Geology of liquefaction-induced lateral spreading, new results from Christchurch, New Zealand

Gregory P. De Pascale, Jeffrey L. Bachhuber, Ellen Rathje, Jing Hu, Peter Almond, Christian Ruegg, & Mike Finnemore

Submitted August 11, 2016, SCEC Contribution #6617, 2016 SCEC Annual Meeting Poster #084

Earthquake triggered liquefaction and lateral spreading was widespread in Canterbury, New Zealand during the 2010-2012 Canterbury Earthquake Sequence (CES) and led to ~$20 Billion NZD of damage to infrastructure. Permanent lateral ground displacements and subsequent vertical subsidence along rivers and streams progressed hundreds of metres back from the river banks, significantly exceeding expected lateral spread distances calculated from empirically-based predictive relationships. Although the causes and timing of liquefaction during the CES are well known, the geological controls on lateral spreading are more poorly-understood. We undertook a multidisciplinary investigation funded by the National Science Foundation (NSF) that included trenching, mapping, satellite displacement models, Multi-channel Analysis of Surface Waves (MASW), Cone Penetration Testing (CPT), and boreholes in and around lateral spreading sites in Christchurch - the main city in the Canterbury region. The focus of the study is to better understand the geologic controls on lateral spread occurrence and distances back from river banks. Preliminary results based on the first lateral spread trench (1 of 2) in our study demonstrate that: 1) displacements observed from satellite-image modeling correspond well with on-the-ground and subsurface displacement measurements and headscarp margins; 2) trenching of Holocene sediments along the headscarp margins of these lateral spreading zones demonstrates that ground cracks and vertical steps at the surface correspond with liquefaction-ejecta filled cracks in the subsurface and a zone of extension indicated by normal faulting and horst and graben structures; 3) translated sediments are composed of laminar, sub-horizontal bedded sands with occasional silt interbeds; 4) discrete displacements are well exhibited by progressive vertical-lateral displacements of silt marker beds in the sand and can be used to measure total movement of the lateral spread mass; 5) radiocarbon dating indicates that the translated sands are late-Holocene (<600 years BP); and 6) displacements at the surface (from the CES) are less than the cumulative displacement of the sand layers measured in the subsurface (trench walls) which may suggest paleo-lateral spreading at the site and therefore recurrence of lateral spreading events. The preliminary results suggest repeated lateral spreading (pre-historic and CES) in essentially the same zones, and which we have found to have similar geologic conditions. These results can provide a sound basis for hazard zonation maps (e.g. liquefaction and lateral spreading), hazard maps) and mitigation strategies (e.g. avoidance or ground improvement).

Key Words
liquefaction, lateral spreading, New Zealand, Christchurch, paleoliquefaction

De Pascale, G. P., Bachhuber, J. L., Rathje, E., Hu, J., Almond, P., Ruegg, C., & Finnemore, M. (2016, 08). Geology of liquefaction-induced lateral spreading, new results from Christchurch, New Zealand. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology