Recent deformation of Quaternary sediments as inferred from GPR images and
shallow P-wave velocity tomograms: Northwest Canterbury Plains, New Zealand
S.F.A. Carpentier
a,
⁎
, A.G. Green
a
, J. Doetsch
a
, C. Dorn
a,1
, A.E. Kaiser
a
, F. Campbell
a
,
H. Horstmeyer
a
, M. Finnemore
b
a
ETH Zürich, Institute of Geophysics, Zürich, Switzerland
b
Southern Geophysical Ltd, 28/3 Tanya Street, Bromley, Christchurch, New Zealand
abstractarticle info
Article history:
Received 15 November 2010
Accepted 9 September 2011
Available online 19 September 2011
Keywords:
GPR
Seismic
Tomography
Palaeoseismicity
Faults
Folds
New Zealand
Prior to the recent highly damaging M 7.1 earthquake near the city of Christchurch on the South Island of
New Zealand, we recorded coincident high-resolution seismic and ground-penetrating radar (GPR) data
across parts of the northwest Canterbury Plains. The seismic reflection images reveal a vast network of inter-
connected faults and folds below a seemingly undisturbed flat surface. To complement the seismic images,
which only provide limited information on the very shallow subsurface (i.e., b20 m), we have now processed
and analysed the GPR data. The migrated GPR images are dominated by complex reflection patterns charac-
teristic of glaciofluvial sediments. Such sediments eroded from the Southern Alps are observed at the surface
throughout our study site. Although it is difficult to distinguish between complexities associated with com-
plicated sedimentation processes and disruptions and offsets of GPR reflections associated with recent move-
ments on faults and folds, we identify a number of regions where the GPR data are consistent with tectonic
deformation of Holocene sediments. Two of these regions straddle an interpolated connection between active
faults mapped at the surface. In a third region, the development of river terraces imaged in the GPR data may
have been affected by slip on newly discovered underlying faults. The most significant near-surface deforma-
tion, which is apparent on a coincident seismic reflection image, P-wave tomogram and GPR image, is ob-
served on the flank of a major anticline that appears to have been thrust close to the surface along a
reverse fault. Some of the faults and folds resolved in our seismic and GPR data may have been reactivated
during the recent period of intense seismicity.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Buried or blind faults that have the potential to rupture as large earth-
quakes make the construction of reliable seismic hazard and risk maps ex-
tremely difficult in numerous regions worldwide (Ambraseys, 1981;
Calais et al., 2010; Dolan et al., 2003; Feng et al., 2010; Guzofski et al.,
2007; Jackson, 2006; Leon et al., 2009; Lettis et al., 1997; MonaLisa et al.,
2007; Shyu et al., 2005; Talebian et al., 2004). Since ruptures on such
faults may cause dislocation or folding of the overlying strata, their pres-
ence at some locations may be deduced from detailed geomorphological
studies and/or paleoseismological and geophysical investigations of the
shallow sedimentary units. In this contribution, we image faults and
folds using a combination of high-resolution seismic reflection and
ground-penetrating (GPR) profiling.
The geology between the Pacific Ocean and the Southern Alps of
New Zealand (Fig. 1) is largely hidden beneath a variably thick veneer
of relatively young (b24 ka) Quaternary glaciofluvial sediments that
creates the remarkably flat and even landscape of the Canterbury
Plains (Fig. 2). Extensive exposures in the foothills of the Southern
Alps and the Malvern Hills together with isolated outcrops along
the banks of the Waimakariri River and adjacent uplifted hills suggest
that the geology underlying the northwest Canterbury Plains (Forsyth
et al., 2008) is extremely complex. These exposures and small out-
crops reveal Permian–Triassic Torlesse Group basement rocks over-
lain by Late Cretaceous–Tertiary interbedded sedimentary and
volcanic layers, all of which are highly faulted and folded. Relatively
old (N59 ka) Quaternary glaciofluvial sediments that are less faulted
and folded are observed at a number of locations.
Because several of the exposed faults and folds offset or disrupt Ho-
locene sediments, they have been classified as active (Campbell et al.,
2000; Dorn et al., 2010; Estrada, 2003; Forsyth et al., 2008; May, 2004;
McLennan, 1981). Low levels of seismicity recorded prior to 2010
(e.g., two M 5+ events since 1974 and two M 4+ events in 2009)
appeared to support the hypothesis that this region of the South Island
is tectonically unstable (Dorn et al., 2010). In September and October
2010, a highly damaging M 7.1 earthquake and a large number of after-
shocks (e.g., twelve M 5.0–5.9 and one hundred and thirty three M 4.0–
Journal of Applied Geophysics 81 (2012) 2–15
⁎ Corresponding author at: Institute of Geophysics, Dep. of Earth Sciences, ETH,
Sonneggstr. 5, CH-8092, Zurich, Switzerland. Tel.: +41 44 6332659.
E-mail address: carpentier@aug.ig.erdw.ethz.ch (S.F.A. Carpentier).
1
Now at the Institute of Geophysics, University Lausanne, Lausanne, Switzerland
0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jappgeo.2011.09.007
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