Hancox, G.T.; Ries, W.F.; Parker, R.N.; Rosser, B. 2016 Landslides caused by the MS 7.8 Murchison earthquake of 17 June 1929 in northwest South Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/42. vii, 131 p. + 4 maps.
Abstract: This report describes a new database of landslides triggered by the MS 7.8 Murchison earthquake of 17 June 1929 in northwest South Island, New Zealand, which was compiled using 1968 aerial photos, historical photos, and 2005-2015 Google Earth images. The mapped earthquake-induced landslides were validated using historical ground and aerial photos, and were compared to records of rainfall-induced landslides in the region, geotechnically assessed, and analysed in ArcGIS. The dataset comprises 5989 landslides larger than ~2500 m2 spread across an area of 16,080 km2, with an average density of 37 landslides per 100 km2. The main area of intense landsliding (highest frequency of failures) extends over of 5150 km2 in which 5356 landslides occurred with an average density of 104/100 km2. Landslides smaller than ~2500 m2 could not be identified using the 1968 aerial photos. The main area of landsliding correlates well with the zone of aftershocks and the seismogenic fault plane, which extends 30 km south and 90 km north of the epicentre, and along the White Creek Fault on which the earthquake occurred. Environmental criteria (size and density of landslides) were used to revise the position and extent of the MM9 and MM10 isoseismals in unpopulated mountain areas where most of the landslides occurred. The MM10 zone covers an area of ~1900 km2 within which 3379 landslides were mapped with an average density of 178/100 km2. Analysis of the database showed that the landslide distribution was controlled mainly by shaking intensity, slope angle (gradient), and geology. Most of the landslides (39%) occurred in Devonian and Cretaceous granites; 36% in Tertiary limestone, sandstone, mudstone, and coal measures; and 23% in Paleozoic greywacke, and argillite. Few landslides were mapped in Quaternary alluvium, possibly because the scars had revegetated when the aerial photos were taken in 1968. The highest density of landslides occurred in Tertiary rocks with an average of 60/100 km2 and up to 340/100 km2 in limestone and sandstone scarps of the Matiri Range. A number of very large (≥1 Mm3) landslides occurred on 20-30° dip-slopes in Tertiary rocks. Landslides in granitic rocks were mainly rock and debris slides and avalanches, and were generally larger and more frequent near the White Creek Fault, particularly on the hanging (upthrown) side of the fault. For example, south of Little Wanganui ~80% more landslides occurred on the hanging wall than on the footwall. Landslide frequency increases with slope angle, with 92% of the landslides having maximum source area slopes angles of 35°–65°, and ~7% have slopes of 35° or less. However only about 2% have source area slopes greater than 65°, mainly because scars on steep to sub-vertical road cuts and escarpments could not be detected by the 10 m digital elevation model (DEM) used in this study. Analysis of landslides in relation to slope aspect showed that 37% occurred on NE–SE-facing slopes, 29% on NW–SW-facing slopes, and 25% on SE–SW-facing slopes. Only 9% were on NW–NE-facing slopes despite the semi-uniform distribution of hill slope aspect in the study area. The prevalence of landslides on east and west-facing slopes is thought to be related to the strong reverse fault movement on the White Creek Fault. Relatively few (167 or ~3%) of the mapped 1929 landslides were small (~103–104 m3), with the majority (3713 or ~62%) classed as moderate (~104–105 m3), 1985 (33%) as large (105–106 m3), and 123 as very large (106–108 m3). Two giant (≥108 m3) rotational slides occurred at Little Wanganui Head (210 Mm3) and Kongahu Point (120 Mm3) in Tertiary mudstone on coastal cliffs 35 km northwest of the epicentre. Very large landslides, mainly rock avalanches and slides formed at least 42 landslide-dammed lakes in narrow mountain valleys across the region, 20 of which have been drained or infilled with alluvium, and 22 still exist today. Overall, this study has shown that the Murchison earthquake triggered the largest and the greatest number of landslides of any historical earthquake in New Zealand since 1840. Although the 2008 MS 8.0 Wenchuan earthquake in China triggered many more landslides over a much wider area (56,000 over 42,000 km2), the Murchison earthquake has a similar power-law trendline and frequency–size distribution curve, especially for landslides of 200,000 m2 and greater. Most of the landslides triggered by the Murchison earthquake were disrupted failures, mainly as rock falls, rock slides, rock avalanches, debris falls, slides, and debris flows. The types of landslides that occurred in 1929 were similar to those during the 1968 Inangahua earthquake, but were greater in number, larger, and more widespread. The largest and most common landslides triggered by the 1929 earthquake formed in Tertiary rocks, with at least 39 (53%) of 123 landslides greater than 1 Mm3 occurring, either as dip-slope bedding plane slides, rotational slides and falls; 29 (39%) were in granitic rocks; and 6 (8%) of the landslides occurred in Palaeozoic greywacke and volcanics. At least 40 very large (> 1 Mm3) rock and debris avalanches were triggered by the earthquake, many of which occurred in granite, and Paleozoic volcanics and conglomerates on high ridges up to 90 km north of the epicentre, and also on dip-slopes and scarps of Tertiary limestone, sandstone, and calcareous mudstone. The distribution of rock avalanches appears to be influenced mainly by steep and high topography, rock types, as well as the site locations in relation to the White Creek Fault source, strong earthquake shaking, and the direction of fault movement. Several significant rock slides also occurred including three fatal failures in the Matakitaki and Maruia valleys near Murchison (Matakitaki, 18.1 Mm3; Gibson’s, 5 Mm3; and Holman’s, 2 Mm3). Liquefaction effects (sand boils, lateral spreading etc.) were not mapped as part of this study. However, reports in newspapers, historical photos, and geotechnical papers show that effects due to soil liquefaction during the 1929 earthquake were widespread in alluvial deposits from Murchison to Westport, and coastal areas from Greymouth to Karamea, Collingwood, Takaka, and Motueka. Typical effects included: ground cracking and fissuring of roads and farm paddocks (lateral spreading); ejections of sand, silt, and water from cracks (sand boils); geysers and waterspouts up to 10 m high; rupture of water and sewer pipes, and tilting of power poles. Overall the damage to buildings, roads and the landscape as a result of liquefaction was minor compared to the damage caused by landslides. Landslides triggered by the Murchison earthquake had great impact on the landscape, closed roads across the region, and had catastrophic social effects. Rock falls, rock slides, and a debris flow, destroyed several houses, and caused the deaths of 14 people, mainly in the Matakitaki and Maruia valleys near Murchison, and 2 miners died in coal mine collapses in Seddonville. Landslide damage to roads was extensive. The Buller Gorge road from Murchison to Inangahua was almost destroyed by rock falls and took 22 months to repair, and slips blocked the lower gorge road from Inangahua to Westport for three days. The Karamea Bluffs section of State Highway 67 was damaged by rock falls and closed for a few weeks. Several weeks after the earthquake, landslide dams in the Karamea, Mokihinui, Little Wanganui, and Kakapo rivers were breached during floods, causing flooding and damage in Seddonville, Little Wanganui and Karamea. Landslide dams on the Matakitaki, Buller, and Maruia rivers were breached without causing serious flooding downstream. The 1929 landslides also did extensive damage to the landscape, forming large scars on mountain ridges that remain visible today, and many valleys are still blocked with debris. Based on these effects, the size and density of the landslides that occurred and the deaths they caused, the 1929 Murchison earthquake is believed to be New Zealand’s most significant historical earthquake-induced landslide event, and provides insight into the threat of similar events in the future, including the next Alpine Fault earthquake. (auth)