Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals

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Reducing the subjectivity in Modified Mercalli
intensity analysis drawing isoseismals

• Warwick Smith

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MM intensities are arguably

• Subjective
• Useless
• Hopeless
• Pointless
• Meaningless
• All of the above
• But they are all we have for our most important
  earthquakes, and ever will have.
• We had better learn all we can from them.

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Hawkes Bay 1931 G.A. Eiby
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Why the Roman numerals?

• Maybe
• 1. To distinguish intensities from magnitudes
• To discourage fractions use integers only
• What is 6.7 in Roman numerals?
• My practice Arabic for individual observations
• Roman for isoseismals

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Hawkes Bay 1931 G.A. Eiby
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Drawing isoseismals

• The developing practice is to model them as
  ellipses in an attenuation function
• Lets try to fit them as ellipses

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Which is the best ellipse for MM VI?
Minimise the sum of distances to anomalous
observations
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Assigning weights for fitting MM VI isoseismal
Intensity Weight if inside Weight if outside
4 2 0
5 1 0
6 0 1
7 0 2
8 0 3
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Then it gets more complicated
Intensity Weight if inside Weight if outside
4-5 1.5 0
5-6 0.5 0.5
6-7 0 1.5
7-8 0 2.5





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Then it gets more complicated
Intensity Weight if inside Weight if outside
4-5 1.5 0
5-6 0.5 0.5
6-7 0 1.5
7-8 0 2.5
4? 1.5 0
5? 0.5 0
6? 0 0.5
7? 0 1.5
8? 0 2.5
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Parameters to fit

• Coordinates of centre (lat, long)
• Axes (major, minor)
• Orientation
• But for a stable solution we can specify the
  centre and require that all the isoseismals have
  the same orientation.

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Hawkes Bay 1931
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Buller 1929
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Why are they not circles?

• Long fault sources?
• Anisotropic propagation?

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Why are they not circles?

• Long fault sources?
• Anisotropic propagation
• Energy propagation parallel to the strike of the
  country seems to be more favourable than across
  it. Source effects should mostly be seen at short
  distances, comparable with the fault length.

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Hope 1888
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Orientation of major axis

• Rather than solving for the strike, should we
  use
• A representative regional value for the outer
  isoseismals
• The fault strike for inner isoseismals
• Interpolation between
• ? ? ? ? ?
• Maybe

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Systematic analysis

• All large events (e.g. Dowrick Rhoades)
• Digitize all reporting locations, with
  intensities
• Fit isoseismals to each
• Fit attenuation model to derived semi-axes

Watch this space
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What happens when ellipses dont fit well?

• Soft ground gives systematically high intensities
  (e.g. Wanganui)
• Strong attenuation gives low intensities (e.g.
  across the TVZ)
• etc
• Procedure
• Fit ellipses
• Look for systematic departures