New on-fault evidence for a great earthquake in A.D. 1717, central Alpine fault, New Zealand
G.P. De Pascale and R.M. Langridge, Dept. of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Posted online 29 June 2012; doi: 10.1130/G33363.1.
The Alpine fault is the major onshore plate-boundary structure between the Australian and Pacific plates on the South Island of New Zealand. No previous study of the central portion of the high-uplift central segment has provided on-fault evidence for the most recent earthquake (MRE). Using LiDAR (light detection and ranging) data and field mapping, G.P. De Pascale and R.M. Landridge discovered the main trace of the fault in the rainforest north of Gaunt Creek (Westland) as a north-trending fault -- a major deviation from the generally northeast-trending fault. They enhanced a natural exposure that revealed evidence for repeated thrust fault movement with a thick clay fault gouge layer juxtaposing hanging-wall (Pacific Plate) bedrock thrust over young footwall (Australian Plate) stream deposits and dated a buried peat on the fault scarp that correlates with the postulated A.D. 1717 earthquake. By comparing other records for the 1717 earthquake with recent research along the fault, De Pascale and Landridge calculate a moment magnitude (Mw) of between Mw 8.0 to 8.2 in 1717, a great earthquake that is larger than any previous estimate. Because the Alpine fault has not ruptured for ~300 years, it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to New Zealand.
Disequilibrium dihedral angles
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