THE GEOLOGIC AND ANTHROPOGENIC INFLUENCES ON THE SURFACE EXPRESSION OF LIQUEFACTION

The geologic controls on the surface expression of seismically-induced liquefaction have been well documented and helpful in mitigating earthquake hazards. However, the impact of anthropogenic infrastructure on the expression of surface liquefaction phenomena has not been as thoroughly explored. As a result, there has been a general failure to document and quantify the degree to which human modifications at the surface and in the shallow subsurface affect the spatial distribution of liquefaction ejecta. Here we investigate geologic and anthropogenic influences on the surface expression of liquefaction during the 2010-2011 Canterbury Earthquake Sequence (CES) in eastern Christchurch, New Zealand. Historical aerial photographs, city council grading and drainage plans, knowledge of local sediment characteristics and processes, and ground penetrating radar (GPR) are used to highlight the effect that old (1970) drainage pipes/trenches and meander scroll bars have in influencing the spatial distribution of liquefaction ejecta. Old drainage pipes (perforated) and surrounding gravel-filled trenches were preferred conduits (i.e. high permeability, pressure lows) for transport and deposition of liquified sediment during the CES. GPR is effective in capturing the location of subdrains and imaging river scroll bar features. In some locations, the scroll bar boundaries intersect at the surface with linear sand blow arrays, suggesting that liquefied sediment is migrating upward along the boundaries between individuals point bars - as conceptually proposed by Almond et al. (2018). Our results highlight the significant role humans play in shaping surface process during earthquakes.