Paleoliquefaction in Christchurch, New Zealand

Abstract

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.