New Fault Line A “Major Surprise”
The fault that ruptured causing the magnitude 7.1 earthquake early Saturday morning was previously unknown, according to
University of Canterbury geologist Associate Professor Tim Davies.
“The earthquake on 4 September exceeded all previous scenarios for seismic shaking at the University of Canterbury. An
event of about the same magnitude on the Porters Pass fault had been anticipated, but the 4 September earthquake was
very much closer – about 30 km away instead of at least 60 km. Thus the shaking intensity on the UC campus was very much
greater than expected. Though significant damage to structures, infrastructure and contents has occurred, the situation
could have been very much worse.
“By comparison with the expected Alpine Fault earthquake, which will be a very much larger event but several times
farther away, the 4 Sept earthquake caused severe damage in Canterbury but did not disrupt infrastructure (e.g. power
generation) island-wide. Further, occurring in the early morning at a weekend was probably optimal in terms of lack of
deaths and injuries,” said Professor Davies.
“The aftershock sequence appears to be fairly normal, and the most likely scenario is that aftershocks will continue;
some may be significant and capable of causing further damage; but a repeat of the M= 7.1 quake is unlikely.
“While it is possible that the ‘new’ fault rupturing on Saturday 4 September could trigger an earthquake on another
fault, for example the Porters Pass fault, the Hope fault or even the Alpine fault, over the next days, months or years,
the probability of this is unlikely to be high.
“The fault that ruptured causing the shake was previously unknown. A fault beneath the Canterbury Plains capable of
generating an earthquake of that size is a major surprise. Building design is based on the NZ Probabilistic Seismic
Hazard Map, which is based on anticipated shaking from known faults. A significant question arises, about whether design
standards should take account of the possibility of such unknown faults.”
ENDS