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Lecture Plate Tectonics and Earthquakes

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Lecture Plate Tectonics and Earthquakes Powered By Docstoc
					           ESYS150
          LECTURE 4
     PLATE TECTONICS AND
        EARTHQUAKES

Introduction - Plate Boundaries
Faulting and Earthquake Fault Plane solutions
Spreading Center Earthquakes
Transform Fault Earthquakes
Subduction Zone Earthquakes
TYPES OF PLATE BOUNDARY
SPREADING, TRANSFORM AND SUBDUCTION

                      Spreading centers have a pull-apart
                         motion and are in a divergent
                         zone (small earthquake).

                      Transform faults have a slide-past
                         motion (medium to large
                         earthquakes)

                      Subduction zones have a down
                         going plate and colliding motion.
                         They are in a convergent zone
                         (Largest earthquakes).
TYPES OF PLATE BOUNDARY
PACIFIC PLATE

                 Pacific plate is the largest in the
                     world.
                 Its eastern and southern edges are
                     mostly spreading centers and
                     short transform faults
                 It has three long continental
                     transform boundaries: San
                     Andreas, Queen Charlotte and
                     Alpine.
                 Subduction zones lie along the
                     western and northern edges of
                     the plate
FAULTING
EARTHQUAKE FAULT PLANE SOLUTIONS
Beach Balls




Relation of “Beach Ball” solutions to actual types of faulting
a) Fault plane solution for a strike-slip fault
b) Fault plane solution for a thrust fault
c) Fault plane solution for a normal fault.
FAULTING
EARTHQUAKE FAULT PLANE SOLUTION
Lower focal sphere projection of first motion at distant stations




a)   Schematic cross section through the Earth. The focal sphere is an imaginary
     small sphere centered on the earthquake focus at O. The ray path for the first
     arrival at S intersects the lower focal sphere (shaded) at an angle i from the
     vertical.
b)   Projection of the lower focal sphere onto the horizontal showing North and
     position of the seismograph station.
c)   Fault plane solution for a strike-slip earthquake. Compression, positive first
     motion, black. Dilation, negative first motion, white.
SPREADING CENTER EARTHQUAKES
GLOBAL DISTRIBUTION OF EARTHQUAKES
Map of earthquake epicenters 1961-1967
Note the linear belts in the oceans associated with the mid-ocean ridges.
SPREADING CENTER EARTHQUAKES
NORMAL FAULTS AT MID-OCEAN RIDGES

Idealized cross section of the mid-Atlantic spreading center showing typical
focal mechanism solutions for three normal faults.
SPREADING CENTER EARTHQUAKES
MID-ATLANTIC RIDGE

Plan view of the mid-Atlantic ridge. Note that the earthquakes on the
spreading centers (red rectangles) are normal faults indicating extension.
SPREADING CENTER EARTHQUAKES
RIFT VALLEYS
East African rift classic example.

                             Spectacular topographic dome
                             with a 5600-km rift valley running
                             down its center.

                             Has steep escarpments and dramatic
                             valleys.

                             South of Ethiopia it separates into two
                             rift valleys

                             Western has very deep lakes.
                             The eastern shallower lakes and
                             spectacular volcanoes ie Kilimanjaro.
SPREADING CENTER EARTHQUAKES
RIFT VALLEYS

East African Rift runs into Red Sea and Gulf of Aden.


                                      In plate tectonic theory the East African
                                      Rift is a spreading center which connects
                                      through a triple junction with the
                                      spreading centers in the Gulf of Aden
                                      and the Red Sea.

                                      It is a zone of extension and the
                                      earthquakes are created by high
                                      angle normal faults like at a mid-
                                      ocean spreading center.
TRANSFORM FAULT EARTHQUAKES
OCEANIC TRANSFORMS
The mid-Atlantic ridge.
All the strike-slip earthquakes form on the transform faults (green
rectangles) connecting the spreading centers.
TRANSFROM FAULT EARTHQUAKES
CONTINENTAL TRANSFORMS
Dead Sea transform fault


                    Between Arabian and African plates

                    Map of the Dead Sea Fault zone
                      showing the transform faults
                      straight lines and the pull-apart
                      basins that are created by steps
                      along the fault.

                    Get strike-slip earthquakes on the
                       actual transform fault

                    Get strike slip and normal faults within
                       the pull-apart basins.
TRANSFORM FAULT EARTHQUAKES
CONTINENTAL TRANSFORMS
Repetition rate: Dead Sea Fault




 Have a historical record going back 3000 years in the Holy Land.
 This record has been pieced together to show that large earthquakes of
    greater than 6.5 magnitude have occurred about every two hundred
    years on this fault system.
 The Dead Sea Fault has been active for all the time the Red Sea has been
    opening. It is moving at around 5-10 mm.yr
TRANSFORM FAULT EARTHQUAKES
CONTINENTAL TRANSFORMS
North Anatolian fault




Accommodates westward movement of Turkey.
Note the time sequence of the fault ruptures. Started in the east in 1942
   with a magnitude 7.1 earthquake and they have progressively moved
   westwards. Next earthquake could be very close to Istanbul.
The ruptures are all strike slip faults.
TRANSFORM FAULT EARTHQUAKES
CONTINENTAL TRANSFORMS
San Andreas Fault




Note the strike-slip earthquakes along the actual San Andreas and San
   Jacinto Faults but also the compressional faulting in the transverse
   ranges. This is due to the bend in the fault.
TRANSFORM FAULT EARTHQUAKES
CONTINENTAL TRANSFORMS
San Andreas Fault 1906 San Francisco Earthquake




                               QuickTime™ and a
                      TIFF (Uncompressed) decompressor
                         are neede d to see this picture.
SUBDUCTION ZONE EARTHQUAKES
SUBDUCTION ZONE EARTHQAUKES
CONTINENT/CONTINENT COLLISION
India being thrust under Eurasia
                          Produces huge earthquakes in
                             China India and the
                             surrounding regions.


                          Shaanxi 1556, Tangshan 1976.

                          The Aceh-Andaman 2004
                             earthquake occurred where
                             the oceanic part of the Indian
                             plate was being subducted
                             below the Sumatran Plate.


                          Kashmir 2005 occurred where
                             huge gap in predicted
                             earthquakes
SUBDUCTION ZONE EARTHQUAKES
CONTINENT/CONTINENT COLLISON
Predicted slip potential for earthquakes along the
Himalayan Mountain Range
SUBDUCTION ZONE EARTHQUAKES
CONTINENT/CONTINENT COLLISON
Harvard and USGS solutions for the Kashmir
Earthquake
SUBDUCTION ZONE EARTHQUAKES
OCEAN/OCEAN COLLISION
The Pacific Plate subducted under the Eurasian plate




 Dotted patterns show severely shaken areas with dates for major
    earthquakes since 1920. Note the seismic gaps these are the areas
    where the next major thrust earthquakes are expected.
SUBDUCTION ZONE EARTHQUAKE
OCEAN/OCEAN COLLISION
Earthquakes in cold down-going slab beneath Japan.




  At shallow depths thrust earthquakes are generated in brittle rocks in
      both subducting and overriding plate.
  At greater depths only the interior of the subducting plate is cold
      enough to maintain the rigidity necessary to produce
      earthquakes.
  Can produce very large shallow earthquakes.
SUBDUCTION ZONE EARTHQUAKES
OCEAN/CONTINENT COLLISION
Created the 9.5 1960 Chilean thrust earthquake

                               Comparison of areas affected by great
                               transform and subduction earthquakes.

                               The 1960 Chilean earthquake was the
                               largest ever recorded.It was a major thrust
                               event

                               The 1906 San Francisco earthquake was
                               caused by a 10m rupture of 430 km of the
                               San Andreas Fault.

                               The 1960 Chilean earthquake involved
                               ~ 20 m movement on a 1000 km by 300 km
                               rupture plane
SUBDUCTION ZONE EARTHQUAKES
OCEAN/CONTINENT & OCEAN/OCEAN COLLISION
Created the 9.2 2004 Aceh-Andaman thrust earthquake
                             Aceh-Andaman slip zone was 900 km
                                long 100 km wide and slipped 15
                                km.

                             Started south off Sumatra and took
                                about 6 minutes to propagate to
                                North.

                             Aftershocks show limit of zone of slip.

                             Arrow shows direction of slip

                             Created huge Tsunami that killed >
                                250,000 people.

				
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