Abstract
This study of attenuation of Modified Mercalli intensity in New Zealand earthquakes used a carefully selected set of isoseismal maps from 30 events with magnitudes ranging from ML= 5.0 to Ms = 7.8, and depths ranging from very shallow to 65 km. A completely revised set of magnitudes was used. A two-step stratified regression analysis was used to evaluate the coefficients of the expression for intensity I = a + bM + cr + dlog10r where r is the mean slope radius corresponding to the distance from the isoseismal to the centroid of the fault rupture surface. The resulting expressions model events of all depths very well, and completely revise the attenuation curves of earlier work done in the 1970’s in which depth was not taken into account. The earlier proposals for regional variations in attenuation were not substantiated, the apparent differences for most of the country being explained by the effects of depth and source mechanism rather than path characteristics. The attenuation character of the Central Volcanic Region and Fiordland remain to be properly established. Attenuation of intensity in New Zealand was found to be very similar to that for Turkey, the two regions being dominated by strike-slip faulting.
Highlights
The subjective felt intensity scales remain a valuable, necessary, measure of strength of ground shaking in earthquakes despite the growth in numbers of strong-motion accelerographs in recent decades
Equation (1) is a form of expression previously used for attenuation of intensity, eg Ambraseys' study (8, 9] of earthquakes in Turkey, or for the attenuation of peak ground acceleration, amax' where log amax is substituted for I, as BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol 24, No 3 September 1991 in Joyner and Boore's study [10) of Californian ground motion recordings
As described elsewhere [21], the coefficients in equation (1) were evaluated using a two-step multiple regression analysis of the type preferred by Joyner and Boore [22]
Summary
This study of attenuation of Modified Mercalli intensity in New Zealand earthquakes used a carefully selected set of isoseismal maps from 30 events with magnitudes ranging from ML= 5.0 to Ms = 7. A completely revised set of magnitudes was used. The resulting expressions model events of all depths very well, and completely revise the attenuation curves of earlier work done in the 1970 1s in which depth was not taken into account. The earlier proposals for regional variations in attenuation were not substantiated, the apparent differences for most of the country being explained by the effects of depth and source mechanism rather than path characteristics. Attenuation of intensity in New Zealand was found to be very similar to that for Turkey, the two regions being dominated by strike-slip faulting
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