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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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
Hawke’s Bay 1931
G.A. Eiby
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NZSEE Annual Conference 2007 GNS Science
Why the Roman numerals?
Maybe ……
1. To distinguish intensities from magnitudes
2. To discourage fractions – use integers only
What is 6.7 in Roman numerals?
My practice: Arabic for individual observations
Roman for isoseismals
VIVII
X
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NZSEE Annual Conference 2007 GNS Science
Hawke’s Bay 1931
G.A. Eiby
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NZSEE Annual Conference 2007 GNS Science
Drawing isoseismals
The developing practice is to model them as ellipses in an attenuation function
Let’s try to fit them as ellipses
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NZSEE Annual Conference 2007 GNS Science
Which is the best ellipse for MM VI?
Minimise the sum of distances to anomalous observations
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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
Hawke’s Bay
1931
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NZSEE Annual Conference 2007 GNS Science
Buller
1929
Very poor control
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NZSEE Annual Conference 2007 GNS Science
Why are they not circles?
• Long fault sources?
• Anisotropic propagation?
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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
Hope
1888
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NZSEE Annual Conference 2007 GNS Science
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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NZSEE Annual Conference 2007 GNS Science
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-axesWatch
this space
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NZSEE Annual Conference 2007 GNS Science
What happens when ellipses don’t 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