The January 2013 Mt Haast rock avalanche and Ball Ridge rock fall in Aoraki/Mt Cook National Park, New Zealand

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Hancox, G.T.; Thomson, R. 2013 The January 2013 Mt Haast rock avalanche and Ball Ridge rock fall in Aoraki/Mt Cook National Park, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/33 iv, 26 p.

Abstract: Two significant landslides occurred in the Aoraki/Mt Cook National Park of New Zealand's Southern Alps in January 2013. The first was a large rock fall on Ball Ridge which occurred during the 9-10 January rainstorm. This was followed on 21 January by much larger rock avalanche from a nearby peak 5 km northeast of Mt Cook. The rock avalanche from the ridge between Mt Haast and Mt Dixon at 2:09 pm on 21 January 2013 originated in very weak rock, associated with a fault zone developed sub-parallel to bedding in the greywacke bedrock. The avalanche was initiated by a collapse of the ridge crest at an elevation of about 3040 m with an estimated volume of up to ~1 million m3. The falling rock debris entrained snow and ice as it flowed down the neve south of Mt Haast with an average velocity of up to 160 km per hour. Although the rock avalanche occurred during fine weather without an obvious trigger, heavy rainfall (>900 mm) during two rainstorms in early January is believed to have strongly influenced the time of the failure. The rock avalanche ran out 2.9 km on to the Grand Plateau and came to rest about 200 m west of Plateau Hut near the top of the Hochstetter Icefall at an elevation of ~2100 m. The avalanche debris (boulders, gravel, and sand up to 5 m thick) covers an area of about 0.8 km2 on the Grand Plateau, with an estimated total volume of ~2 million m3, of which about half is snow and ice. The shaking caused by the rock avalanche was recorded on several GeoNet seismographs at distances of 29¨C139 km from the source, and is calculated to have had an average local magnitude of ML 2.25. This recent collapse appears to have reduced the risk of future large failures from the ridge between Mt Haast and Mt Dixon because of the reduced source area. Repeat failures of similar size to the recent collapse are unlikely, but minor retrogressive rock falls should be expected in the future. There are several sites where small rock falls could occur in the future, such as the overhanging headscarp on the western side, and the areas of loose debris on the upper failure zone. The annual risk (loss of life) at Plateau Hut from future rock avalanches from the source area on Mt Haast is estimated to be very low (~10-7). The ~150,000 m3 rock fall on the popular Ball Ridge tramping track to Ball Pass occurred in an area of weak, argillite-dominated rock during the 9-10 January 2013 rainstorm when ~383 mm rain was recorded at Mt Cook. Although the rock fall was triggered by rainfall, the slope was made more susceptible to failure by erosion and glacial retreat over the last 100 years. The rock fall area on the northwest side of the ridge is expected to increase in size over time, but is not expected to prevent use of the Ball Ridge track if further large collapses occur, although the route may become more difficult to use. The Ball Ridge rock fall and Mt Haast rock avalanche in January 2013 are the most recent large rock slope failures in the Southern Alps, providing further evidence of increased slope instability in the last 50 years. Given the marked climatic and physiographic changes that have occurred, large slope failures in alpine areas are expected to continue in the future, particularly during rainstorms and strong earthquake shaking (¡ÝMM8). The next Alpine Fault earthquake is expected to trigger numerous large rock falls, rock avalanches, and other types of landslides in the Southern Alps. (auth)