SPECIAL FEATURE Geologic and geomorphic influence on the spatial extent of lateral spreading in Christchurch, New Zealand
Lateral spreading occurred proximal to waterways during the 2010 to 2011 Canterbury Earthquake Sequence (CES) resulting in severe damage to overlying infrastructure. Published empirical and semi-empirical models for predicting lateral spread displacements have been shown produce displacement that varied by a factor of <0.5 to >2 from those measured in parts of Christchurch. Post-CES studies have indicated that the spatial distribution and extent of lateral spreading was strongly influenced by geomorphic and topographic features. These features are not explicitly accounted for in the current predictive models and likely account for some of the discrepancy between predicted and measured displacements. This study aims to examine the influence of geomorphic features on the spatial extent of lateral spreading and associated ground movement for a study area proximal to the Avon River.
Extensive LiDAR and satellite derived horizontal displacement datasets are available following the main CES earthquakes. These datasets provide an indication of the extent of lateral spreading and associated ground movements. However, each dataset has inherent limitations including spatial resolution, accuracy, and acquisition errors (e.g. flight line offset). It is therefore impossible to accurately derive the maximum extent of lateral spreading from a single dataset. The summation of measured crack widths, LiDAR derived ground subsidence, documented land damage, and cracking observed from post-event aerial photography provide additional information on the extent of lateral spreading.
In this study, the maximum extent of lateral spreading was derived by combining observations from the CES datasets. Relatively narrow zones of lateral spreading ranging from 0 to 300 m were identified and associated with maximum horizontal movements between 0 and 2.5 m. Zones were spatially correlated and with geomorphic maps (Figure 1). The width of these zones is shown to be strongly influenced by local geomorphic features; spreading is shown to terminate adjacent to paleo-cut banks and terrace risers irrespective of distance from the free-face. In comparison, areas underlain by point-bar deposits exhibit high horizontal displacements, while in other areas underlain by recent fluvial sediments spreading appears to gradually decay inland. Detailed geotechnical characterization of the subsurface sediments will be undertaken within each geomorphic area to identify prevalent soil types, stratification, and the thicknesses and lateral extent of critical layers. Soil types in areas on either side of the spreading extent will also be examined to determine factors influencing the extent.
This work aims to enhance engineering evaluation of lateral spreading, based the exceptional datasets available from the CES. The results will be incorporated into the revised geotechnical guidelines for assessing lateral spreading. This project is funded by QuakeCoRE (project #E6471/16056)
Figure 1: Geomorphic map of the study area overlain with the derived maximum extent of lateral spreading