Reconnaissance report on liquefaction effects and landslides caused by the ML6.5 Cook Strait earthquake of 21 July 2013, New Zealand

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Hancox, G.T.; Archibald, G.C.; Cousins, W.J.; Perrin, N.D.; Misra, S. 2013 Reconnaissance report on liquefaction effects and landslides caused by the ML 6.5 Cook Strait earthquake of 21 July 2013, New Zealand. Lower Hutt, N.Z.: GNS Science GNS Science report 2013/42 20 p.

Abstract: This report presents the results of GeoNet reconnaissance inspections of landslides and liquefaction effects caused by the ML 6.5 Cook Strait earthquake on 21 July 2013. A field inspection and ground surveying at the CentrePort Wellington container storage area, carried out on 23 July 2013, showed that the earthquake caused extensive slumping and ground cracking with minor sand ejection over a ~50,000 m3 area of 1970s reclamation fill. The earthquake also caused sub-aqueous sand boils in shallow water and small earth falls at Kaiwharawhara Point, minor cracking on the wharf promenade at Te Papa, and a small rock fall on old quarry slope on the western side of Lyall Bay. The liquefaction effects that occurred during the 21 July 2013 Cook Strait earthquake are the most extensive recorded in the Wellington City area since the 1855 and June 1942 Wairarapa earthquakes. The liquefaction damage observed at the container storage reclamation area in Wellington Harbour is a timely reminder of the type of effects that could occur in reclaimed areas, especially during a magnitude 7.5 earthquake on the Wellington Fault, the subduction zone interface, or other active faults in the Wellington area. The damage that occurred suggests that reclamation fills are vulnerable to lateral spreading and collapse due to liquefaction of the underlying unconsolidated harbour silts and muds. A helicopter reconnaissance flight was undertaken on 25 July 2013 to locate and photograph landslides and other ground damage caused by the earthquake. The landsliding that occurred was restricted mainly to the steep (35°) coastal cliffs within ~15 km of the epicentre, and terrace edges in the middle Awatere Valley. Only one small rock fall occurred on the south coast of Wellington 40 km northeast of the epicentre. The largest landslide that occurred was a ~100,000 m3 rotational slide on the mudstone cliffs 1 km west of Cape Campbell. A number of debris falls occurred on White Bluffs and other similar cliffs in the area, but most were small (10-1000 m3). A 19 m high earth dam 12 km southwest of Seddon was slightly damaged (cracking on the upstream side of crest) by the earthquake. Most of the coseismic landslides are on the coastal cliffs on the edge of the modelled MM8 isoseismal zone, about 16 km from the epicentre. The number and size of landslides that occurred during the earthquake is at the lower end of what can be expected for a shallow M 6.5 earthquake in New Zealand. It is regarded, therefore, as a threshold event for earthquake-induced landsliding in the Wellington and Marlborough areas. The relatively limited landsliding and liquefaction damage caused by the earthquake is probably partly due to the fact that the earthquake was located offshore and about 15 to 50 km away from the affected areas. It may also reflect the fact that natural and engineered (cut) slopes in the area are less susceptible to failure for the maximum level of ground shaking that occurred (pga 0.21g at Ward). Slope angle, slope height, and material susceptibility appear to have strongly influenced the limited distribution and severity of both the landsliding and the liquefaction effects caused by the earthquake. (auth)