Landslides and ground damage associated with the M6.2 Eketahuna earthquake, 20 January 2014

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Rosser, B.J.; Townsend, D.B.; McSaveney, M.J.; Ries, W. 2014 Landslides and ground damage associated with the M6.2 Eketahuna earthquake, 20 January 2014. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/51 34 p.

Abstract: Widespread but small-scale landsliding occurred during the Mw 6.2 (Me 6.4) Eketahuna earthquake on 20 January 2014. A total of 177 instances of ground damage and landslides (consisting of rockfalls, slumps, slides, ground cracking and liquefaction), were identified as being triggered by the Eketahuna earthquake. The main area of landslides occurred over an area of about 1,000 km2, and the total area affected by landslides was about 5,500 km2. Isolated rockfalls were triggered as far north as the Parapara Range, Whanganui district, about 120 km from the epicentre. Earthquake-induced landslides occurred on moderately steep to steep slopes (21-35). A few large deep-seated (rotational) landslides occurred on slopes underlain by mudstone, and there were isolated areas of small shallow soil slides within 5 km of the epicentre. Small rockfalls from steep slopes and cuttings were common, affecting many roads in the region. Many of the landslides were reactivations of older existing landslides. There was some incipient cracking on slopes and ridge crests, as well as on roads near the epicentre. The majority of failures were located on steep or very steep (~35°-45° or >) natural (cliffs, terrace edges) slopes and over-steepened man-made slopes (mainly road cuttings) on a variety of lithologies. There were two reports of minor liquefaction associated with the event (sand boils) at Castlepoint and in the Manawatu River. The area affected by landsliding is well above the area/magnitude mean regression line for historical New Zealand earthquakes of similar magnitude. This is probably because of the very high stress drop associated with the earthquake and the relatively deep (34 km) epicentre which propagated strong shaking over a large area. The density of landslides triggered by the Eketahuna earthquake in the main area of landsliding was 13/100km2, and the surface area to volume ratio conforms to a power law of 1.1 – 1.2. Based on the observed type, size and number of landslides and other ground damage attributed to the Eketahuna earthquake, the maximum Modified Mercalli (MM) felt intensity in the epicentral area is estimated to have been about MM 7. (auth)