The transpressional boundary between the Australian and Pacific plates
in the central South Island of New Zealand comprises the Alpine Fault
and a broad region of distributed strain concentrated in the Southern
Alps but encompassing regions further to the east, including the
northwest Canterbury Plains. Low to moderate levels of seismicity
(e. g., 2 > M 5 events since 1974 and 2 > M 4.0 in 2009) and Holocene
sediments offset or disrupted along rare exposed active fault segments
are evidence for ongoing tectonism in the northwest plains, the surface
topography of which is remarkably flat and even. Because the geology
underlying the late Quaternary alluvial fan deposits that carpet
most of the plains is not established, the detailed tectonic evolution
of this region and the potential for larger earthquakes is only poorly
understood. To address these issues, we have processed and interpreted
high-resolution (2.5 m subsurface sampling interval) seismic data
acquired along lines strategically located relative to extensive
rock exposures to the north, west, and southwest and rare exposures
to the east. Geological information provided by these rock exposures
offer important constraints on the interpretation of the seismic
data. The processed seismic reflection sections image a variably
thick layer of generally undisturbed younger (i.e., < 24 ka) Quaternary
alluvial sediments unconformably overlying an older (> 59 ka) Quaternary
sedimentary sequence that shows evidence of moderate faulting and
folding during and subsequent to deposition. These Quaternary units
are in unconformable contact with Late Cretaceous-Tertiary interbedded
sedimentary and volcanic rocks that are highly faulted, folded, and
tilted. The lowest imaged unit is largely reflection-free Permian
Triassic basement rocks. Quaternary-age deformation has affected
all the rocks underlying the younger alluvial sediments, and there
is evidence for ongoing deformation. Eight primary and numerous secondary
faults as well as a major anticlinal fold are revealed on the seismic
sections. Folded sedimentary and volcanic units are observed in the
hanging walls and footwalls of most faults. Five of the primary faults
represent plausible extensions of mapped faults, three of which are
active. The major anticlinal fold is the probable continuation of
known active structure. A magnitude 7.1 earthquake occurred on 4
September 2010 near the southeastern edge of our study area. This
predominantly right-lateral strike-slip event and numerous aftershocks
(ten with magnitudes >= 5 within one week of the main event) highlight
the primary message of our paper: that the generally flat and topographically
featureless Canterbury Plains is underlain by a network of active
faults that have the potential to generate significant earthquakes.