A dendrogeomorpholgical study of aggradation hazards in Westland, New Zealand. (2021)
Type of ContentTheses / Dissertations
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
A major earthquake along the Alpine Fault, or on a different major fault within the western Southern Alps, will have a profound effect upon the west coast of New Zealand’s South Island. Large numbers of coseismic landslides, as well as the hazards due to the interactions of these landslides with the river systems into which most of them will fall, threaten commerce, livelihoods, towns and infrastructure. Landslide sediment input to rivers causes landslide dams and sediment waves, which in turn lead to river aggradation and avulsions on the mountain-front river fans and lowlands (floodplains) of the west coast. This may continue as an active process for perhaps decades following an earthquake, with far-reaching and long-term effects on west coast society and commerce. This project aims to develop a realistic picture of the spatial extent and duration of such deposits on Westland river floodplains in an effort to aid in preparation for, and recovery from, events of this type.
A tree-coring campaign in the Wanganui, Whataroa, Omoeroa, Karangarua, and Waiatoto river catchments yielded data from 14 new even- aged cohorts from 535 cored trees. Combined with previous published and unpublished datasets, a total of 132 tree cohorts from multiple catchments allowed the extent, timing, and duration of aggradation events in Westland to be estimated.
The outcomes of this research showed that in the decades following the c.1460 AD earthquake, substantial aggradation extended nearly to the coast in some catchments (Waiho, Karangarua, and Ohinemaka) while others had survivor trees of that time period amongst cohorts dating to a younger event. There is also some evidence to support the suggestion that this event was perhaps two events separated by 50-75 years. The large extent of this aggradation may suggest that this event was either an exceptionally large earthquake on the Alpine fault, or a less exceptional earthquake on a fault within the western Southern Alps.
Following the c.1620 AD earthquake, aggradation was extensive in the Wanganui and Whataroa catchments, but was less apparent farther south, and not found at all on the Karangarua floodplain. It is hypothesized that this event may have been a rupture of a fault within the Southern Alps rather than on the Alpine fault, as previously assumed, which may explain the question of why some catchments are more affected than others.
The aggradation episode following the 1717 AD Alpine fault earthquake, although affecting catchments across Westland, did not reach the lower floodplains of any of the catchments studied. It may be that not enough time had elapsed between this event and the one previous for substantial source material to have been generated, or perhaps the energy from this range-front event did not affect the Southern Alps as much as previous events within the orogen. A fourth aggradation event can be seen in the data from the southern catchments to as far north as the Karangarua River, perhaps even the Omoeroa or Waiho, and is believed to have resulted from an 1826 AD Fiordland earthquake.
These aggradation episodes correlate well with the known occurrence of major earthquakes in Westland, and although spatial and temporal distribution of aggradation suggests that they are not all due to a rupture on the Alpine fault, they all imply a significant threat to the population centers, industry and livelihoods in Westland as a result of future earthquakes. These data allow the potential extent and duration of aggradation following the next major earthquake in the area to be anticipated, and this information will be useful in planning societal response to, and recovery from, such an event.