Geophysical characterisation of the Alpine Fault within the Haast Valley

Technical Abstract

The subsurface geometry of the Alpine Fault, the major boundary between the Pacific and Australian Plates on the South Island of New Zealand is enigmatic due to the scarce amount of information that we have from the subsurface. This is particularly so in the southern section of the Alpine Fault, near Haast, where we know the fault is capable of very large earthquakes – but have never seen one happen. Geological and geophysical techniques can be used to improve our knowledge of such a fault and how it will rupture in the future. Two seismic lines were acquired in regions of relatively easy access along the Haast and Turnbull Rivers in late January 2009. These lines were acquired by detonating small explosions in holes that were ~1.2 m deep and then recording the resulting echoes using sensitive microphones deployed on the surface. The resulting images are analogous to ultrasound images used in medicine except that the scales in question are 100’s of metres rather than centimeters. Interpretations of the seismic sections identify the base of the ancient glacially-eroded valley and coastal plain that have now been filled in with as much as 800 m of sediments carried down the valley by the glaciers and rivers.

The data also identify sedimentary rock layers (strata) that are broken by the Alpine Fault in the subsurface. Even though there are no reflections from the Alpine Fault itself, the offset strata enable the positioning of the fault at depth. This suggests that the fault is steeply dipping to the southeast. Quantitative measurements of the offset in strata and the erosional base of the old valley can also be used to estimate the uplift rate on the fault. This is seen to agree with earlier work, which set the rate at 5.9 mm per year. Interpretations would be improved by the addition of other seismic lines to better constrain the 3D geometry of the ancient glacial valley in a location where strike-slip motion of 23.1±0.8 mm per year on the Alpine Fault rapidly changes the downstream shape of the valley.