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Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals

Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals. Warwick Smith. 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.

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Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals

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  1. Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals Warwick Smith

  2. 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. NZSEE Annual Conference 2007

  3. Hawke’s Bay 1931 G.A. Eiby NZSEE Annual Conference 2007

  4. VII X VI 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 NZSEE Annual Conference 2007

  5. Hawke’s Bay 1931 G.A. Eiby NZSEE Annual Conference 2007

  6. Drawing isoseismals The developing practice is to model them as ellipses in an attenuation function Let’s try to fit them as ellipses NZSEE Annual Conference 2007

  7. Which is the best ellipse for MM VI? Minimise the sum of distances to anomalous observations NZSEE Annual Conference 2007

  8. Assigning weights for fitting MM VI isoseismal NZSEE Annual Conference 2007

  9. Then it gets more complicated NZSEE Annual Conference 2007

  10. Then it gets more complicated NZSEE Annual Conference 2007

  11. 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. NZSEE Annual Conference 2007

  12. Hawke’s Bay 1931 NZSEE Annual Conference 2007

  13. Very poor control Buller 1929 NZSEE Annual Conference 2007

  14. Why are they not circles? • Long fault sources? • Anisotropic propagation? NZSEE Annual Conference 2007

  15. 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. NZSEE Annual Conference 2007

  16. Hope 1888 NZSEE Annual Conference 2007

  17. 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 NZSEE Annual Conference 2007

  18. 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 NZSEE Annual Conference 2007

  19. 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 NZSEE Annual Conference 2007

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