article: Feb 20 2008 - Isoseismal maps estimate the impact

The modelled effects of an earthquake are shown on the new isoseismal map. It can be found on the "maps" tab of any earthquake report.

A deep magnitude 5.4 earthquake felt across the top of the South Island, November 2007. No damage expected.



The shallow magnitude 6.7 earthquake in Fiordland, October 2007. Capable of causing serious damage, but the red zone is offshore.

When we hear about an earthquake, we often want to know who is going to have been affected by it. ShakeNZ on the GeoNet home page tells us the shaking intensities at individual locations within minutes. What if we want a broad view of the zone of impact? This is where the new isoseismal map can help. Isoseismal contours join up points of equal Modified Mercalli intensity, much like an isobaric map shows the high and low pressure areas on a weather map. A model¹ is used to predict the likely level of shaking caused by an earthquake of a certain magnitude, depth, region and fault rupture type.

A model is necessarily a simple view of the more complex reality, and the contours shown will not exactly correspond to the spot intensities reported by instruments and people. Nevertheless the contours give a quick visual appreciation of the anticipated zone of impact.

The model has been derived from the contours of "felt" reports of many past New Zealand earthquakes. The reports you make after feeling an earthquake add to the data that can be used to improve this model in the future. The model also allows planners to set up "what if?" scenarios and be better prepared for actual earthquakes when they happen.

The accompanying images show two recent examples of isoseismal maps: the top one is a "felt only" earthquake, with damage unlikely; the bottom one is potentially serious, but the structural damage zone is out to sea, and the lesser damage zone is largely in wilderness. We have used three coloured zones to help understand the overall seriousness of the earthquake: green for areas likely to have felt the earthquake, yellow for areas where household contents and fittings may be damaged, and red for areas that may experience damage to buildings.

In the past two years there have been three earthquakes that caused shaking in the red zone: the Fiordland earthquake (shown) and one of its aftershocks, and the December 2007 earthquake off the coast of Gisborne. By using colour to highlight the expected seriousness of shaking, we can better understand whether an earthquake is significant or not.

¹Dowrick, D.J.; Rhoades, D.A. 2005 Revised models for attenuation of Modified Mercalli intensity in New Zealand earthquakes. Bulletin of the New Zealand Society for Earthquake Engineering, 38(4): 185-214.