i.3 the earths structure & seismicity
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geophysicsTRANSCRIPT
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Session I.3
The Earth’s Structure and Seismicity
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The Earth’s Layers
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0 – 70 km deep 300 + km deep
70 - 300 km deep
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Causes of intermediate and deep earthquakesBody 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 AsiaCOMPLEX
PLATE BOUNDARY
ZONENorthward motion of
India deforms the region
Eastward motion in China & SE Asia
Many small plates (microplates) and
blocks
India subducts beneath Burma microplate
Molnar & Tapponnier, 1977
SUNDA PLATE
INDIAN PLATE
EURASIAN PLATE
BURMAMICROPLATE
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Partitioning of strain between thrust motion at the trench and strike-slip motion on the Sumatra fault
Stein & Wysession 2003
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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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1. Crustal rocks at rest
2. Deformation; dilatancy and development of cracks
3. Instant rupture (earthquake)
4. 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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Brittle
Ductile
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Normal faulting
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Ductile
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Strike-SlipFault
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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 featuresalong a Strike-Slip Fault
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San Andreas FaultSan Andreas FaultW
alla
ce C
reek
Wal
lace
Cre
ek
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Faults atSea!!
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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.