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rise. We also highlight the complex interactions within coexisting occurred in response to channel incision of older fans upstream
natural and built environments, where anthropogenic activities and by sediment from basin headwaters. Significant river avulsion
designed to mitigate regional flood hazard may deprive subsiding has occurred periodically as a result of river sediment bed-load
coastal areas of replenishing sediment that would have provided overtopping natural channel levees, leading to the river mouth
natural mitigation of coastal and flooding hazards. using the Avon-Heathcote Estuary at least several times over the
past 6.5 ka. This includes avulsion north and south of Banks
GEOLOGY AND GEOMORPHOLOGY OF CHRISTCHURCH, Peninsula several times through the Holocene, with the latest
NEW ZEALAND northward migration commencing in the last millennium (Soons
et al., 1997; McFadgen and Goff, 2005) (Fig. 1). The co-evolution
Christchurch (population ~350,000 at latest census) is located of floodplain and coastal landscapes produced significant spatial
on the eastern coast of New Zealand’s South Island, adjacent to heterogeneity in Holocene sediments underlying Christchurch,
the Pacific Ocean. Previously a seasonal resource-gathering area with alluvial gravels dominating the west of the city and coastal
for Maori, development of the built environment began with dunes and estuarine/tidal wetland sediments dominating the east,
English colonial settlement in the 1850s. Most of the city resides with finer alluvial overbank deposits from the Avon and
upon late Quaternary alluvial sediments derived from Mesozoic Heathcote Rivers superimposed on these accumulations (Brown
quartzo-feldspathic metasediments (graywackes and argillites) in and Weeber, 1992).
source catchments in the Southern Alps. The city is bounded to
the south by Banks Peninsula, comprised largely of Neogene Pre-CES subsidence rates across Christchurch are poorly
volcanic rocks, and to the north by the large, braided Waimakariri constrained, but the dominant processes would have been long-
River. Two smaller spring-fed tidally influenced rivers, the Avon term sediment loading and periodic settling through local and
and Heathcote, flow through the city into the Avon-Heathcote regional earthquakes. Minimum earthquake peak ground accel-
Estuary and out to Pegasus Bay via an inlet to the south (Fig. 1). erations (PGA) required to initiate liquefaction manifestations at
the ground surface and surface subsidence (0.1–0.2 g) have esti-
Following the last deglaciation, marine transgression reached mated return periods of 40 to 170 years for Christchurch shallow
its furthest inland extent ~10 km west of the modern coastline soil sites (Stirling et al., 2008). A local earthquake (MW 4.7–4.9) in
ca. 6.5 ka (Brown and Weeber, 1992). Since 6.5 ka, the coastline in 1869 caused pervasive damage in parts of Christchurch consistent
the vicinity of central Christchurch has prograded through with PGA ≥0.2 g shaking (Downes and Yetton, 2012) and may
episodic coastal and alluvial deposition, fed by sediments from have caused surface subsidence; it was reported after the earth-
the continental shelf and Waimakariri River. Basher et al. (1988) quake that “the tide runs higher up the Heathcote River than
give a comprehensive geomorphologic overview of Waimakariri formerly” (Weekly News, 26 June 1869).
alluvial fan evolution over the Holocene. The lower floodplain
comprises a set of nested alluvial fans with each fanhead lower Subsidence in the Christchurch region has been counteracted
and farther downstream than the previous one. Fan-building over geological time scales by sediment delivery from Waimakariri

Figure 1. Geologic and seismic context of Christchurch through the 2010–2011 Canterbury Earthquake Sequence (CES). Shown are inferred causative fault planes GSA TODAY | www.geosociety.org/gsatoday/
and associated largest moment magnitudes (MW) for events on 4 Sept. 2010, 22 Feb. 2011, 13 June 2011, and 23 Dec. 2011. Also shown are MW 4–6 epicenters in the
months following each major event. The Greendale Fault surface rupture coincident with the 4 Sept. 2010 events is shown, after Quigley et al. (2012). Also shown
are the Waimakariri River with adjacent stopbanks and former channel locations evidenced by alluvial gravels, Avon River (AR), Heathcote River (HR), and
Avon-Heathcote Estuary (AHE). The blue dashed line is the 6.5-ka maximum inland extent of postglacial marine transgression, after Brown and Weeber (1992).
Inset map shows location of the study region in New Zealand, the Alpine Fault (AF), and wider tectonic boundary (red lines) between the Indo-Australian plate
(IAP) and Pacific plate (PP).

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