Seismic hazard from cross-faulting in North Canterbury: broader implications from the Arthur's Pass earthquake sequence of 18 June 1994

Technical Abstract

The 18 June 1994 Arthur's Pass earthquake of magnitude 6.7 (ML) occurred in a geographically remote area of the central Southern Alps of north Canterbury, New Zealand. The well developed aftershock sequence followed the largest historical earthquake in the central Southern Alps area and highlighted a NNW trending zone of seismic activity developed at a high angle to the regional structural trend. The well documented aftershock sequence enabled several aspects of this previously unidentified cross-fault to be investigated.

On a broad scale, aftershock seismicity defines the edge of an upper crustal block, highlighted by concentrated seismic activity along a NNW trend between the Avoca River and the Wilberforce River. A second major structure is identified in the upper Waimakariri River area along an ENE trend. Within these large scale seismically defined blocks, internal deformation as a result of regional dextral motion is accommodated by a complex and interconnecting series of structures, clearly illustrated by the temporal and spatial distribution of seismicity in both a NNW and NE - SW direction. Clockwise block rotation is permitted by a seismically defined mid crustal level detachment. The Harper Fault to the southeast served as a clear barrier to aftershock development, while the Bruce Fault was not significantly involved with the aftershock process.

eismicity estimates based on a b-value approach yield recurrence intervals for a similar magnitude event at between 250 and 850 years, while minimum recurrence intervals can only be based on the historical earthquake record. In terms of estimates of regional seismic hazard, the presence of crossfault structures close to active major faults necessitates a review of the regional seismic hazard, in view of evidence for triggering between adjacent structures clearly demonstrated from overseas examples and the widespread effects of ground shaking from the 18 June, 1994 Arthur's Pass earthquake sequence.

The Harper Fault has remained inactive in terms of an identified surface rupture trace during the Holocene. Its nature as a southwest dipping thrust fault has previously been unrecognized. Ridge rent scarps commonly observed in the area investigated by this study are considered to have formed as a result of glacially induced defect controlled rock mass creep. The seismic response of ridge rent structures is considered to be site specific. In the case of such structures within this study area, the ground shaking effects as a result of the magnitude 6.7 Arthur's Pass earthquake may be just below the required threshold to activate (or reactivate) movement.