Using Cluster Analysis to Optimize Tsunami Evacuation Zones

William Power, Biljana Luković

GNS Science, Lower Hutt, New Zealand

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New Zealand tsunami sources

Background Figures from: Integrated Tsunami Database for Pacific

Distant/Regional EarthquakesLocal Earthquakes

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Tsunami warnings

• Divide the coast into zones

• Assign a threat level for each zone, based on maximum predicted water level

• Example is based on shipping forecast zones – not optimised for tsunami

Tsunami threat levels

Source: Mw 9.1 Southern Peru (1868)

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Tsunami threat levels Tsunami threat levels

Source: Mw 9.1 Southern Peru (1868)

Source: Mw 9.1 Southern Peru (1868)

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The Basic idea

Fault 1 Fault 2Max water level Fault 1

Max

wat

er le

vel F

ault

2

Cluster 1

Cluster 2

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Source locations

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Colours indicate clusters

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Problem

• The standard algorithms for computing clusters do not require the members of the cluster to be contiguous

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No relationship between separated clusters of the same colour

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Conclusions

• New Zealand is exposed to tsunami from many directions

• Different parts of the coast are more/less susceptible to different source regions

• In a warning system based around zones it is beneficial if the coast within each zone has a similar pattern of susceptibility

• Cluster Analysis is one route for classifying stretches of coast according to their susceptibility to different sources

• A drawback of conventional cluster analysis is that it does not constrain the clusters to be contiguous around the coast

• Approaches to adding the contiguity constraint are possible, but more work is required

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Acknowledgments

• NOAA – use of MOST and FACTS

• Diana Greenslade (BOM) – discussions about warning zones

• David Rhoades (GNS) – discussions about statistical analysis