Phys Org reports on the most extensive record of Earthquakes in the article 8000-year quake record improves understanding of Alpine Fault.
The investigation, which was centred on a remote river terrace near Lake McKerrow about 35km northeast of Milford Sound, and supplemented with information from the Haast area, found evidence of 24 surface ruptures of the Alpine Fault dating back to 6000BC.
Scientists used a range of investigation techniques, including radiocarbon dating of seeds, leaves, and reeds contained in swampy sediments, to determine the ages of the ruptures.
The dating measurements were made at GNS Science’s Rafter Radiocarbon Laboratory in Lower Hutt.
The findings dramatically improve the known earthquake history of the Alpine Fault. Previously scientists had determined the ages for only the last four earthquakes dating back to about 1000AD.
The project has produced one of the longest continuous earthquake records of any on-land plate boundary fault in the world. Knowing the ages of so many major earthquakes enables a better understanding of the fault’s behaviour.
Earthquakes are not normally considered cyclical or periodic phenomena, although there are sometimes hints of periodicity. Long term periods are not normally able to be determined because of the lack of suitable data. So this study is quite unique.
The investigation found the mean interval between large earthquakes on the fault is 330 years and two thirds of the intervals were between 260 and 400 years. As with many natural systems, there was a spread of intervals with the longest being about 510 years and the shortest about 140 years.
The Earthquakes were typically magnitude 8 and so represent very severe events. We are 295 years into the average 330 year interval, but the cycle is not highly regular, so the conclusion is:
Based on this research, scientists have estimated there is a 30% probability of a large earthquake on the Alpine Fault in the next 50 years. This is at the higher end of existing estimates, which were based on only the last four large quakes on the fault. Scientists have more confidence in this new estimate as it is based on a much longer earthquake record.