Lecture

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Lecture 13- Focal Mechanisms
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Faulting

• We have learned that the three basic styles of
  faulting
• Normal
• Reverse
• Strike-slip
• are associated with the three main types of
  plate boundaries
• Divergent
• Convergent
• Conservative

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Faulting

• We also have learned that these three mechanisms
• Normal
• Reverse
• Strike-slip
• are associated with the three types of stress
  regimes
• Extension (vertical stress greater than
  horizontal stresses)
• Compression (vertical stress less than horizontal
  stresses)
• Translation (vertical stress in between
  horizontal stresses)

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Faulting
There are special graphics, called focal
mechanisms or beachballs that we use as
shorthand to describe the style of faulting.
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Faulting and Plate Tectonics
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Faulting

• So far we have talked about the faulting of
  shallow earthquakes, which are well explained by
  plate tectonics.
• What about the faulting style of deep earthquakes
  ?
• Do similar principles hold true?

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Faulting

• We sometimes see normal faulting at depths of
  100 km or so in subduction zones

Earths surface
The slab can break under the extensional bending
stresses.
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Faulting

• We sometimes see reverse faulting for the
  deepest earthquakes at about 600 km depth

Earths surface
670 km Discontinuity
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Faulting

• So, what are focal mechanisms (beachballs)
  anyway?
• Graphical shorthand for a specific faulting
  process (strike, dip, slip)
• Projections of a sphere onto a circle (the lower
  focal hemisphere)
• Representations of the first motion of seismic
  waves

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Focal Mechanism

• When mapping the focal sphere to a circle
  (beachball) two things happen
• Lines (vectors) become points
• Planes become curved lines

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Focal Mechanisms

• Beachballs always have two curved lines
  separating the quadrants. That means that
  beachballs show two planes.
• But there is only one fault plane.
• The other plane is called the auxiliary plane.
• Seismologists cannot tell which is which from
  seismograms alone, so we always show both of the
  possible solutions.

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Focal Mechanisms

• Although seismologists cannot tell which plane
  ruptured from the seismograms, we can tell the
  type of faulting (reverse, normal, strike-slip)

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Focal Mechanisms

• It is often possible to make an educated guess as
  to which of the two possible planes is the actual
  fault plane
• Normal earthquakes work with gravity so are
  often steep
• Thrust events work against gravity so are often
  shallow

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A normal faulting mechanism. Which plane do you
think is the fault plane ?
A reverse faulting mechanism. Which plane do you
think is the fault plane ?
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Focal Mechanisms

• For strike slip focal mechanisms we can often
  determine the fault plane by its orientation with
  respect to the fault.

Ridge
Plate A
Plate B
(right or left lateral ??)
Ridge
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Summary

• The focal mechanism of an earthquake is a
  graphical description of the type of faulting
• Although the fault type can be determined from
  seismograms, the particular fault plane cannot be
  determined.
• There are always 2 possibilties
• The real plane, known as the fault plane
• The fake plane, known as the auxiliary plane

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Summary

• The fault plane can be distinguished from the
  auxiliary plane by considering
• The steepness of the fault
• Reverse shallow
• Normal steep
• Nearby morphological features
• The strike of the transform segment for instance
• The distribution of aftershocks
• Aftershocks tend to line up along the fault plane
  (but not always of course)