Paleoseismic investigation of the north and west Wairau sections of the Alpine Fault, South Island, New Zealand
Author: Mark D Yetton
Paper number: 3705 (EQC 99/353)
Abstract
The Alpine Fault is the onshore boundary of the Pacific and Australian crustal plates in the South Island of New Zealand. The Alpine Fault commences at the southwest coast at Milford Sound and forms a major active fault line along the west boundary of the Southern Alps for approximately 400 km to the Lewis Pass area. It then continues as a less active fault a further 200 km through the Nelson Lakes area to the Pacific coast at Blenheim.
Although the Alpine Fault has the highest long term movement rate of any active fault in New Zealand there have been no historical earthquakes on any part of it including the most active central area. This project has been undertaken to determine the approximate timing of the most recent prehistoric earthquake on the Alpine Fault in the less active area in the north (the north section of the Alpine Fault) and in the northeast (the Wairau section of the Alpine Fault). By knowing the approximate timing of the most recent prehistoric earthquake, and how much the fault moved in the last earthquake, it is possible to estimate the likelihood of a future earthquake of a similar size.
Excavations have been undertaken at four locations, three on the north Alpine Fault southwest of Nelson Lakes, and one on the Wairau section of the fault near Tophouse. These excavations of the surface soil and gravel at the fault line have provided wood, charcoal and peat that can be radiocarbon dated to determine the approximate age of the soil and gravel, and in some cases the time of the last earthquake rupture.
The results from the three locations on the north Alpine Fault indicate that the most recent earthquake in this area was between AD 1440 and AD 1700. It is most likely to have been between AD 1600 and 1670 and closely matches previous research on the fault further southwest that suggests a large earthquake occurred at around AD 1620 that extensively damaged the natural forests.
The results from the excavation on the Wairau section of the fault indicate there has been an earthquake in this area since AD 200, but the new results do not indicate when. If this is the same earthquake rupture that has been identified in previous work 50km northeast (which is likely) then the earthquake occurred between AD 200 and AD 1000.
Although the fault has been locked near the ground surface since these prehistoric earthquake ruptures, the steady movement of the plates at lower crustal depths has been accumulating strain near the fault. These new results suggest there is already sufficient stored energy to generate an earthquake of a similar size to the most recent earthquake rupture along virtually the entire Alpine Fault, including the Wairau section of the fault near Blenheim.
Technical Abstract
locations that bracket the north and Wairau sections, between the Blue Grey River and Tophouse Saddle. Six trenches and pits have been excavated at four localities along one hundred kilometres of fault strike and a total of fifteen radiocarbon dates provide age constraints on the timing of the most recent earthquake.
At the three localities along the north Alpine Fault (Blue Grey River, Maruia River, Matakitaki River) there is consistent evidence that the most recent earthquake occurred between AD 1450 and 1700, and it is most likely to have occurred between AD 1600 and 1700. This is in close agreement to the results obtained from previous paleoseismic excavations on this north section of the Alpine Fault at two localities further southwest. Previous analysis of the timing of forest disturbance suggests that the earthquake occurred in the southwest area in AD 1620 ± 10yr, and this may be the same earthquake that is recorded at these three new locations. The surface offsets at the fault scarp during the most recent event at the new locations were relatively small (horizontal offset 1.3 ± 0.6m, vertical offset 0.25± 0.1 m).
The trench site at Tophouse Saddle on the Wairau section of the Alpine Fault indicates there has been an earthquake rupture since AD 200 but how long after this date can not be determined. If this is the same earthquake rupture as that recorded in previous work on this same section of fault, then the rupture occurred between AD 200 and AD
1000, and is likely to have been nearer to the AD 200 end of this range.
The new paleoseismic data from this investigation indicates there is an earthquake rupture segment boundary between the Matakitaki River and Tophouse Saddle. Longterm slip rates suggest sufficient elastic strain has accumulated since the last earthquakes to generate new earthquake ruptures with surface offsets at the fault scarp comparable to those of the most recent events. On this basis, earthquakes on both these sections of the Alpine Fault should be considered possible within traditional planning time periods (i.e. 50 -100 years).
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