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Plate tectonics: overview of important techniques: magnetic anomalies
paleomagnetism; seismology for normally magnetized oceanic crust…
…value adds to current field: positive anomaly
websites from which images are drawn: for reversely magnetized oceanic crust…
http://www.geo.lsa.umich.edu/~crlb/COURSES/270 …value subtracts from current field: negative anomaly
http://www.earth.nwu.edu/personal/seth
http://www-personal.umich.edu/~vdpluijm/gs205.html
http://vcourseware5.calstaela.edu/cgi-bin
http://www.pmel.noaa.gov/vents/coax/coax.html
http://pubs.usgs.gov/publications/text
http://www.geo.cornell.edu/geology/classes/geol388/intro388.html
http://wwwneic.cr.usgs.gov
sources:
Kearey, P. and F. Vine, 1996, Global tectonics, second edition, Blackwell
Scientific, 333 p.
Rowland, S. and E. Duebendorfer, 1994, Structural analysis and synthesis,
2nd edition, Blackwell Scientific, 279 p.
from: http://www.earth.nwu.edu/personal/seth
anomalies symmetric about ridge axis
to determine
spreading rate:
measure width of
magnetized stripe
and age of reversal
rate = distance
time
from: http://www-personal.umich.edu/~vdpluijm/gs205.html from: http://pubs.usgs.gov/publications/text
paleomagnetism… magnetic anomalies allow dating of oceanic crust…
…Earth’s current magnetic field… …for basalts intensity of remanent magnetism > induced
magnetic field flow lines anomalies will vary with latitude and ridge orientation
yield different if oceanic crust acquires its magnetism at high latitudes…
inclinations and declinations magnetization vector dips steeply…
for each spot on Earth in northern latitudes…
normal:
I = tan-1(2 tan λ) …dips steeply north
λ = tan -1[(tan I)/2] reversed:
…points steeply up/south
I: inclination
…closer to equator,
λ : latitude relative to pole
magnetization vector not as steep
…at equator,
from: Rowland, S. and E. Duebendorfer, 1994 magnetization vector horizontal
negative anomaly coincides
with normal blocks from: Kearey and Vine, 1996
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reversal occurs in ~1200 years magnetohydrodynamic models of Earth’s magnetic field
…during reversals, many poles likely exist
http://yubanet.com/artman/publish/article_6834.shtml
apparent polar wander sample sites and paleomagnetic poles
time-averaged position of Earth’s magnetic poles
coincides with geographic poles
use paleomagnetic inclination to determine
paleolatitude of an area of interest
paleomagnetic investigations can document changes
in north-south position (latitude) of an area
…cannot do changes in longitude
fixed plate southward-moving plate
if a plate moves only east-west through time,
(or east-west motion)
rocks of all ages will show same pole pole appears to move north
if a plate moves toward equator with time,
younger rocks will show more gently plunging inclinations cannot generally determine normal or reversed
…either 30°S or 30°N latitude
from: Rowland, S. and E. Duebendorfer, 1994
an overview of seismology
APWs paths hold information about collision
much of the information we have for plate tectonics and
whole Earth structure comes from earthquakes
two pieces moving separately on Earth… …earthquakes generate seismic waves that travel through Earth
…apparent polar wander paths --their interaction with material as they pass through
will differ the interior gives us information about
…layering and physical properties
some terms:
magnitude--measure of energy release
hypocenter (focus)--point of origin of earthquake
epicenter--point on surface directly above hypocenter
two pieces collide and move together epicentral angle, Δ:
…apparent polar wander paths angle defined by hypocenter and point of recording
will coincide
hypocenter seismograph
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
Δ
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strain energy released by earthquake transmitted as equations for velocities
several types of waves which propagate by elastic deformation
of rock through which they move-- body waves: P and S
1/2
Vp = k + 4/3µ ρ density
P: longitudinal or compressional S: shear or transverse
ρ
µ shear modulus (rigidity)
k bulk modulus (rigidity)
1/2
Vs = µ
ρ
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
from: http://www.earth.nwu.edu/personal/seth
other waves generated by earthquakes: earthquake location
surface waves--restricted to vicinity of free surface detected by seismographs; global network set up in 1962
Rayleigh waves: particles move in ellipse in World-Wide Standardized Seismograph Network (WWSSN)
vertical plane
earthquakes at large distances from seismographs: teleseismic
Love waves: horizontally polarized shear waves;
…located by arrivals of various phases on records
transmitted when S wave velocity of
e.g. P vs. S
surface is lower than that of layer below
• assume a standard model for velocity layering of Earth
• use many different seismic phases and seismographs
surface waves move more
slowly than body waves focal depth for teleseismic calculated by arrival time difference
between P and pP
pP: P wave reflects at surface of Earth above focus
P: red trajectory
pP: red and blue
trajectory
from: Kearey and Vine, 1996
sample seismogram
P, S, L, R are arrivals of P, S, Love and Rayleigh waves
from same earthquake
seismograph
from: http://www.geo.cornell.edu/geology/classes/geol388/intro388.html
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Earthquakes in the last month
from: http://neic.usgs.gov from: http://neic.usgs.gov
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://neic.usgs.gov
from: http://vcourseware5.calstaela.edu/cgi-bin
examine 3 seismograms from Japan
relationship of P and S wave velocities and S-P interval
and measure S-P interval in seconds
find time of arrival of
S and P waves
use time difference between
S and P arrivals
Akita
Pusan Tokyo from: http://vcourseware5.calstaela.edu/cgi-bin from: http://vcourseware5.calstaela.edu/cgi-bin
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for our Japan example:
plot circles centered
S-P interval: on stations with
Tokyo: 44 sec radii of
Pusan: 56 sec appropriate distance
Akita: 71 sec
distance intersection of circles
Tokyo: 434 km is epicenter
Pusan: 549 km
Akita: 697 km
from: http://vcourseware5.calstaela.edu/cgi-bin from: http://vcourseware5.calstaela.edu/cgi-bin
determining earthquake magnitude measure maximum amplitudes of S waves from 3 seismograms
once again, use seismograms… (these are the same one)
measure maximum amplitude of S wave
(this is one method; others exist) Akita: 30 mm
Pusan: 90 mm
Tokyo: 170 mm
Akita
from: http://vcourseware5.calstaela.edu/cgi-bin Pusan Tokyo from: http://vcourseware5.calstaela.edu/cgi-bin
use figure to left
distance
which plots
Tokyo: 434 km
distance
Pusan: 549 km
magnitude
Akita: 697 km
amplitude
amplitude
Akita: 30 mm
Pusan: 90 mm
Tokyo: 170 mm
magnitude ~ 6.8
from: http://vcourseware5.calstaela.edu/cgi-bin from: http://vcourseware5.calstaela.edu/cgi-bin
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more information is contained on seismograms… earthquakes occur along faults…
consider that… happen according to elastic rebound theory
different types of faults will yield different displacements • block traversed by pre-existing fracture (fault) (A)
• strain along fracture as two sides want to move
XY marker shows strain in system (B)
• small strains accommodated by rock (B and C)
• rock broken when strain too large and earthquake occurs (D)
fault movement is instantaneous; reduces strain to zero
X
X
X
X Y Y
look at geometry of fault plane and sense of slip Y Y
A: time 1 B: time 2 C: time 3 D: time 4
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270 example is for a right-lateral strike-slip fault
seismic waves radiate from hypocenter and recorded by seismograms the push or pull is recorded on seismogram as either
an upward or downward displacement of the arrival of P-wave
…block motion on either side of fault is characteristic of that fault
…look at ‘first motion’ of P-wave on seismogram
and yields characteristic pattern for P wave propagation
up
example of right-lateral strike-slip fault
gray quadrants first experience
fault is red down
a push, or compression,
auxiliary plane from P-wave
is yellow blue quadrants first experience
a pull, or dilatation, up
from P-wave
fault/auxiliary plane
divide area into 4 quadrants down
no P wave propagation
along fault/aux. planes look at first motions from many geographically dispersed
--movement is shear there-- seismograms to identify quadrants of compression, dilatation
called nodal planes from: http://www.earth.nwu.edu/personal/seth and delineate nodal planes
technique is complicated by focal mechanism … commonly known as ‘beachball’
• spheroidal shape of Earth
• progressive increase of seismic velocity with depth
…seismic waves follow curved paths
between foci and seismograms
problem overcome by from: Kearey and Vine, 1996
• considering directions in which seismic waves left focus
• assuming a standard model for velocity structure of Earth
first motions plotted on equal-area projection of lower half of
focal sphere centered on earthquake focus…focal mechanism from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
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to generate focal mechanism… what type of earthquake along what fault orientation is this?
…use equal area lower hemisphere projection
center is earthquake focus
ambiguity:
station reports location relative to focus
• which is fault plane?
azimuth: relative to focus
• which is auxiliary plane?
angle of incidence
angle between ray vector and vertical
can be either:
• right-lateral on EW fault
• left-lateral on NS fault
from: Kearey and Vine, 1996
azimuth and angle of incidence plot on net from: Kearey and Vine, 1996
and first motion noted
from: Rowland and Duebendorfer, 1994
thrust faults normal faults
shaded: compressional shaded: compressional
(a): W dipping fault (a): W dipping fault
(c): E dipping fault (c): E dipping fault
(b): focal mechanism
(b): focal mechanism
same for both
same for both
use geological setting use geological setting
to determine most to determine most
reasonable reasonable
also Anderson’s theory: also Anderson’s theory:
thrusts dip < 45° normal faults dip > 45°
from: Kearey and Vine, 1996 from: Kearey and Vine, 1996
let us reconsider transform faults…
summary how would you prove the nature of a transform fault?
i.e. sense of displacement different from transcurrent fault
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
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perhaps you would look at data for an earthquake along a transform… Mid-Atlantic transform faults
focal mechanism for transform fault (Sykes, 1967)
mid-Atlantic ridge
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
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