Abstract - The recent Mw 7.1 Darfield Earthquake produced rupture of the ground surface along a fault that was not known to exist prior to the earthquake. How many more active faults remain undiscovered, how best to identify these faults and what hazard they might pose are all important questions arising from the Canterbury earthquakes. In this talk we use aerial photograph, fault trenching, seismic reflection, LiDAR and historical seismicity information post 1845 to cast light on the first two of these questions for New Zealand. On the Taranaki Peninsula, where active normal faults generally have slow displacement rates <0.5 mm/yr, seismic reflection lines reveal that <50% of active faults produce mappable traces and that even where active traces have been mapped these constitute <50% of the sub-surface fault length. Similar statistics also apply on the Rangitaiki Plains in the Bay of Plenty, where a recently published LiDAR digital elevation model reveals many more active normal faults with longer traces than was previously identified from 1:16000 aerial photographs and field mapping. In common with the Taranaki example the rates of sedimentation over much of the plains are comparable to the fault displacement rates (0.2-2 mm/yr) and fault scarp burial may be common. To further test the incompleteness of the geological record historical large magnitude earthquakes since 1845 are considered for all of New Zealand. Of the onshore events with magnitudes >7 only about half show evidence of surface rupture along the primary fault surface and have the potential to be recorded in the future geological record. For the historical data, however, the main reason active faulting was not observed is that either ground surface rupture did not occur or was below the detection threshold. Together, lack of surface rupture and scarp concealment (or destruction) by surface processes are likely to mean that many active faults, particularly with slower displacement rates (<0.5 mm/yr), have not yet been discovered. The Taranaki and Bay of Plenty studies suggest, however, that acquisition of data, including LiDAR and seismic reflection lines, can significantly reduce this knowledge gap.
Abstract of talk given to:
Geological Society of New Zealand Annual Conference, Geological Society of New Zealand, Nelson, November 2011.