Earthquakes
Earthquake Hazards
Earthquake Waves
Earth Structure
Introduction
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Violent & frightening phenomenon
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Movement of the solid Earth
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Psychologically disturbing
Effects
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Ground shaking
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Trivial in and of itself
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Causes most other effects
Damage to structures
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Caused by shaking
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This is what kills people
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Greatest on unconsolidated materials and fill
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Good design reduces damage
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No such thing as an earthquake-proof structure
Effects
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Fire
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Can do more damage than the shaking
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Must plan for fighting it
Landslides
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Destructive to structures and sometimes, lives
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Effects can be minimized by proper zoning
Ground Cracks
Effects
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Liquefaction
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Granular material turns into a liquid state
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Triggered by vibration
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Material can no longer support structures
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Buildings sink or tilt
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Water expelled
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Buried objects float
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Effects can be minimized by proper zoning
Nigata, Japan—1964
Loma Prieta Earthquake--1989
Effects
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Tsunami, tidal waves, seismic sea waves
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Not tidal
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Caused by sudden, generally vertical, motion of the sea floor
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Travel 400-500 mph in open ocean
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Low amplitude in open ocean
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Lose velocity but gain height in shallow water
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First is not always the largest
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Tsunami Warning System, zoning, and education can reduce loss of life and
property damage
Alaskan Earthquake of 1946
Tsunami in Hilo, Hawaii
Wave Advancing
Wave Over Seawall
Wave Engulfs Seawall
Wave Arrives
Wave on Shore
Bayfront Damage
Damage at Hilo Harbor
Damage to House and Railroad
Boat Washed 400 Feet
Alaska Earthquake—1964
Tsunami Damage in Alaska and California
Crescent City, CA
Nichol’s Pontiac Showroom
Dock Damage, Whittier, AK
Whittier, AK
Railroad Damage at Seward
Seward, AK
Seward, AK
What Causes These Effects?
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What causes the shaking of the ground?
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What causes the wiggles we see on a seismogram?
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There is general agreement that the cause is motion on faults
Introduction
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Today the link between earthquakes and faults is taken for granted
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Nearly all geologists will tell you that earthquakes are caused by motion on faults
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Wasn’t really established until 1906 California earthquake
H.F. Reid of Johns Hopkins University proposed the Elastic Rebound Model based on
observations of the 1906 quake
Background
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All scientific theories are shaped by the data upon which they are built
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When Reid arrived in the Bay area he had the results of prior surveys with which to work
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1851-1865
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1874-1892
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1906-1907 — Reid & colleagues
Reid’s model had 2 pre-earthquake stages and 1 post earthquake stage
Elastic Rebound
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Elastic deformation means the deformation is recoverable
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Reid’s model recognizes that the Earth can store elastic deformation or strain for some
time and then release it suddenly
Elastic Rebound Model
Elastic Rebound Model
Elastic-Rebound Conclusion
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Reid’s Elastic Rebound Model has been used to explain earthquakes and the generation
of seismic waves ever since
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The model works very well
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It runs into trouble with deep-focus earthquakes because temperatures and pressures at
depth make elastic behavior impossible
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Everyone recognizes this weakness, but it applies in such a small fraction of quakes, it is
generally ignored
Seismic Waves
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Generated by motion along faults
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Body waves—Pass through the body of the Earth
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Surface waves—Travel around the Earth following a surface or a layer
Body Waves
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P waves
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From the Latin undae primae or first waves
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Like sound waves—longitudinal
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Particles vibrate in the direction the wave is travelling
Body Waves
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P waves
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Velocity
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k = Bulk Modulus or 1/compressibility
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 = Shear modulus or resistance to shear
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 = Density
Body Waves
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P Waves
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The fastest of all seismic waves
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K is positive but has a small range
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 ranges from 0 for liquids to 1022 viscosity units
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 ranges from 1 to 16 g/cm3
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 has the largest effect
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Velocity depends only on the properties of the material through which the wave is
passing
Body Waves
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S Waves
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From the Latin undae secundae or second waves
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Shear waves, not like sound waves
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Particles vibrate perpendicular to the direction the wave travels
Body Waves
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S Waves
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Same  as in P wave discussion
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Same  as in P wave discussion
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Velocity depends only on the properties of the material through which the wave is
passing
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Since  for liquids is zero, velocity of S waves in liquids is 0
Summary
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Fastest of all seismic waves
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Passes through liquids
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Velocity depends only on properties of the material through which the wave is passing
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Slower than P waves
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Do not pass through liquids
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Velocity depends only on properties of the material through which the wave is passing
Surface Waves
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Rayleigh Waves
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The most destructive of all seismic waves
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Responsible for ground roll
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Velocity is about 0.9 Vs
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Similar to ocean waves except
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Retrograde particle motion in Rayleigh waves but not in ocean waves
Surface Waves
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Velocity and wavelength
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Longer waves affect a thicker layer
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Velocity generally increases with depth
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Thicker layers imply the wave is traveling at a greater depth
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Therefore, longer wavelength waves affect thicker layers and have higher
velocities
In the case of Rayleigh waves, this means
Velocity depends on properties of the material through which the wave is passing and
varies with wavelength
Surface Waves
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Love Waves
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Really a special kind of S wave
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S waves that travel along a particular surface
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Particles vibrate perpendicular to the direction the wave is traveling and in the
plane of the surface along which the wave is traveling
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Velocity is the S wave velocity in the particular material
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Velocity depends on properties of the material and also wavelength
Behavior of Seismic Waves
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When the body waves from an earthquake strike a boundary with a different kind of
material several things can happen
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The energy can be reflected
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The energy can be refracted
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The refraction or reflection can be as the same type of wave or it can change type
The Moho
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This is precisely the method that Andrija Mohorovicic used to find the boundary between
the crust and the mantle
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He saw a velocity change represented by a change in slope on a time-distance
graph
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He interpreted this change in slope to be due to a change in composition of the
material through which the waves were passing
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We now recognize that boundary as the Moho
Waves Through the Earth
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If we make the layers thin enough, we get a smooth curve in the path of the seismic wave
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This is what we see as waves travel through the Earth
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The waves follow curved paths
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They are reflected and refracted at each of the major boundaries in the Earth
Waves Through the Earth
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Wave Names
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P = longitudinal wave in the mantle
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S = transverse wave in the mantle
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K = longitudinal wave in the outer core
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I = longitudinal wave in the inner core
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J = transverse wave in the inner core
How Big Was That Earthquake?
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Intensity
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Measured by the effects
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Assigned to numerical categories
Magnitude
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Instrumental measure
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Based on ground motion
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Energy
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Calculated from magnitude
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Calculated from fault area
Intensity
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Observer’s description of ground shaking
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Reports of damage to structures or disturbance of objects
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Requires collection of reports
Rossi-Forel Scale
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De Rossi in Italy
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Forel in Switzerland
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Joined forces and set up the Rossi-Forel Scale in 1883
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Widely adopted
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Version still in use today in Europe
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Uses Roman numerals
Mercalli Scale
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Developed in the United States
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Original version in 1902
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Modification of 1931 generally used
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Richter made minor modifications 1956, which were generally not accepted
Modified Mercalli Scale
Magnitude
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Instrumental measure
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Richter recognized
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Larger earthquakes produce larger wiggles on a seismogram
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For any earthquake, the size of the wiggles is smaller away from the epicenter
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The range of wiggle size was going to be huge, so he used logarithms
Magnitude
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Plotted log of size of largest wiggle versus distance from epicenter
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Found curves were nearly parallel
Magnitude
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At any distance, difference between curves is the same
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Establish a “Zero” earthquake
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Initially small enough that there would be no negative magnitudes
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Fixed the zero curve by specifying its value at a particular distance
Magnitude
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The Zero earthquake
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One-thousandth of a millimeter at a distance of 100 kilometers
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An earthquake recording with a trace amplitude of 1 millimeter measured on a
standard seismogram at 100 kilometers is magnitude 3
Magnitude
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There is no minimum magnitude
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There is no maximum magnitude
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In practice, the strength of rocks dictates that the largest earthquakes will have a
magnitude of about 10
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Original scale was local magnitude confined to California and was based on largest
waves, no matter what phase
Magnitude
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To get out of California, scale was modified to consider surface waves and body waves
on different scales
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mb= Body wave magnitude based on P waves
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Ms = magnitude based on the largest surface wave
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Produced a variety of magnitude scales all of which are related
Magnitude Problems
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Magnitudes are given on a variety of scales
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None is the original Richter Scale
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Scales gave significantly different values for large earthquakes
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Scales failed to discriminate among large earthquakes
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A new magnitude calculation called Moment Magnitude seems to solve these
problems
Comparing Magnitudes
What is moment?
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Imagine a couple (two equal but opposite forces) acting on a beam
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The longer the distance between the two forces the easier it is to move the beam
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Multiplying the value of one of the forces by the distance between them gives the
moment
Seismic Moment
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If you think back to the Elastic Rebound Model, you will realize that it is easy to apply
the moment description to earthquakes
Moment Magnitude
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Seismic moment
(M0) = shear modulus x area x displacement
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Shear modulus (rigidity)
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Area of the fault that ruptured during a quake
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Average displacement across the fault
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Moment magnitude
(Mw) = 2/3 log10 M0 - 6.0
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Moment magnitude is becoming the current standard
Energy
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Can be calculated from magnitude
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log E = 4.8 + 1.5 Ms in Joules
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This is only one estimate; there are a variety of others
Let’s look at how this works
Magnitude/Energy Comparison
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Ms = 5
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log E = 4.8 + 1.5 (5)
= 4.8 + 7.5
= 12.3
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E = antilog 12.3
= 2.0 x 1012 Joules
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One magnitude point is 10 times the ground motion
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Ms = 6
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log E = 4.8 + 1.5(6)
= 4.8 + 9.0
= 13.8
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E = antilog 13.8
= 6.31 x 1013 Joules
= 63.1 x 1012 Joules
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One magnitude point is about 30 times the energy
Energy
Comparisons
First Motion Studies
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Sometimes called Focal Mechanism Studies
First Motion Studies
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Use data from seismograph stations to determine the direction of first ground motion at
each station
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Plot these data on a map
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Observe pattern
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Determine the direction of motion on a fault
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Requires geological information
First Motions
First Motions
First Motions
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Takes data from many stations
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Must use geologic knowledge not obtained from seismograms
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Can be used for any kind of fault to determine the type of motion
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Allows identification of type of fault from seismogram and some knowledge of local
geology