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The sinking city: Earthquakes increase flood hazard in
Christchurch, New Zealand
GSA TODAY | MARCH/APRIL 2015 Matthew W. Hughes, Dept. of Civil & Natural Resources Engineering,
University of Canterbury, Private Bag 4800, Ilam, Christchurch, New
Zealand; Mark C. Quigley, Dept. of Geological Sciences, University of
Canterbury, Private Bag 4800, Ilam, Christchurch, New Zealand;
Sjoerd van Ballegooy, Bruce L. Deam, Tonkin & Taylor Ltd, PO Box
5271, Wellesley Street, Auckland 1141, New Zealand; Brendon A.
Bradley, Dept. of Civil & Natural Resources Engineering, University of expected to greatly increase exposure and loss to the impacts of rela-
Canterbury, Private Bag 4800, Ilam, Christchurch, New Zealand;
Deirdre E. Hart, Dept. of Geography, University of Canterbury, Private coastal flooding (Hanson et al., 2011; Hallegatte et al., 2013) through
Bag 4800, Ilam, Christchurch, New Zealand; and Richard Measures,
National Institute of Water & Atmospheric Research (NIWA), PO Box
8602, Christchurch, New Zealand
ABSTRACT
Airborne light detection and ranging (LiDAR) data were
acquired over the coastal city of Christchurch, New Zealand, prior
to and throughout the 2010 to 2011 Canterbury Earthquake
Sequence. Differencing of pre- and post-earthquake LiDAR data
reveals land surface and waterway deformation due to seismic
shaking and tectonic displacements above blind faults. Shaking
caused floodplain subsidence in excess of 0.5 to 1 m along tidal
stretches of the two main urban rivers, greatly enhancing the
spatial extent and severity of inundation hazards posed by
100-year floods, storm surges, and sea-level rise. Additional
shaking effects included river channel narrowing and shallowing,
due primarily to liquefaction, and lateral spreading and sedimen-
tation, which further increased flood hazard. Differential tectonic
movement and associated narrowing of downstream river chan-
nels decreased channel gradients and volumetric capacities and
increased upstream flood hazards. Flood mitigation along the
large regional Waimakariri River north of Christchurch may have,
paradoxically, increased the long-term flood hazard in the city by
halting long-term aggradation of the alluvial plain upon which
Christchurch is situated. Our findings highlight the potential for
moderate magnitude (MW 6–7) earthquakes to cause major topo-
graphic changes that influence flood hazard in coastal settings.
INTRODUCTION
Approximately 10% of the world’s population inhabits low-
lying (≤10 m above sea level) coastal areas, and most of this popu-
lation is contained within densely populated urban centers
(McGranahan et al., 2007). Cities constructed on low-lying coastal
and river plains are highly vulnerable to ocean-sourced hazards
GSA Today, v. 25, no. 3–4, doi: 10.1130/GSATG221A.1.
E-mails: Hughes: matthew.hughes@canterbury.ac.nz; Quigley: mark.quigley@canterbury.ac.nz; Ballegooy: svanballegooy@tonklin.co.nz; Deam: BDeam@tonklin
.co.nz; Bradley: brendon.bradley@canterbury.ac.nz; Hart: deirdre.hart@canterbury.ac.nz; Measures: richard.measures@niwa.co.nz.
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(e.g., sea-level rise, storm surges, tsunamis) and terrestrial hazards
(e.g., surface subsidence and compaction, flooding, erosion, sedi-
ment supply changes, groundwater table changes) induced by
natural and/or anthropogenic processes (Syvitski et al., 2009;
Nicholls and Cazenave, 2010). Coastal population growth and
concentration, economic development, and urbanization are
tive sea-level rise (Nicholls and Cazenave, 2010; IPCC, 2014) and
the next century, defining one of society’s greatest challenges.
Geospatial data, such as satellite-based synthetic aperture radar and
airborne light detection and ranging (LiDAR), are increasingly
being used to measure surface subsidence and delineate areas prone
to flood and sea-level rise hazards (Dixon et al., 2006; Wang et al.,
2012; Webster et al., 2006), thereby assisting land-use planning
and management decisions (Brock and Purkis, 2009).
Great (MW ≥ 8.5) earthquakes on subduction zones may cause
abrupt and dramatic elevation changes to coastal environments.
The 1964 MW 9.0 Alaska earthquake caused tidal marshes and
wetlands to subside up to 2 m (Shennan and Hamilton, 2006); the
2005 MW 8.7 Nias earthquake caused up to 3 m in coastal uplift
proximal to the trench and 1 m of more distal coastal subsidence
(Briggs et al., 2006); and the 2011 MW 9.0 Tohoku earthquake
caused subsidence up to 1.2 m along the Pacific Coast of north-
eastern Japan (Geospatial Information Authority of Japan, 2011,
cited in IPCC, 2014). However, the influence of moderate magni-
tude (i.e., MW 6–7) earthquakes, which can occur in both inter-
plate and intraplate settings, on coastal flood and sea-level
hazards is not well characterized and not typically included in
studies that assess the future vulnerability of coastal populations
(McGranahan et al., 2007).
In this paper, we summarize differential vertical and horizontal
ground movements in Christchurch, New Zealand, using airborne
LiDAR survey data captured prior to, during, and after the 2010 to
2011 Canterbury Earthquake Sequence (CES). Differential LiDAR
applications in earthquake studies have been used to map defor-
mation along fault zones (e.g., Duffy et al., 2013; Oskin et al.,
2012); however, this is the first differential LiDAR study showing
the cumulative surface effects of earthquake shaking and faulting
on an urban environment. Here we show that earthquakes
sourced from blind and/or previously unrecognized faults, in
addition to those from known seismic sources, have the ability to
create profound landscape changes that impact current and future
flood hazards associated with urban rivers and relative sea-level