Sarah Bastin

Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence

Continuous observational monitoring of a study site in eastern Christchurch, New Zealand, following the 2010 M w 7.1 Darfi eld earthquake has recorded ten distinct liquefaction episodes in the mainshock‐aftershock sequence. Three nearby accelerometers allow calibration between the geological expressions of liquefaction and the intensity of earthquake-induced surface ground motion at the site. Sand blow formation was generated by M w 5.2‐7.1 earthquakes with M w 7.5‐normalized peak ground accelerations (PGA 7.5 ) of ≥ 0.057 g (acceleration due to gravity). Silt drapes between successive sand blow deposits provide markers for delineating distinct liquefaction-inducing earthquakes in the geologic record. However, erosion quickly modifi es the surface of sand blows into alluvial and aeolian forms that complicate geologic diagnosis. The two feeder-dike generations identifi ed in subsurface investigations signifi cantly underrepresent the number of liquefaction-inducing earthquakes due to extensive dike reactivation. New constitutive equations enable PGA 7.5 variations to be estimated from the thickness and areal extent of sand blows.

Paleoliquefaction in Christchurch, New Zealand

Liquefaction during the 2010 moment magnitude (Mw) 7.1 Darfield earthquake and large aftershocks (known as the Canterbury earthquake sequence) caused severe damage to land and infrastructure in Christchurch, New Zealand. Liquefaction occurred at Mw-weighted peak ground accelerations (PGA7.5) as low as 0.06 g at highly susceptible sites. Trenching investigations conducted at two sites in eastern Christchurch enabled documentation of the geologic expressions of recurrent liquefaction and determination of whether evidence of pre–Canterbury earthquake sequence liquefaction is present. Excavation to water table depths (∼1–2 m below surface) across sand blow vents and fissures revealed multiple generations of Canterbury earthquake sequence liquefaction “feeder” dikes that crosscut Holocene-to-recent fluvial and anthropogenic stratigraphy. Canterbury earthquake sequence dikes crosscut and intrude oxidized and weathered dikes and sills at both sites that are interpreted as evidence of pre–Canterbury earthquake sequence liquefaction. Crosscutting relationships combined with 14C dating constrain the timing of the pre–Canterbury earthquake sequence liquefaction to post–A.D. 1660 to pre–ca. A.D. 1905 at one site, and post–A.D. 1415 to pre–ca. A.D. 1910 at another site. The PGA7.5 of five well-documented historical earthquakes that caused regional damage between 1869 and 1922 are approximated for the study sites using a New Zealand specific ground motion prediction equation. Only the June 1869 Mw ∼4.8 Christchurch earthquake produces a median modeled PGA7.5 that exceeds the PGA7.5 0.06 g threshold for liquefaction. Prehistoric earthquakes sourced from regional faults, including the 1717 Alpine fault Mw ∼7.9 ± 0.3 and ca. 500–600 yr B.P. Mw ≥ 7.1 Porters Pass fault earthquakes, provide additional potential paleoseismic sources for pre–Canterbury earthquake sequence liquefaction. The recognition of pre–Canterbury earthquake sequence liquefaction in late Holocene sediments is consistent with hazard model-based predicted return times of PGAs exceeding the liquefaction triggering threshold in Christchurch. Residential development in eastern Christchurch from ca. 1860 to 2005 occurred in areas where geologic evidence for pre–Canterbury earthquake sequence liquefaction was present, highlighting the potential of paleoliquefaction studies to predict locations of future liquefaction and to contribute to seismic hazard assessments and land-use planning.

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

On February 22, 2011, a magnitude Mw 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.