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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, (e.g., sea-level rise, storm surges, tsunamis) and terrestrial hazards
University of Canterbury, Private Bag 4800, Ilam, Christchurch, New (e.g., surface subsidence and compaction, flooding, erosion, sedi-
Zealand; Mark C. Quigley, Dept. of Geological Sciences, University of ment supply changes, groundwater table changes) induced by
Canterbury, Private Bag 4800, Ilam, Christchurch, New Zealand; natural and/or anthropogenic processes (Syvitski et al., 2009;
Sjoerd van Ballegooy, Bruce L. Deam, Tonkin & Taylor Ltd, PO Box Nicholls and Cazenave, 2010). Coastal population growth and
5271, Wellesley Street, Auckland 1141, New Zealand; Brendon A. concentration, economic development, and urbanization are
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; tive sea-level rise (Nicholls and Cazenave, 2010; IPCC, 2014) and
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, the next century, defining one of society’s greatest challenges.
National Institute of Water & Atmospheric Research (NIWA), PO Box Geospatial data, such as satellite-based synthetic aperture radar and
8602, Christchurch, New Zealand airborne light detection and ranging (LiDAR), are increasingly
being used to measure surface subsidence and delineate areas prone
ABSTRACT to flood and sea-level rise hazards (Dixon et al., 2006; Wang et al.,
2012; Webster et al., 2006), thereby assisting land-use planning
Airborne light detection and ranging (LiDAR) data were and management decisions (Brock and Purkis, 2009).
acquired over the coastal city of Christchurch, New Zealand, prior
to and throughout the 2010 to 2011 Canterbury Earthquake Great (MW ≥ 8.5) earthquakes on subduction zones may cause
Sequence. Differencing of pre- and post-earthquake LiDAR data abrupt and dramatic elevation changes to coastal environments.
reveals land surface and waterway deformation due to seismic The 1964 MW 9.0 Alaska earthquake caused tidal marshes and
shaking and tectonic displacements above blind faults. Shaking wetlands to subside up to 2 m (Shennan and Hamilton, 2006); the
caused floodplain subsidence in excess of 0.5 to 1 m along tidal 2005 MW 8.7 Nias earthquake caused up to 3 m in coastal uplift
stretches of the two main urban rivers, greatly enhancing the proximal to the trench and 1 m of more distal coastal subsidence
spatial extent and severity of inundation hazards posed by (Briggs et al., 2006); and the 2011 MW 9.0 Tohoku earthquake
100-year floods, storm surges, and sea-level rise. Additional caused subsidence up to 1.2 m along the Pacific Coast of north-
shaking effects included river channel narrowing and shallowing, eastern Japan (Geospatial Information Authority of Japan, 2011,
due primarily to liquefaction, and lateral spreading and sedimen- cited in IPCC, 2014). However, the influence of moderate magni-
tation, which further increased flood hazard. Differential tectonic tude (i.e., MW 6–7) earthquakes, which can occur in both inter-
movement and associated narrowing of downstream river chan- plate and intraplate settings, on coastal flood and sea-level
nels decreased channel gradients and volumetric capacities and hazards is not well characterized and not typically included in
increased upstream flood hazards. Flood mitigation along the studies that assess the future vulnerability of coastal populations
large regional Waimakariri River north of Christchurch may have, (McGranahan et al., 2007).
paradoxically, increased the long-term flood hazard in the city by
halting long-term aggradation of the alluvial plain upon which In this paper, we summarize differential vertical and horizontal
Christchurch is situated. Our findings highlight the potential for ground movements in Christchurch, New Zealand, using airborne
moderate magnitude (MW 6–7) earthquakes to cause major topo- LiDAR survey data captured prior to, during, and after the 2010 to
graphic changes that influence flood hazard in coastal settings. 2011 Canterbury Earthquake Sequence (CES). Differential LiDAR
applications in earthquake studies have been used to map defor-
INTRODUCTION mation along fault zones (e.g., Duffy et al., 2013; Oskin et al.,
2012); however, this is the first differential LiDAR study showing
Approximately 10% of the world’s population inhabits low- the cumulative surface effects of earthquake shaking and faulting
lying (≤10 m above sea level) coastal areas, and most of this popu- on an urban environment. Here we show that earthquakes
lation is contained within densely populated urban centers sourced from blind and/or previously unrecognized faults, in
(McGranahan et al., 2007). Cities constructed on low-lying coastal addition to those from known seismic sources, have the ability to
and river plains are highly vulnerable to ocean-sourced hazards create profound landscape changes that impact current and future
flood hazards associated with urban rivers and relative sea-level
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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