Appliances firmly bolted down
Gas lines flexible
Cupboards and shelving attached to walls
Heavy objects at low levels; anchor heavy furniture
Beds away from windows to avoid broken glass
Earthquake Forecasting:
Short-term prediction:
- Noticeable ground deformation can precede earthquakes
Changes in electrical conductivity of rocks:
- An increase in conductivity suggests groundwater movement
- Groundwater conducts electricity better than rock
Strange animal behavior:
- Certain types of earth movement may produce sounds or vibrations that
are detected by animals
Increased radon gas emission:
- Gas produced in the crust by the radioactive decay of uranium
- Increased emission can be due to presence of rock fracturing that might
precede an earthquake
Enough ground deformation can cause foreshocks:
- Small earthquakes occurring before a large one
- 1975 Haicheng earthquake (China; moment magnitude 7.3) was
successfully predicted because of foreshock observation
- Area was evacuated days beforehand
- City underwent significant destruction
-Changes in the water table (observing the water level in a well) can indicate
fault movement preceding an earthquake
Warning and prediction:
-
Precursory seismicity
Precursory deformation
Changes in physical properties of rocks near a fault
Changes in water levels, soil gases
Unusual behaviour of animals
Earthquake Prediction:
Important concepts:
1) Earthquake recurrence interval…seismic gap
2) Role of paleoseismology
- Yet our predictive ability is rudimentary, so we use probabilities
- e.g., 86% probability that a destructive quake of M>7 will hit southern
California in the next 30 years (1994 estimate)
Causes Of Earthquakes
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Where are the world’s earthquakes in terms of plate tectonics?
- The great majority of earthquakes are located at plate margins
- Plate margins are where magnetism, friction, faulting, etc… are most
intense
- Earthquakes in plate interiors are rare comparatively speaking
Pacific Rim of Fire:
- Most violent earthquakes (& volcanoes) occur here
- Notorious zone is characterized by subduction zones (where one tectonic
plate moves underneath another one and sinks in the process)
- The friction from the movement of the plates against each other produces
large destructive earthquakes
Earthquake Generation Along A Fault
- The earthquake focus is its point of origin along a fault plane
- Its epicenter is the vertical projection of the focus to the surface
- The energy stored within the rock along the fault will be released at the
focus (point where the energy begins to be released)
-The energy generated at the focus is liberated and travels to the surface
(energy travels through the shortest distance to the focus from the surface
to the epicenter)
-Epicenter is the point that is located on shortest distance from the focus to
the surface
Rock Behavior & Formation
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Rock Behavior and Deformation
3 Types of forces that act on rocks:
Stress: Any force that is applied on an object
Strain: Response to/Effect of the stress that we can see on the object or
material
Strength: Every object has its inherent strength, in order for something to
happen to the object you must overcome the strength of that object.
Strength is the maximum stress that a material can withstand without
breaking by rupture or continuous plastic flow
Response of rock to stress depends on:
- Type of stress
- Amount of pressure
- Temperature
- Type of rock
- Length of time rock subjected to stress
Types of Stress:
Compressional Stress
- Forces directed toward one another
- Decreases volume of material
- Lithostatic pressure, example of all-sided confining pressure produced by
burial
Tensional Stress
- Stretching stress that tends to increase volume of a material
Shear Stress
- Shearing: Two parallel forces operating in different directions
- Results in displacement of adjacent layers along closely spaced planes
Stress (force/area) and strain
Types of strain:
Extension  Stretching
Compression  Shortening
Rock Response to Stress
Elastic deformation:
- Strain is proportional to stress
- Rock returns to original volume/shape if the stress is removed
-Any object that undergoes deformation must first go through elastic
deformation
- Object will absorb energy, store energy, then release energy in any form of
energy that does work
Plastic deformation:
- Permanent deformation caused by flowing and folding at stresses above
elastic limit
- Happens in very high pressure or temperature
- All the warm rocks tend to deform plastically, don’t break
- Rocks in the astinosphere (much deeper in the earth) behave plastically;
absorb energy and become permanently deformed
Brittle deformation:
-Rocks at or near the surface (cold, low pressure) tend to deform by brittle
rupture
-Rocks exceed the elastic limit and break if the stress is too great, causing
permanent deformation
-This is how cracks in the earth’s crust form
-If the elastic limit is just slightly exceeded we have a crack in the surface
due to the fact that its stored too much energy
-The excess energy has to be dissipated at the surface so we need more
surface area to liberate the excess energy beyond the elastic limit
Types Of Waves
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Earthquakes generate two types of waves
1) Body waves (travel through the earth)
P and S waves
- Body waves generated when the energy is released at the focus
2) Surface waves (travel along the surface)
Rayleigh and Love waves
Body Waves: Primary waves (P Waves)
- Primary waves, or P waves, travel through solid, liquid, and gas
-They are alternately compressional and expansive
-Their speeds are ~5 km/s
- Rock vibrates parallel to the direction of wave propagation
Body Waves: Shear waves (S Waves)
-
Shear, or S waves, travel only through solids
They push material at right angles to their travel path
Their speeds are 2-3 km/s
Rock vibrates perpendicular to the direction of wave propagation
A sample seismogram:
A seismogram is the graphical representation of Earth movement
-S&P body waves are generated at the focus as soon as the energy is
released
- Primary waves arrive first the surface because they travel with a very high
speed
Surface waves: Love Waves
- Surface waves, such as Love waves, are restricted to Earth’s surface
- They cause sideways shaking of the ground
- Their speed is slightly less than S waves
-Surface waves cause sideways shaking of the ground when they reach the
surface
-Love waves cause more damage than Rayleigh waves
Surface waves: Rayleigh Waves
- Rayleigh waves are similar to Love waves
- But instead of causing shaking, they produce rolling motions of the
ground
Raleigh Waves behave like ocean waves
Case Study: San Andreas Fault
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San Andreas Fault:
- Along much of the west coast, the plate boundary is a transform margin
- The San Andreas is a right-lateral strike-slip or transform fault
-Transform boundary between the Pacific plate and the North American plate
- Pacific plate moves northwest
- North American plate moves southeast (relative to fault)
- Some parts of the fault lock up and store energy
- Release it in abrupt motions, large earthquakes
- Other parts of the fault move smoothly
- Cause ground deformation, but only small earthquakes
-
Over 1,200 km long
At least 16 km deep in places
One of the most studied faults in the world
Has produced some infamous earthquakes
Earthquakes on the San Andreas
1) San Francisco area
-
1906 San Francisco quake
M 7.7-7.9 ( magnitude)
3000 dead, $400 million US damage
225,000 homeless (pop. at the time was 400,000)
2)1989 Loma Prieta quake
- M 6.9 ( magnitude)
- 57 dead
- $6 billion in damage
3) Parkfield area
- 1857 Los Tejos quake
- M 8.0 (moment magnitude)
- 2 dead (it hit a then-sparsely populated part of California)
- After 1857, earthquakes M >6.0 occurred in
- 1881, 1901, 1922, 1934, 1966 and 2004
- Very active area of fault
1989 Loma Prieta Earthquake
- The Marina district of San Francisco was very hard hit
- Unconsolidated, water-saturated materials were liquefied and mobilized by
the shaking
- The lower picture shows a “volcano” of liquefied sand (not actual volcano)
Caste Study: Cascadia
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- In the Pacific Northwest, the tectonic regime is subduction-related,
rather than transform as we have seen in California
Here, there is evidence for very large earthquakes over the last several
thousand years…the most recent is 300 years ago
Case Study: Quebec
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- The St. Lawrence region has high levels of seismicity for a zone in the
interior of a tectonic plate
- This seismicity may be related to old, aborted rifts (openings in the crust)
about 200 Ma ago
What is an aborted or failed rift?
- Failed rifts are ancient to modern features where continental rifting began,
but then failed to continue.
- Rifts are distinct from Mid-ocean ridges, where new oceanic crust and
lithosphere is created by seafloor spreading.
- In rifts, no crust or lithosphere is produced. If rifting continues, eventually
a mid-ocean ridge may form, marking a divergent boundary between two
tectonic plates.
- There are three main groups of theories that have been proposed to
explain the spatial occurrence of intraplate earthquakes: stress
concentration, zone of weakness, and high heat flow.
- Aborted rift is when the rift suddenly stops opening
Effects Of Earthquake
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1) Aftershocks:
- Aftershocks normally occur after a major earthquake
- There may be many thousands of aftershock events over the space of
months or even years
- Although their magnitudes generally decrease with time, aftershocks have
potential to cause significant damage to already weakened materials
(e.g., rocks, soils, buildings, power and gas lines)
-Aftershocks are the result of the need of the earth’s crust to readjust itself
to pre quake form
-Usually not very strong but can cause some landslides, weakening of
buildings
2) Liquifaction:
- Occurs on sediments during earthquake shaking
- Groundwater can move upwards due to the shaking
- Water lubricates contact between sediment grains
- Weakens sediments
- Liquefaction has two consequences
- Amplifies shaking in structures
- Causes buildings to sink into sediment
- Shaking causes the water and the groundwater to come to the surface
which causes the sediment to become water saturated and amplifies the
shaking of the structures and causes them to sink into the ground
3) Landslides:
- The ground vibrations and severe shaking associated with an earthquake
can induce landslides in mountainous areas
4) Tsunamis:
- Tsunamis are ocean waves caused by displacements from earthquakes,
landslides, etc.
- They can be devastating at great distances from the epicenter
5) Building Destruction:
- Buildings are damaged or destroyed by ground vibrations and shaking
- The magnitude and duration of shaking are important factors in the
extent of damage
- Liquefaction and aftershocks increase the damage
Effects on building materials:
- Masonry (brick, stone, tile etc…)is not capable of withstanding significant
bending stresses
- Wood is more resistant because it is more yielding but wood is vulnerable
to fires
6) Fires:
- The ground shaking will rupture power and gas lines and damage to water
mains prevents or hinders fire fighting efforts
7) Personal Loss:
- We are examining earthquakes from a scientific perspective, but we must
not forget the human element and the pathos