Attenuation of peak ground accelerations in New Zealand earthquakes

Abstract

The main result of this study is the development of attenuation expressions for peak ground accelerations (PGAs) in New Zealand earthquakes, in terms of magnitude Mw and shortest distance from the source. Other factors which are modelled are depth, focal mechanism, ground class and tectonic type of earthquake i.e. crustal, interface or dipping slab. As well as being implied in the source distance, the substantial effect of depth is modelled well with a separate linear depth term. For crustal events, focal mechanisms which are predominantly reverse are found to cause PGAs that are 28 percent stronger on average than for other mechanisms. PGAs at soil sites (ie those having soil deposits >3 m thick) are found to be 53 percent stronger on average than at other (ie rock) sites, but the difference between PGAs on rock and soil sites in large amplitude shaking remains uncertain, ie near the source o f events of Mw ≥ 7. Earthquakes occurring on the interface between the Pacific and Australian tectonic plates give rise to smaller PGAs than do crustal or slab events of the same magnitude, depth and distance. Comparisons are made between our New Zealand model and some for Europe, Japan and the Western USA.