The New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3-D seismic data from the DFDP-2 drill site in Whataroa to constrain valley structures that were obscured in previous 2-D seismic data. The new data consist of a 3-D extended vertical seismic profiling (VSP) survey using three-component and fiber optic receivers in the DFDP-2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3-D P wave velocity model by first-arrival traveltime tomography. We identify a 100-460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3-D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3-D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.Viewed on a regional scale, the central Alpine Fault appears as a straight boundary (e.g., Norris & Cooper, 2001;Sutherland et al., 2006). Crustal-scale seismic reflection data show a single oblique fault striking northeastward and dipping 40−60 • to the southeast at depths of 15-30 km (e.g., Davey et al.

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<p>The large variety of subglacial landforms observed on Earth are due to a complex interplay between the overlying ice sheet and the solid Earth below. While the ice cover thermally isolates the subglacial region, hence shields it from any influence by variations in the atmosphere, spatially varying geothermal heat fluxes from below may lead to the formation or reinforcement of existing subglacial landform patterns, such as tunnel valleys. An observed spatial correlation between tunnel valleys and underlying salt structures in the North German Basin is often explained mechanically. In this work, we alternatively focus on the role of heat transfer for the formation of tunnel valleys, which has not been holistically investigated until now. As salt has a higher thermal conductivity than the surrounding rocks, a local concentration of geothermal energy above salt structures may lead to increased subglacial melting rates of the overlying ice sheet. In particular, it is our goal to investigate to which extent the resulting meltwater discharge and corresponding erosion has the potential to reinforce tunnel valley formation. For our analysis, we develop a coupled computational strategy capable of determining the interplay between the temperature distribution within the heterogeneous subsurface including heat transport and ground water flow, and the overlying ice sheet. Modelling the interfacial heat flux from the subsurface into the ice sheet then allows us to infer on subglacial melt rates, which can be further assessed with respect to their role in the formation of tunnel valleys. In this contribution, we present results of a scaling analysis that takes into account the ice sheet with its internal horizontal and vertical velocity fields, the subsurface and the subglacial interfacial area. We furthermore describe a 1D computational strategy to combine the heat transport including subglacial phase change into a coupled process model allowing for investigating feedback mechanisms. Finally, we discuss strategies how this can be integrated into a full dimensional computational subsurface model, such as SHEMAT-Suite. Preliminary results for two tunnel valleys overlying salt structures in the German North Sea show that the local concentration of geothermal energy solely basing on heat conduction is only slightly augmented. The role of hydrothermal flow processes still remains to be quantified. We can therefore conclude that the geothermal distribution has a complementary effect to mechanical processes together leading to the formation of tunnel valleys.</p>

<p>The Alpine Fault along the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth.  </p><p>Here we present results from a 3D seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole penetrated almost down to the fault surface. Within the glacial valley, we collected 3D seismic data to constrain valley structures that were obscured in previous 2D seismic data. The new data consist of a 3D extended vertical seismic profiling (VSP) survey using three-component receivers and a fibre optic cable in the DFDP-2B borehole as well as a variety of receivers at the surface.</p><p>The data set enables us to derive a reliable 3D P-wave velocity model by first-arrival travel time tomography. We identify a 100-460 m thick sediment layer (average velocity 2200±400 m/s) above the basement (average velocity 4200±500 m/s). Particularly on the western valley side, a region of high velocities steeply rises to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3D structures implied by the velocity model on the upthrown (Pacific Plate) side of the Alpine Fault correlate well with the surface topography and borehole findings.</p><p>A reliable velocity model is not only valuable by itself but it is also required as input for prestack depth migration (PSDM). We performed PSDM with a part of the 3D data set to derive a structural image of the subsurface within the Whataroa Valley. The top of the basement identified in the P-wave velocity model coincides well with reflectors in the migrated images so that we can analyse the geometry of the basement in detail.</p>