8000-year Earthquake Record Forms Basis of Seismic Forecasting
Recent research by scientists from GNS Science, New Zealand and the University of Nevada-Reno, Reno provides insights into how isolated, geometrically simple strike-slip faults can produce a near-regular earthquake pattern – having implications for earthquake prediction.
The scientists, led by Kelvin Berryman of GNS Science, studied the Alpine Fault in southwest New Zealand. The Alpine Fault separates the Pacific tectonic plate from the Australian plate and is one of the longest, straightest and fastest-moving faults of its kind on Earth. Previous research on the Alpine fault had only produced age-estimates for the last four major earthquakes; Berryman and the team of scientists increased this to a record of 24 major earthquakes, stretching over a period of 8000 years – one of the longest continuous records of fault activity on Earth.
Researchers found that a pattern of cyclic stratigraphy in fault-adjacent deposits recorded the seismic history of the area. This allowed for a reliable, composite record of major earthquakes to be established. The layers in the sediments corresponding to earthquakes, known as event horizons, were dated using radiocarbon dating of leaves and seeds. All-in-all, eighty-two radiocarbon ages were used to produce the earthquake sequence.
The results of the paper, published in Science, estimate that there is a mean recurrence interval of 329 years for the 24-event data set. There hasn’t been a major earthquake on the Alpine Fault since written records began, around 170 years ago, but multiple lines of evidence suggest that it has produced large (moment magnitude (Mw) >7) earthquakes and poses a substantial seismic hazard.
By utilising an unusually long earthquake record, the scientists were able to categorise the Alpine Fault as ‘quasi-periodic’; meaning that – due to its simple geometry and isolation from other faults – it can be used as an end member for the characterisation of faults around the world that threaten major earthquakes. In other words, features observed in the Alpine Fault can be recognised in areas in which seismic records are much shorter. This has implications for earthquake hazard perception and forecasting. The researchers suggest that by studying features of faults; such as slip-rate, total slip, geometric complexity and interaction with other faults, and by relating them to long earthquake records such as that of the Alpine Fault, more informed decisions can be made in situations of earthquake prediction and hazard analysis.
Millions of people live near to major fault zones and studies such as this one can help create safer, more prepared communities when disaster does strike.
Original paper: http://www.sciencemag.org/content/336/6089/1690