Geophysical Imaging of Subsurface Earthquake-induced Liquefaction Features at Christchurch Boys High School, Christchurch, New Zealand

Abstract

On February 22, 2011, a magnitude M w 6.2 earthquake affected the Canterbury region, New Zealand, resulting in many fatalities. Liquefaction occurred across many areas, visible on the surface as ''sand volcanoes'', blisters and subsidence, causing significant damage to buildings, land and infrastructure.Liquefaction occurred at a number of sites across the Christchurch Boys High School sports grounds; one area in particular contained a piston ground failure and an adjacent silt volcano. Here, as part of a class project, we apply near-surface geophysics to image these two liquefaction features and determine whether they share a subsurface connection. Hand auger results enable correlation of the geophysical responses with the subsurface stratigraphy.The survey results suggest that there is a subsurface link, likely via a paleo-stream channel. The anomalous responses of the horizontal loop electromagnetic survey and electrical resistivity imaging highlight the disruption of the subsurface electrical properties beneath and between the two liquefaction features. The vertical magnetic gradient may also show a subtle anomalous response in this area, however the results are inconclusive. The ground penetrating radar survey shows disruption of the subsurface stratigraphy beneath the liquefaction features, in particular sediment mounding beneath the silt ejection (''silt volcano'') and stratigraphic disruption beneath the piston failure.The results indicate how near-surface geophysics allow the characteristics of liquefaction in the subsurface to be better understood, which could aid remediation work following liquefaction-induced land damage and guide interpretation of geophysical surveys of paleoliquefaction features.

Micromorphological analysis of liquefaction features in alluvial and coastal environments of Christchurch, New Zealand

“…Some of the abandoned river channels can be traced through the wider area of Christchurch city and were probably active within the last 500 years (Basher et al ., ). These channel deposits were particularly prone to liquefaction, as was shown in some of the main studies undertaken to understand the correlation between liquefaction features and geomorphic setting of the CES (Almond et al ., ; Wotherspoon et al ., ; Nobes et al ., ; Bastin et al ., ; Villamor et al ., ).…”

Paleoliquefaction Studies and the Evaluation of Seismic Hazard

“…Results showed clear reflectors similar to what one might achieve with seismic reflection surveys ( Figure 12 [122]). Nobes et al [128], in a post-earthquake liquefaction study, combined GPR imaging with electrical imaging and demonstrated good agreement between the two methods. A key advantage to GPR surveys is the speed with which surveys can be accomplished relative to seismic surveys.…”

An Overview of Clastic Dikes: Significance for Earthquake Study

“…While a final solution about the seismogenic origin of the injection dikes must be accepted, based on geological and structural indicative criteria, preliminarily these structures and other liquefaction features might be find on GPR images. Most of GPR works conclude that sand blows on radargrams are shown as stratigraphic disruptions of the radar events and disturbance of the GPR facie [Liu, Li, 2001;Al-Shukri et al, 2006;Nobes et al, 2013;Baradello, Accaino, 2016]. Charac-teristic anomalies, the shape of which is like a funnel or a pipe with disordered and stronger reflection are additional indicators of liquefaction on GPR images [Hsu et al, 2005;Lunina, Gladkov, 2016].…”