Chapter 10: Earthquakes

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Chapter 10 Earthquakes
photo credit USGS
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elastic rebound theory
what happens along a fault to generate an
earthquake?
3 m offset
1906 San Francisco earthquake
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elastic rebound animation
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offset lettuce rows - El Centro, CA
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earthquake focus (hypocenter) and epicenter
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earthquake focus animation
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when faults rupture to cause earthquake, energy
is released
energy is in form of seismic waves

• types and behavior of seismic waves
• body waves
• primary or compressional
• secondary or shear
• surface waves
• Raleigh (large vertical displacements)
• Love (shear)

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seismic wave types
body waves
surface waves
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displacement parallel to wave motion
P wave
displacement normal to wave motion
S wave
from http//www.personal.umich.edu/vdpluijm/gs20
5.html
P waves faster than S waves
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equations for velocities
Vp k 4/3µ r
1/2
density
r
µ
shear modulus (rigidity)
Vs µ r
k
bulk modulus (rigidity)
1/2
because shear modulus (rigidity) for fluid is
zero, S waves cannot propagate through a fluid
consequence of equations is that P waves are 1.7x
faster than S
can infer physical properties from P and S waves
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wave animations
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locating and measuring earthquakes

• determining the location of an earthquake
• measuring the size of an earthquake

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vertical component seismometer
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horizontal component seismometer
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seismograph animation
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seismograph animation 2
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sample seismogram
P, S, L, R are arrivals of P, S, Love and
Rayleigh waves from same earthquake
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determining distance to earthquake from
seismograms
use arrival times of S and P waves on 3
seismograms (triangulation problem)
remember that P waves travel faster than do S
waves
note time between P and S wave arrivals (S-P
interval)
from http//vcourseware5.calstaela.edu/cgi-bin
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examine 3 seismograms from Japan and measure
S-P interval in seconds
Akita
Pusan
Tokyo
from http//vcourseware5.calstaela.edu/cgi-bin
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relationship of P and S wave velocities and S-P
interval
find time of arrival of S and P waves use time
difference between S and P arrivals
from http//vcourseware5.calstaela.edu/cgi-bin
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for our Japan example S-P interval Tokyo
44 sec Pusan 56 sec Akita 71 sec
distance Tokyo 434 km Pusan 549
km Akita 697 km
from http//vcourseware5.calstaela.edu/cgi-bin
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plot circles centered on stations with radii
of appropriate distance
intersection of circles is epicenter
from http//vcourseware5.calstaela.edu/cgi-bin
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measuring earthquake magnitudes

• Three distinct methods to measure earthquakes.
  Two based on energy and one based on intensity.
• Richter Magnitude Scale originally developed for
  southern California. Log scale, which has no
  upper bound. Small earthquakes may yield
  negative values. Tends to be inaccurate at gt7
  magnitudes.
• Moment Magnitude Scale measurement of the amount
  of work done during the earthquake. Based on rock
  strength, area of rupture, and displacement
  during event.
• Modified Mercalli Intensity Scale based on the
  damage associated with a particular event at a
  particular location. Ranges from I (less damage)
  to XII (more damage).

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determining earthquake magnitude (generalized)
once again, use seismograms
measure maximum amplitude of S wave
(this is one method others exist)
from http//vcourseware5.calstaela.edu/cgi-bin
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measure maximum amplitudes of S waves from 3
seismograms (these are the same one)
Akita 30 mm Pusan 90 mm Tokyo 170 mm
Akita
Pusan
Tokyo
from http//vcourseware5.calstaela.edu/cgi-bin
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use figure to left which plots distance magnitude
amplitude
from http//vcourseware5.calstaela.edu/cgi-bin
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distance Tokyo 434 km Pusan 549
km Akita 697 km
amplitude
Akita 30 mm Pusan 90 mm Tokyo 170 mm
magnitude 6.8
from http//vcourseware5.calstaela.edu/cgi-bin
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modified Mercalli Intensity Index
1886 Charleston, SC earthquake
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frequency-intensity-magnitude relations
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effects of earthquakes

• ground displacement
• liquefaction/landslides
• seiches
• tsunamis
• fires

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pancaked building - 1985 Mexico City
Earthquakes dont kill people - buildings do!
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ground rupture, 1906 Olema, CA
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surface displacement - 1964 Alaska
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deformed fence - Gallatin County, MT
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buckled concrete - 1971 San Fernando, CA
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soil liquefaction - 1964 Nigata, Japan
shaking disturbs clay particles in soil and they
collapse like a house of cards
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tsumani generation
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tsunami devastation - 1964 Alaska earthquake
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tsunami animation
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tsunami wave propagation times
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earthquakes and plate tectonics
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