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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Today
•  Lecture:
–  Seismology
•  Lab:
–  Elastic Rebound on the San Andreas Fault
5/7/15
Cal Poly Pomona
Florida Sinking
•  “From a survey of GPS data, for a decade or less
and for five stations, Florida may be sinking at a
rate of about –0.5 ± 1.6 mm/yr. This should be
viewed very cautiously, but a sinking Florida is in
general agreement with geophysical models of
Earth’s changing shape due to post-glacial
rebound from the last ice age”
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Elastic
Rebound and
Earthquakes
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
1
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Elasticity and
Earthquakes
Elastic Behavior
•  Similarly for stress σ and strain
ε for linearly elastic medium:
σ = µε or σ/ε = µ
–  µ: elastic constant (units?)
•  Most rocks, when subjected to
near surface conditions, can be
described as linearly elastic
solids:
–  deformation remains
proportional to applied stress
and
–  strain is reversible
5/7/15
•  Earthquake occurs when tectonic strain exceeds frictional forces
that are preventing slip
•  Away from region of failure, seismic waves spread out by elastic
deformation of rocks through which they travel
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Strain Leads to
Stress: Elastic
Rebound
Coseismic and Interseismic Slip
Elastic rebound model of
transform fault that releases
all strain that accumulates
between earthquakes
(interseismically)
•  Material at fault only
experiences relative motion
coseismically (during quake)
•  Both sides away from fault are gradually moving past one
another, whereas fault itself is locked, accumulating strain.
•  This flexure places greater and greater stress on fault.
•  When it exceeds strength of fault, fault slips => earthquake.
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
2
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
San Andreas Fault
FAR FIELD SLIP RATE
D ~ 35 mm/yr
https://www.youtube.com/watch?v=_YLjIvJXhpg
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/7/15
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Cal Poly Pomona
5/7/15
Cal Poly Pomona
3
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Lab Exercise
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Earthquake History on SAF
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Southern SAF
•  Has not produced great earthquake for at
least 250 years.
•  Slip rate of few cms per year => minimum
slip deficit accrued ~ 7–10 m, comparable
to maximum co-seismic offset ever
documented on SAF.
•  Observed strain rates confirm
that “southern SAF may be
approaching end of interseismic phase of earthquake
cycle”. [Fialko, Nature, 2006]
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
http://www.iris.edu/hq/programs/
education_and_outreach/animations
4
GSC307
Introduction to Global Geophysics
Earthquake Location
Seismic Waves
•  Hypocenter: where the
earthquake starts
(latitude, longitude and
depth)
–  Also called: focus
•  Epicenter: projection of
the hypocenter on the
Earth’s surface (latitude
and longitude)
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Seismometers
Seismometers
Seismometers detect and
record Earth motions
as a function of time
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
5
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Seismograms
Wave Parameters
•  Each seismogram is a composite of:
•  Amplitude
•  Wavelength
•  Period = time between
waves in seconds
•  Frequency = number of
waves passing a given
point in 1 second
(measured in cycles per
sec: Hz)
•  Velocity (seismic ~ km/
sec)
–  Earthquake source effects
–  Propagation effects => Earth structure
5/7/15
Cal Poly Pomona
5/7/15
GSC307
Introduction to Global Geophysics
Seismogram
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Dog Predicts Earthquake?
Seismic Waves
•  Body Waves
–  Travel through Earth’s interior
–  Fastest
–  Dominate at short distances
–  High frequencies (1-30 Hz)
•  Short period waves
SURFACEWAVE
•  Surface Waves
–  Travel near Earth’s surface
–  Arrive after body waves
–  Long period
–  Dominate at larger distances
5/7/15
Cal Poly Pomona
0
R
WAVEFRONT
BODYWAVE
5/7/15
Cal Poly Pomona
6
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Ray and Wavefront
WAVEFRONT
Z
36
Y
3(
0
X
•  Surface in which all
particles vibrate with same
phase = wavefront
•  Ray path = direction
perpendicular to
wavefront, direction of
wave propagation
SEISMIC
RAY
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Body Waves: P-wave
•  Primary wave, first to arrive
•  Compression, no rotation
•  Particle motion in direction of wave
propagation
•  Can travel through any material
5/7/15
5/7/15
Cal Poly Pomona
Body Waves: S-wave
•  Secondary wave, second to arrive
•  Transverse, shearing, no volume change
•  Particle motion at right angles to wave
propagation
•  Can travel only through solids
5/7/15
Cal Poly Pomona
7
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Particle Motion
WAVEFRONT
•  P-wave particle motion
is along ray
Z
36
Y
3(
•  Shear wave particle
motion is in plane of
wavefront
0
X
SEISMIC
RAY
http://web.utah.edu/thorne/animations.html
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Surface Waves: Love waves
•  Horizontal, shearing motion
•  Do not travel through water
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
8
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
All Wave Types +
Particle Motions
Surface Waves: Rayleigh waves
• 
• 
• 
• 
Backward-rotating, elliptical motion
Vertical as well as horizontal motions
Travel through both ground and water
Amplitude decays exponentially with depth
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/7/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Horizontal Seismometer
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
9
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Full
Seismograms
5/7/15
Cal Poly Pomona
5/7/15
Cal Poly Pomona
10