A revision of the modified Mercalli intensity scale

Abstract

The Modified Mercalli intensity scale has been used to categorise non-instrumental observations of earthquakes in New Zealand since the mid-1940s when it replaced the Rossi-Forel Scale then in use. A version modified to improve its usefulness for New Zealand conditions was published by Eiby in 1966.  Its principal strengths have been its proven utility and its universality. Its main weakness is the great difference between the current stock of structures in New Zealand and that for which the scale was original constructed, a difference made greater by the steady improvement in the earthquake performance of both commercial and domestic buildings since the 1930s.

This weakness, and other concerns about internal consistency of some of the effects, led to the NZ National Society for Earthquake Engineering commissioning a study group in 1990 to revise the scale so that:

1. It is internally consistent
2. It is still consistent with overseas practice
3. It represents the smallest possible departure from the existing (1965) and the original 1931 scales.