Page 6 - gt1503-04
P. 6

GSA TODAY | MARCH/APRIL 2015  avulsions. This natural sediment replenishment entails the rapid            In 2003, the Christchurch City Council commissioned an aerial
                              advance of coarse alluvium along relict and newly excavated chan-         LiDAR survey for hydrological modeling purposes. Following the
                              nels, driven by high river flows and accompanied by extensive             4 Sept. 2010 Darfield earthquake, another LiDAR survey was
                              flooding. Such avulsions pose a severe physical threat to the built       commissioned and flown on 5 Sept. 2010 by the New Zealand
                              environment. Extensive flood protection works, including gravel           Ministry of Civil Defense and Emergency Management to quantify
                              extraction, were first established in 1928, with three subsequent         property subsidence and to facilitate insurance assessments and
                              flood events breaching the primary stopbank (levee) system,               reconstruction work. Further LiDAR campaigns were flown typi-
                              resulting in floodplain inundation. Throughout the majority of            cally one month after each subsequent major CES earthquake to
                              European settlement, the city has been spared from major floods           provide time for ejected sand and silt to be removed from most
                              from the Waimakariri, although stopbank failure remains a hazard.         properties and streets, so that measurements recorded ground
                              Christchurch has also long been vulnerable to localized flooding          surface level. LiDAR capture equipment had a horizontal accuracy
                              from its urban rivers, exacerbated by low-lying, relatively flat terrain  of 0.44 to 0.55 m, with a vertical accuracy of ±0.15 m for the 2003
                              with low gradients and high groundwater levels, extreme tides, and        survey and ±0.07 m for the post-earthquake surveys. These errors
                              storm surge. Urban expansion since the 1880s imparted distinct            exclude Global Positioning System network error and approxima-
                              anthropogenic signatures on local hydrology. Widespread drainage          tions within the New Zealand Quasigeoid 2009 reference surface,
                              works undertaken for urban development caused ground surface              which has an expected vertical accuracy of ±0.07 m. From each
                              subsidence due to reduction of the groundwater levels, leading to         LiDAR dataset a bare-earth 5-m-resolution Digital Elevation
                              historical surface flooding and ponding in low-lying areas. In            Model (DEM) was generated; the 5-m-resolution was determined
                              parallel, separate underground storm water and waste water systems        to be optimal for interpolation of pre- and post-earthquake
                              were established, with the latter long recognized as being “leaky”        LiDAR ground returns in the urban environment. The accuracy
                              —that is, allowing infiltration into pipes with associated draining of    of LiDAR data and bare-earth DEMs were assessed against refer-
                              groundwater and suppression of local water tables (Wilson, 1989).         ence geodetic survey control benchmarks and topographic surveys
                              The storm water system, originally integrating open channels and          conducted pre-CES on roads and subdivisions at suburb-level in
                              buried pipes and then incorporating roadside gutters, was developed       August 2011 and on residential properties in January 2012. These
                              to manage overland flow runoff exacerbated by expansion of imper-         assessments showed reasonable accuracy as a whole, with hard
                              meable surfaces through suburban development.                             surfaces providing smaller standard deviations of errors for roads
                                                                                                        than for residential properties, reflecting the differing roughness
                              THE CES AND URBAN LANDSCAPE EVOLUTION                                     of the two types of terrain. Here we show total vertical elevation
                                                                                                        changes (∆ETot), elevation changes due to liquefaction (∆ELiq),
                                Between September 2010 and December 2011, Christchurch was              lateral ground movements due to liquefaction (∆XLiq), and vertical
                              damaged by six earthquakes: 4 Sept. 2010 (MW = 7.1); 22 Feb. 2011         tectonic changes (∆ETec) (Fig. 2). Tectonic movements were deter-
                              (MW = 6.2, 185 fatalities); 13 June 2011 (two earthquakes:                mined using satellite interferometry synthetic aperture radar data
                              MW = 5.3 at 1 p.m. and MW = 6.0 at 2:20 p.m.) and 23 Dec. 2011            (see Beavan et al., 2011, 2012b), which we subtracted from ∆ETot as
                              (two earthquakes: MW = 5.8 at 1:58 p.m. and MW = 5.9 at 3:18 p.m.)        determined by LiDAR-derived DEMs to produce ∆ELiq.
                              (Fig. 1; for detailed reviews of the geologic and seismic aspects of
                              the CES, see Beavan et al., 2010, 2011, 2012a, 2012b; Duffy et al.,         We also present pre-/post-earthquake differential elevation anal-
                              2013; Quigley et al., 2012; Bradley et al., 2014). The close proximity    ysis (∆ETot) for the Avon-Heathcote Estuary, based on 1-m-resolution
                              of causative faults to Christchurch generated strong ground motions       DEMs interpolated from LiDAR data (area of bed exposed above
                              (Bradley and Cubrinovski, 2011; Bradley, 2012) that caused exten-         water surface during survey), supplemented by ground survey and
                              sive damage to residential and commercial properties (Bech et al.,        depth-sounder survey data for areas covered by estuarine waters
                              2014; Fleischman et al., 2014; Moon et al., 2014) and infrastruc-         during LiDAR surveys (Measures et al., 2011; Measures and Bind,
                              ture lifelines, particularly potable water, waste water, and road         2013). Pre-/post-earthquake ground surveys and echo-sounder
                              networks (Cubrinovski, et al., 2014a, 2014b, 2014c; O’Rourke et           surveys were conducted using Real-Time Kinetic Global Navigation
                              al., 2014). Much of the damage to the city’s built environment was        Satellite System positioning, on foot or with a boat-mounted depth
                              caused by widespread soil liquefaction that occurred predomi-             sounder, and calibrated to local benchmarks.
                              nantly in saturated, unconsolidated alluvial and marine fine sedi-
                              ments in east Christchurch, in the region of late Holocene coastal          The 4 September 2010 Darfield earthquake caused 74% of
                              progradation. In susceptible soils with high water tables (e.g.,          central and eastern Christchurch to subside; 60% of this area
                              suburbs adjacent to the Avon River), liquefaction was manifested          subsided up to 0.2 m (Fig. 2A). Vertical tectonic displacements
                              at the ground surface in earthquakes as low as MW 5.0 and PGAs            of 0.8 to 1.8 m along the associated surface rupture ~50 km west
                              as low as 0.08 g (Quigley et al., 2013). Less-susceptible soils           of Christchurch caused partial river avulsion and flooding
                              required higher shaking intensities for liquefaction initiation           (Duffy et al., 2013). The 22 February 2011 Christchurch earth-
                              (Tonkin & Taylor, 2013; van Ballegooy et al., 2014b). Liquefaction        quake caused 83% of eastern and central Christchurch to
                              caused significant ground deformations, ejection of groundwater           subside further; 78% subsided up to 0.3 m, with localized areas
                              and sediments on to the ground surface, and lateral spread around         exceeding 1 m. This event also caused a clear signature of
                              rivers (Cubrinovski et al., 2014c; Quigley et al., 2013; Green et al.,    tectonic uplift (~0.45 m) around the Avon-Heathcote Estuary
                              2014; van Ballegooy et al., 2014b). In some areas, loadings from          caused by blind faults (Fig. 2A and 2E). Compared to pre-
                              structures and preferential ejecta pathways through roads and             earthquake elevations, 86% of central and eastern Christchurch
                              buried infrastructure imparted distinct anthropogenic signatures          subsided through the CES; 10% subsided more than 0.5 m, with
                              on surface ejecta patterns.                                               some localized locations exceeding 1 m. Cumulative tectonic
                                                                                                        subsidence through the CES reached 0.18 m (Fig. 2E). Both

6
   1   2   3   4   5   6   7   8   9   10   11