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