22 September 2012
On the western side of South Island in New Zealand the Southern Alps are bounded my a major active fault, known as the Alpine Fault. In this area the Alpine Fault has been responsible for many earthquakes in the past, and it is thought to be at high risk of a rupture again. The last major rupture was in 1717; typical return periods are in the range of 140 to 350 years. The fault is a right lateral strike-slip fault, although there is some thrusting associated with it as well.
To the south of the town of Franz Josef the fault is exposed in the banks of a small river called Hare Mare. Here, up on the walls of the creek, there is an outcrop of bedrock thrust over young river gravels. In the image below, the bedrock is about 5 m thick. The contact between the two is the fault:
Major faults in mountain chains trigger large numbers of landslides. In addition, the tectonic disturbance and uplift leave the landscape prone to mass movements. Throughout its length the Alpine Fault has large numbers of landslides in close proximity. At Hare Mare these are clearly evident. A view upstream on the hanging wall side (based upon the thrusting evident in the image) the landscape shows multiple landslide scars:
Although not immediately recent given the amount of revegatation, these are two young to date from the last Alpine Fault rupture. The landslides generate a vast amount of sediment – this is one of the smaller recent fans developed from landsliding in small gully systems on the flanks of the creek:
Note Chris Smart for scale. When the next Alpine fault rupture occurs there are likely to be many landslides, which in themselves will be a substantive hazard. However, they will also present a long-term legacy (as is now occurring in Sichuan) that will be very difficult to manage as the rate of sediment production and movement will increase dramatically across the mountain range. In turn this will cause the rivers to aggrade, which will cause huge disruption right along the mountain front for years or possibly decades.