Rockfall Activity Rates Before, During and After the 2010/2011 Canterbury Earthquake Sequence.

The effects of strong ground shaking on hillslope stability can persist for many years after a large earthquake, leading to an increase in the rates of post earthquake land sliding. The factors that control the rate of post‐earthquake land sliding are poorly constrained, hindering our ability to reliably forecast how landscapes and landslide hazards and risk evolve. To address this, we use a unique data set comprising high‐resolution terrestrial laser scans and airborne lidar captured during and after the 2010–2011 Canterbury Earthquake Sequence, New Zealand. This earthquake sequence triggered thousands of rock falls, and rock and debris avalanches (collectively referred to as "rockfall"), resulting in loss‐of‐life and damage to residential dwellings, commercial buildings and other infrastructure in the Port Hills of Christchurch, New Zealand. This unique data set spans 5 years and includes five significant earthquakes. We used these data to (a) quantify the regional‐scale "rockfall" rates in response to these earthquakes and the postearthquake decay in rockfall rates with time; and (b) investigate the site‐specific factors controlling the location of seismic and nonseismic rockfalls using frequency ratios and logistic regression techniques. We found that rockfall rates increased significantly in response to the initial earthquake that generated the strongest shaking in the sequence—The MW 6.2 22 February 2011 event—Compared to the long‐term background rates derived from the dating of pre‐2010 talus piles at the toe of the slopes. Non seismic rockfall rates also increased immediately after the 22 February 2011 earthquake and decayed with time following a power‐law trend. About 50% of the decay back to the pre‐earthquake rockfall rates occurred within 1–5 years after the 22 February 2011 earthquake. Our results show that the short‐term decay in rockfall rates over time, after the initial earthquake, was attributed to the subsequent erosion of seismically damaged rock mass materials caused by environmental processes such as rain. For earthquake‐induced rockfall at the regional‐scale, the peak ground accelerations is the most significant variable in forecasting rockfall volume, followed by the relative height above the base of the slope. For both earthquake and non‐seismic conditions at the site‐specific scale, the probability of rockfall increases when the adjacent areas have failed previously, indicating that accrued damage preconditions localized areas of the slope for subsequent failure. Such preconditioning is a crucial factor driving subsequent rockfalls; that is, future rockfalls are likely to cluster near areas that failed in the past. Plain Language Summary: Evidence from previous earthquakes suggests that the frequency of land sliding after a large earthquake is significantly higher than before it. Strong earthquakes cause slope cracking and generate landslide debris, which can be more readily remobilized post earthquake, creating new hazards, including further landslides and landslide dams. These hazards may persist for years and decades and represent a prolonged risk that the impacted communities must consider. Currently, the relative increase in land sliding and rate of decay during and after a major earthquake is rarely quantified, thus posing a knowledge gap for those rebuilding after a major earthquake. This paper explores high‐resolution terrestrial laser scan models of slope surfaces and how these surfaces changed during and after strong earthquake shaking during the 2010–2011 Canterbury Earthquake Sequence (CES) in New Zealand. These surface "change" models were used to quantify the volumes of debris–rock and soil–that fell from these slopes during and after the CES. These data were used to establish a regional‐scale, physical relationship between the volume of debris falling from the cliffs per earthquake or unit of time, per unit area of slope. Using the change models, we investigated the factors that control the temporal and spatial distribution of the rockfalls at the regional‐ and site‐specific scales. At the regional scale, we found that the size of the slope and the relative increase in rockfall rates above pre‐CES rates controlled the subsequent non‐seismic rockfall decay time. At the site‐specific scale, the main conclusions from this study are: (a) for earthquake triggers, the peak ground acceleration (a measure of earthquake ground shaking) was the most important variable in forecasting the probability of failure, followed by the relative elevation or height above the base of the slope; and (b) for both earthquake and nonseismic triggers, the probability of failure increases when the adjacent areas have failed previously, indicating that preconditioning of the slope to failure is a key factor driving subsequent rockfalls. Key Points: We quantify rock slope changes during and after a major earthquake sequence through repeat high‐resolution terrestrial laser scan surveysWe analyze landslide rates over 5 years, during and after the earthquakes at the regional‐ and site‐specific scalesNon seismic landslide rates are heightened immediately after the initial earthquake and decayed with time (1–5 years) following a power‐law trend [ABSTRACT FROM AUTHOR]