.<\/p>\n In assessing future rockfall hazard, a key element is evaluating the likely occurrence of the various trigger events.\u00a0 In Christchurch there were three key processes under consideration – a repeat of a local earthquake like the Canterbury sequence; a distant but larger earthquake, which in South Island is most likely to be a rupture on the Alpine Fault; or a large rainfall event.\u00a0 In each case much work has been undertake to evaluate the return periods of these trigger events, but for the Alpine Fault earthquake a key question was the degree to which a large earthquake would induce collapse events.\u00a0 This is challenging – we would expect that a distant very large earthquake would induce less intense shaking but for a longer duration when compared with a local event, but to what extent is this likely to trigger a large-scale rockfall?<\/p>\n In a paper that is in press for Geology, but is available for download<\/a> from the author’s website<\/a>, Benjamin Mackey and Mark Quigley from the University of Canterbury<\/a> have tried to address this problem by dating prehistoric rockfalls at a site in Rapaki in the Port Hills.\u00a0 This site was one of the locations in which rockfalls caused eyebrow-raising damage, in particular when a boulder went straight through a house, out the other side and on down the hill.\u00a0 Interestingly, the slope was also littered with other boulders from previous rockfalls.\u00a0 Mackey and Quigley identified boulders from previous rockfalls that were of a similar size to those that detached in 2011 and undertook a dating exercise on them.\u00a0 The approach used is new and novel dating technique that gives a duration over which a rock surface has been exposed to cosmic rays.\u00a0 The reasoning is that a boulder is shielded from cosmic rays until the rockfall event.\u00a0 From this point onwards the top surface is bombarded; the dating system allows this length of time to be determined.\u00a0 Whilst that might sound trivial, in reality the technique is very challenging and experimental.<\/p>\n The Rapaki site studied in this research. The boulder in the foreground originated from the bluff in the distance, passing through a house (now demolished) and fance (visible in the image) en route<\/p><\/div>\n .<\/p>\n The results are really interesting.\u00a0 In essence the technique suggests that the last big rockfall event at this site occurred about 7,000 years ago, with the possibility of an earlier one at about 13,000 years ago.\u00a0 Earlier rockfall events than this are likely to have occurred, but these boulders are now buried in the slope.\u00a0 The big, distant earthquakes happen much more frequently than this – for example the Alpine Fault generates a large earthquake about every 300-400 years and the Porters Pass Fault generates one every 1500-2000 years.\u00a0 Thie implication is that none of these events in the last 7,000 years has generated wide scale rockfall activity at Rapaki.<\/p>\n So, it seems likely that the prehistoric rockfalls at Rapaki were generated by local earthquakes similar to the Canterbury earthquake sequence.\u00a0 And of course the data might indicate that these events only occur every 6,000 to 7,000 years on average, which is reassuring.\u00a0 Note however that a single site gives limited information about the seismic hazard as a whole – Mackey and Quigley point out that the fault generating a Mw=6.0 earthquake would have to be within 10 km and a Mw=7.0 within 20 km of the site to generate sufficient shaking to trigger rockfalls at Rapaki.\u00a0 Other faults in this area but beyond these distances would not generate enough shaking to generate rockfalls at Rapaki.\u00a0 So the return period of earthquakes of this size across the Canterbury area may be shorter than this if there are other active faults.<\/p>\n![\"prehistoric](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2014\/10\/DSCF0422-e1412324865320.jpg\")
<\/a><\/p>\nDating prehistoric rockfalls<\/h5>\n
![\"prehistoric](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2014\/10\/DSCF0722-e1412325166945.jpg\")
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