[1] 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 PermianTriassic 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.

S U M M A R YThe possibility that parts of the Earth's continental lower crust can be described with stochastic geological models has been suggested for some time. Recent studies of deeper well logs also indicate a possible stochastic structure at mid-crustal levels. This motivates a closer examination of the relation between the statistics of reflection wavefields and that of the lower crust. Such a relation can put important constraints on possible lower crustal models. This study follows up earlier efforts to quantify the statistics of both stochastic lower crustal models and the reflected wavefield. Since modelling of the seismic response of stochastic (von Karman) fields implies the usage of the impedance contrast field of the latter, we wish to compare the second-order statistics of both types of fields (velocity and impedance). This study concludes that the vertical derivative operator on a von Karman velocity field, implicitly present in the impedance contrast field, alters the second-order horizontal von Karman statistics of the velocity fields in a profound way. Wavefield effects, undoubtedly present in observed seismic data, which have earlier been proposed as possible causes for the aforementioned change, seem to play a secondary role. The vertical derivative operation, inherent in the impedance contrast field, reduces the estimated horizontal scale length and Hurst number by a factor of 2-22 and 1-3, respectively. Original vertical scale length and Hurst number of the velocity fields have a (quasi-)linear influence on this underestimation. Horizontal scale lengths and Hurst numbers were also estimated from the seismic response (Primary Reflectivity Section) of the von Karman fields. The values obtained are close to those obtained from the causative impedance contrast fields, and are similarly underestimated. This suggests a dominant role for the vertical derivative operator in the underestimation of horizontal scale length and Hurst number. This attempt to quantify the relation between the horizontal spatial statistics of von Karman fields and the estimates derived from their seismic response, may be useful in upscaling the latter.

Combining S‐wave data, resulting from surface‐wave dispersion analysis with P‐wave tomographic data, is a valuable tool to improve the understanding of near‐surface soil properties and allows the estimation of soil mechanical parameters and the determination of the depth of the water table. To achieve this combination of methods in a complex fault zone setting, active‐source seismic data were acquired at Inchbonnie, New Zealand across the Alpine Fault. This is a major transpressional strike‐slip fault that has generated magnitude > 7.8 earthquakes in the past. In this study, we focus on the surface‐wave component of these data, to determine elastic parameters for the shallow (~60 m) subsurface as well as the depth of the water table. We achieve this by combining S‐wave velocity models from surface‐wave dispersion curve inversion and P‐wave velocity models obtained from traveltime tomographic inversion in a previous study. The surface‐wave dispersion curve inversion is done by means of a laterally constrained inversion algorithm. As a result, we are able to obtain elastic parameters and map the water table and the geology around the Alpine Fault at Inchbonnie, New Zealand. The Alpine Fault itself appears as a relatively sharp lateral discontinuity in all investigated parameters.

[1] High-resolution GPS and ground-penetrating radar (GPR) data are used to detect and identify hidden faults along a stretch of the transpressional Alpine Fault (South Island, New Zealand) immediately north of its junction with the Hope Fault. At this location, the Alpine Fault emerges from the basement into a sequence of variably thick late Holocene gravel deposits. Geomorphology and trenching already mapped three principal fault strands and two distinct step over zones at the study site. Our GPR images reveal numerous additional secondary fault strands throughout the region, only some of which are obvious at the surface or in the trench walls. According to the GPR data, the main fault-generated disturbance zone has a width ranging from $40 to $200 m. The secondary fault strands outside of the step over zones likely represent the branches of positive flower structures, whereas the faulting pattern around the step over zones is best explained in terms of linked Riedel shears. Systematic northeastward increases in the width of the main fault-generated disturbance zone and corresponding increases in principal fault-scarp height are the likely consequences of older terraces in the northeast being disrupted and offset by more earthquakes than younger terraces in the southwest. The pattern of complex faulting in this region is distinct from the system of alternating strike-slip and reverse faults characteristic of the Alpine Fault to the south and from the rather simple sequence of faults mapped to the north. GPR surveying has added new information on the distribution and nature of faulting at our study site.

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