The performance of rock slopes during the 2010/11 Canterbury Earthquake sequence, New Zealand

Abstract:

The 2010-2011 Canterbury earthquakes triggered many landslides in the Port Hills including rockfalls, rock and debris avalanches and slides and associated cliff-top cracking. The most abundant, and the highest risk to people and buildings, were rockfalls and rock/debris avalanches.

Volumes of rock leaving several cliffs during the earthquake sequence were determined from terrestrial laser scan change models. Relationships between volume leaving cliffs during the earthquakes, and site peak ground acceleration, Arias intensity, peak ground velocity, and shaking duration above a given amplitude threshold were compared for different sites. There were no instrumented cliff sites and some distance between the cliffs and nearest strong-motion sites. Therefore, we synthesised free-field rock-outcrop seismograms by employing a stochastic approach controlled by source models and regional parameters derived using spectral inversion of the extensive strong motion data set (Holden et al., 2014; Oth and Kaiser in press; Kaiser et al., 2013).

Rockfall volumes fit a power law with horizontal peak ground accelerations, with the exception of the 4 Sept. 2010 (Darfield) earthquake (Figure 1) which generated much smaller failures at Port Hills sites than in later earthquakes. This is presumed to be because the rock slopes became more fractured in the 22 Feb. 2011 earthquake. Ground conditions likely weakened further after 22 Feb. 2011. Fracturing and rock-mass strength degradation during subsequent earthquakes further degraded rock-mass quality, but the amount of degradation in each is unknown.

We suggest strong ground motion removes the mass most susceptible to failure, whilst reducing the strength of other remaining rock by dilating existing and generating new fractures. Shaking episodes thus increase the slope’s susceptibility to failure. This process repeats, such that rock slopes experience a continual cycle of coseismic weakening and failure (Parker et al. 2012). Immediately after an earthquake sequence, the new rock exposures are also more prone to static failure than was the former slope. The measured post-earthquake sequence static rockfall rates appear to be decaying with time, but they are still significantly higher than the static rates estimated before the earthquakes.