Paleoseismicity and active earth deformation, Lake Rotoiti to West Wairau Valley section of the Alpine Fault (2002)
AuthorsFougere, Samuel Richardshow all
The Wairau Fault, the northern extension of the Alpine Fault, has not ruptured historically. Bounding the Marlborough Fault System to the north the Wairau Fault is over 100 km long and capable of generating a large earthquake rupture. From Cloudy Bay SW to Lake Rotoiti the fault is defined by a linear valley-bound trace and narrow zone of deformation. The main objective of this paleoseismic investigation of a section of this fault was to establish the pre-historic rupture history. Trenching excavations, geomorphic mapping, differential GPS surveying and weathering rind dating techniques were used to investigate the rupture history of the Wairau Fault. From trenching data at Tophouse, one earthquake rupture on the Wairau Fault is recorded since AD 200. No upper limit bounds this rupture date. Integration with a similar paleoseismological study of Zachariasen et al, (2001) refines the last rupture event on the Wairau Fault to between AD 200 - 1000. A branch caught in the shear zones indicates a dated event approximately 12 500 years ago but other events intervene on stratigraphic evidence. Liquefaction deposits are recognised in the trenches, in conjunction with small thrust flaps on the footwall side, suggesting seismic rupture as the dominant translation mechanism, with no significant aseismic creep based on offset linear terraces. Dextral slip rates on the fault of 4.2 ± 1.4 mm/yr compares well with pre-existing data. A single event offset of 4.4 ± 0.5 m is consistent with others further NE along the Wairau Valley. A temporal disparity exists between the last rupture brackets at the Tophouse and Matakitaki River trenches (Yetton, 2002), indicating that, at least for the last rupture event an earthquake segment boundary was operational between the two trench sites. Defining an earthquake rupture segment boundary by a specific structure or location is difficult as any number of faults could dissipate radiated seismic energy and several possibilities are reviewed. If a rupture segment does exist between Matakitaki River and Tophouse, it is most likely about Lake Rotoiti. An estimated earthquake magnitude for the onland section of the Wairau Fault for this rupture segment is M = 7.4 ± 0.25. In addition to aspects of seismicity of the fault strike, both the large scale topographic setting and detailed morphology of the fault zone are discussed. In particular the complexity of interaction between fluvial processes and secondary deformation associated with the fault zone has lead to some insights into both the evolution of secondary structures and to the response of rivers behaviour to seismic cycles. On the basis of elapsed time since the last fault rupture, and slip rates on the fault, an earthquake rupture on the Wairau Fault in the near future is a distinct possibility.