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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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