Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals

Warwick Smith

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.

NZSEE Annual Conference 2007 GNS Science

Hawke’s Bay 1931

G.A. Eiby

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

NZSEE Annual Conference 2007 GNS Science

Hawke’s Bay 1931

G.A. Eiby

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

NZSEE Annual Conference 2007 GNS Science

Which is the best ellipse for MM VI?

Minimise the sum of distances to anomalous observations

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

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

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

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.

NZSEE Annual Conference 2007 GNS Science

Hawke’s Bay

1931

NZSEE Annual Conference 2007 GNS Science

Buller

1929

Very poor control

NZSEE Annual Conference 2007 GNS Science

Why are they not circles?

• Long fault sources?

• Anisotropic propagation?

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.

NZSEE Annual Conference 2007 GNS Science

Hope

1888

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

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

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