Characteristics of Some Rockfalls Triggered by the 2010/2011 Canterbury Earthquake Sequence, New Zealand

Abstract

The Canterbury earthquake sequence triggered thousands of rockfalls in the Port Hills of Christchurch, New Zealand, with over 6,000 falling on 22 February 2011. Several hundred families were evacuated after about 60 homes were hit and penetrated by boulders. We characterised the rockfalls by boulder-size distribution, runout distance, and source-area dimensions. We identified boulder-fall sources and boulder-fall paths, using those triggered by 22 February earthquake. The temporal probability of rockfall is dependent on the triggering cause (e.g, earthquake, rainfall). The probability density distribution of triggered boulder sizes was estimated using the mapped rockfall distributions that were triggered by the earthquakes on 22 February 2011. About 4,000 mapped rockfalls were used in the assessment. Boulder volumes range from a few cm 3 to greater than 10 m 3 . The 50th percentile volume of mapped boulders was about 0.7 m 3 . The spatial probability component of rockfalls is dependent on the distribution of boulder-fall sources, size and shape of the boulder, topography and substrate conditions along the rockfall-path, and was assumed to be largely independent of the triggering cause. These factors were assessed using the 6,000 boulders that fell from slopes during the 22 February 2011 earthquakes.