Report on the landslide that blocked SH1 and the railway line near Rosy Morn Stream south of Kaikoura on 10 September 2010

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Hancox, G.T. 2010 Report on the landslide that blocked SH1 and the railway line near Rosy Morn Stream south of Kaikoura on 10 September 2010 Lower Hutt, N.Z.: GNS Science. GNS Science report 2010/59 33 p.

Abstract: This report describes and evaluates the cause of the landslide which blocked SH 1 and the railway line near Rosy Morn Stream 12 km south of Kaikoura on 10 September 2010. The landslide was a moderately large (about 50,000 m3) debris and rock slide, and occurred on the toe of a large prehistoric landslide on the steep (45-50°) coastal slope. This slope is formed of indurated (hard), interbedded, well jointed greywacke sandstone and mudstone (argillite) bedrock, and is overlain by thin colluvial deposits. The 2010 landslide was a significant event justifying a response and report under the GeoNet Project of GSN Science, mainly because of the disruption it caused to transport by closure of an important road and rail link to Christchurch at a time when supplies were urgently needed there after the Darfield earthquake. Normal road and rail access was blocked for one week while the debris was cleared away.The results of this study indicate that the landslide cannot be attributed to any one triggering event, but instead it appears to have been caused by a combination of factors. These factors include unfavourable geological conditions, shaking caused by the Darfield earthquake mainshock and the long sequence of aftershocks, greater than average rainfall over the preceding four months, and low to moderate rainfall over three days before the failure, all combined to cause the landslide on 10 September. Because of the timing of the aftershocks, rock falls at the site, and rainfall in the area, which stopped about 12 hours before the main failure, the cumulative effect of the mainshock and aftershocks is seen as a particularly important factor in the initiation of the landslide and in bringing the slope to failure. (auth/DG)