Sub-surface deformation of a fast-moving earthflow was studied in the East Coast Region of the North Island, New Zealand, for a period of 2 years. Tiltmeter profiles indicated that earthflow materials were subjected to a variety of movement mechanisms, including internal deformation by gravity-shearing flow, extension flow, compression flow, rotation over curved slopes, rotation of material beneath the earthflow*, and sliding. Overall, internal deformation accounted for less than 25% of the total surface movement, the remaining 75% being the result of sliding movement along the basal shear plane. Gravity-shearing flow occurs along the basal shear plane, which is thought not to be more than a few centimetres thick. Micro-topography (features on a scale of 1-10 m) largely affected tilting behaviour (internal deformation) of fast-moving earthflows. In the longitudinal direction, tiltmeter profiles on curved slopes were monoclinal and rotation was the principal mechanism causing tilt. On concave slopes the tiltmeter profile had a forward (downslope)-convex shape, with compression being the principal deformation mechanism. On even (planar) slopes. extension flow (creep) predominated, with the profile showing little evidence of rotation. However, minimal deformation (tilting) was likely to be caused by a combination of either sub-earthflow rotation, extension flow, or compression flow. In the lateral direction, topographic influences produced considerable deformation, particularly on concave slopes as a result of compression flow. There was little or no lateral deformation on even slopes.
