Plate tectonics overview of important techniques paleomagnetism by mercy2beans119


									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…                                                                      …value subtracts from current field: negative anomaly

    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.


                                                                                                             anomalies symmetric about ridge axis

                                                                                                                                                                                     to determine
                                                                                                                                                                                    spreading rate:
                                                                                                                                                                                   measure width of
                                                                                                                                                                                   magnetized stripe
                                                                                                                                                                                  and age of reversal

                                                                                                                                                                                      rate = distance

                                                  from:                                                  from:

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…
                                                                  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

 reversal occurs in ~1200 years                                       magnetohydrodynamic models of Earth’s magnetic field

                                                                               …during reversals, many poles likely exist

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


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
                                                                                                                           Vp =      k + 4/3µ         ρ density
    P: longitudinal or compressional                             S: shear or transverse
                                                                                                                                                      µ    shear modulus (rigidity)
                                                                                                                                                      k    bulk modulus (rigidity)
                                                                                                                           Vs =         µ

                                                                                                                  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

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
                                                             from: Kearey and Vine, 1996

                                                                                                                  sample seismogram

                                                                                                                       P, S, L, R are arrivals of P, S, Love and Rayleigh waves
                                                                                                                                from same earthquake



        Earthquakes in the last month
                                                         from:                                                            from:

                                                                                              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)


                               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


Pusan                             Tokyo     from:                                           from:

                                       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:                                        from:

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


                                       from:   Pusan                       Tokyo    from:

                                                 use figure to left
                                                   which plots
                                                                                                                                             Tokyo: 434 km
                                                                                                                                             Pusan: 549 km
                                                                                                                                             Akita: 697 km
                                                                                                                                           Akita:  30 mm
                                                                                                                                           Pusan: 90 mm
                                                                                                                                           Tokyo: 170 mm

                                                                                                                                      magnitude ~ 6.8

                                       from:                                        from:

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                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:                      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
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:               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:

to generate focal mechanism…                                                                what type of earthquake along what fault orientation is this?
       …use equal area lower hemisphere projection
                                  center is earthquake focus
                                   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


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



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