Sri Lanka Earthquake and Tsunami Warning Training Program

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Session I.3 The Earths Structure and
Seismicity
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The Earths Layers
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0 70 km deep
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Causes of intermediate and deep earthquakes
Body forces, with increased resistance caused by
lower mantle density increase
variation in negative buoyancy caused by altered
depth of phase transitions
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Cause of shallow thrust earthquakes
Stein Wysession, 2003
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Transform boundaries
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The San Andreas Fault System is a Major
Transform Fault
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South East Asia
COMPLEX PLATE BOUNDARY ZONE Northward motion of
India deforms the region Eastward motion in
China SE Asia Many small plates (microplates)
and blocks India subducts beneath Burma
microplate
EURASIAN PLATE
INDIAN PLATE
SUNDA PLATE
BURMA MICROPLATE
Molnar Tapponnier, 1977
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Stein Wysession 2003
Partitioning of strain between thrust motion at
the trench and strike-slip motion on the Sumatra
fault
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Regions of compression, tension and shearing
stresses accommodated by faulting
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The relationship between faults and earthquakes
Elastic Rebound Theory
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• Crustal rocks at rest
• Deformation dilatancy and development of cracks
• Instant rupture (earthquake)
• Sudden drop in stress after earthquake

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Classification of faults
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Reverse faulting
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Faults occur at many scales
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Ductile
Brittle
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Normal faulting
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Ductile
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Strike-Slip Fault
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Identifying faults - Fault Scarps
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Normal and reverse faults both generate scarps
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Block Diagram showing features along a
Strike-Slip Fault
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Wallace Creek
San Andreas Fault
Wallace Creek
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Faults at Sea!!
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Review

• The Earth lithosphere is broken into numerous
  plates, which move independently of each other.
• This results in regions of divergence
  (destructive boundaries), convergence
  (constructive boundaries) and regions where the
  plates slide past one another (transform or
  conservative boundaries)
• The vast majority of earthquakes occur at plate
  boundaries. The nature of these earthquakes is
  related to the processes at the boundaries.
• Earthquakes occur when strain built up over many
  years is released rapidly as brittle failure
  Elastic Rebound Theory.
• 3 end members of fault strike-slip, normal,
  reverse/thrust.