Chapter 11 Earthquakes

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Chapter 11 Earthquakes
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What is an earthquake?

• An earthquake is the vibration of Earth produced
  by the rapid release of energy
• Energy released radiates in all directions from
  its source, the focus
• Energy is in the form of waves
• Sensitive instruments around the world record the
  event

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Locating the source of earthquakes

• Subsurface versus surface location
• Focus - the place within Earth where earthquake
  waves originate
• Epicenter location on the surface directly
  above the focus

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What is an earthquake?

• Foreshocks and aftershocks
• Foreshocks Earthquakes that sometimes precede a
  major earthquake along the same fault zone by
  days to months, but are smaller than the main
  shock.
• Aftershocks Smaller earthquakes that always
  follow a major earthquake due to the following
• Minor adjustment along or around the fault plane
• Release of strain not released during the main
  shock
• The largest magnitude aftershocks generally occur
  shortly after the main shock, while smaller
  aftershocks may continue for several years

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Seismology

• Seismology The study of earthquake waves or other
  man-made vibrational waves that travel through
  the earth
• Seismographs, instruments that record seismic
  waves
• Early devices recorded the movement of Earth in
  relation to a stationary mass on a rotating drum
• Modern devices use a magnet and coil to generate
  a current proportional to ground motion and
  record it digitally or on paper

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Seismology

• Types of seismic waves
• Body waves
• Travel through Earths interior
• Two types based on mode of travel, P and S
• Primary (P) waves
• Fastest seismic wave, first to arrive
• Push-pull (compress and expand) motion, changing
  the volume of the intervening material
• Travel through solids, liquids, and gases
• Generally, in any solid material, P waves travel
  about 1.7 times faster than S waves
• Generally the lowest amplitude seismic wave

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Seismology

• Secondary (S) waves
• Second fastest seismic wave, second to arrive
• Shaking motion at right angles to the direction
  of travel
• Travel only through solids
• Slower velocity than P waves
• Slightly greater amplitude than P waves

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Seismology

• Types of seismic waves
• Surface waves
• Complex motion
• Travel along outer part of the Earth
• Cause greatest destruction
• Exhibit greatest amplitude and slowest velocity
• Waves have the greatest periods (time interval
  between crests)
• Two types based on mode of travel, Love and
  Rayleigh

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• Types of Surface Waves
• 1) Love waves - slower than body waves, particle
  motion is side to side perpendicular to wave
  travel, with amplitude dying out with depth. Can
  be a very destructive type of wave.
• 2) Rayleigh waves - slower than body waves,
  particle motion is like a water wave, up and down
  rolling motion in direction of wave travel, with
  amplitude dying out with depth. Can be a very
  destructive type of wave.

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Locating the source of earthquakes

• Locating the epicenter of an earthquake
• Three station recordings are needed to locate an
  epicenter
• Each station determines the time interval between
  the arrival of the first P wave and the first S
  wave at their location
• A travel-time graph is used to determine each
  stations distance to the epicenter

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Locating the source of earthquakes

• Locating the epicenter of an earthquake
• A circle with a radius equal to the distance to
  the epicenter is drawn around each station
• The point where all three circles intersect is
  the earthquake epicenter

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Earthquakes Evidence for plate tectonics

• A good fit exists between the plate tectonics
  model and the global distribution of earthquakes
• Zones of earthquake activity indicate areas of
  more intense plate movement.
• Plate tectonic model can be used to predict areas
  of likely earthquake activity and what
  earthquakes are likely to occur, even if none
  have occurred there historically.

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Locating the source of earthquakes

• Earthquake belts
• About 95 percent of the energy released by
  earthquakes originates in a few relatively narrow
  zones that wind around the globe
• Intraplate earthquakes
• Occur within the interior of tectonic plates
• Are not associated with distinct plate boundaries

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Locating the source of earthquakes

• Earthquake depths
• Earthquakes originate at depths ranging from 5 to
  nearly 700 kilometers
• Earthquake foci classified as
• Shallow (surface to 70 kilometers)
• Intermediate (between 70 and 300 kilometers)
• Deep (over 300 kilometers)

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Locating the source of earthquakes

• Earthquake depths
• Definite patterns exist
• Shallow focus occur along the oceanic ridge
  system, areas of continental rifting and
  collision, transform faults, and the upper parts
  of subduction zones.
• Almost all intermediate and deep-focus
  earthquakes occur in the circum-Pacific belt,
  particularly in regions situated landward of
  deep-ocean trenches.

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Measuring the size of earthquakes

• Two measurements that describe the size of an
  earthquake are
• Intensity a measure of the degree of earthquake
  shaking at a given locale based on the amount of
  damage. Intensity generally decreases away from
  the epicenter
• Magnitude estimates the amount of amplitude of
  ground motion at a given distance from the
  earthquake. Only one magnitude for any given
  earthquake

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Measuring the size of earthquakes

• Intensity scales
• Modified Mercalli Intensity Scale was developed
  using California buildings as its standard.
  Measured on a scale of I-XII with I being the
  least damage.
• The drawback of intensity scales is that
  destruction may not be a true measure of the
  earthquakes actual energy release. This is due
  to factors such as building strength, ground
  conditions, proximity to epicenter.

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Measuring the size of earthquakes

• Magnitude scales
• Richter magnitude - concept introduced by Charles
  Richter in 1935
• Richter scale
• Based on the amplitude of the largest seismic
  wave recorded (generaly S-waves for more local
  earthquakes or surface waves for distant ones)
• Accounts for the decrease in wave amplitude with
  increased distance

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Measuring the size of earthquakes

• Magnitude scales
• Richter scale
• Magnitudes less than 2.0 are not felt by humans
• Each unit of Richter magnitude increase
  corresponds to a tenfold increase in wave
  amplitude.
• Example A 7.0 quake has 10 times more wave
  amplitude than a 6.0 and a 100 times more wave
  amplitude than a 5.0.
• Larger magnitude earthquakes occur less often
  than smaller ones

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Earthquake destruction

• Destruction from seismic vibrations
• Ground shaking
• The amount of ground shaking generally decreases
  with increased distance from the epicenter.
  However, destruction varies considerably due to
• Intensity and duration of ground shaking
• The nature of the ground on which the structures
  are built (unconsolidated worse)
• The type of structure (material, height,
  earthquake resistant design or not)
• The direction of ground motion (horizontal worse)
• Areas of wave amplitude interference.

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Earthquake destruction

• Fire
• Seiches
• The rhythmic sloshing of water in lakes,
  reservoirs, and enclosed basins
• Waves can weaken reservoir walls and cause
  destruction
• Landslides, subsidence, faults, fractures
• Liquefaction of the ground
• Unconsolidated materials saturated with water
  turn into a mobile fluid

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Earthquake destruction

• Tsunamis, or seismic sea waves
• Destructive waves that are often inappropriately
  called tidal waves
• Result from vertical displacement along a fault
  located on the ocean floor or a large undersea
  landslide triggered by an earthquake
• On shore, water withdraws rapidly from coast,
  then rapidly returns with a large distructive
  wave.

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Earthquake destruction

• Tsunamis, or seismic sea waves
• In the open ocean height is usually lt 1 meter
• In shallower coastal waters the water piles up to
  heights that occasionally exceed 30 meters
• Can be very destructive

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Can earthquakes be predicted?

• Short-range predictions
• Predict the location and magnitude of a large
  earthquake within days to months
• Research has concentrated on monitoring possible
  precursors phenomena that precede earthquakes
• foreshocks
• uplift or subsidence
• rapid changes of strain in the rock
• Currently, no reliable method exists for making
  short-range earthquake predictions

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Can earthquakes be predicted?

• Short-term warnings
• Notify residents at some distance from an
  earthquake that the waves are coming
• Automatically shut down power plants, trains,
  etc.
• Requires education, coordination, warning system
• Intermediate to long-range forecasts
• Determines the probability that a certain
  magnitude earthquake will effect a given area on
  a time scale of 30 to 100 years, or more
• Indicate the maximum amount of ground shaking a
  given area is likely to feel over the next 10-50
  years
• Outlines areas of likely future earthquake
  activity
• Currently the best method to minimize damage

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Can earthquakes be predicted?

• Methods to determine long-range forecasts based
  on the premise that earthquakes are repetitive or
  cyclical and occur along active faults
• Using historical records, trenching and dating of
  faulting events
• Using epicentral maps of prior earthquake
  activity
• Using earthquake hazard and ground shaking maps
• Using plate motion velocities to determine the
  likely frequency of various magnitude events
• Indicate seismic gaps on faults where future
  earthquakes are likely to occur

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End of Chapter 11