$1½ Million to Research Alpine Fault

The Marsden Fund has allocated $1½ million to fund two studies into New Zealand’s Alpine Fault. One project will focus on the history of Alpine Fault earthquakes, the other will study the fault’s structure.

The Alpine Fault runs between Marlborough and Milford Sound, tracing the edges of the collision zone between the Pacific and Australian tectonic plates. To the north-east and south-west of the fault, the two tectonic plates slide one under the other but, along the Alpine Fault, they grind past each other and form the Southern Alps.

The massive fault represents one of three major seismic hazards for New Zealand. The Wellington Fault, the risk of sudden slippage near the south-eastern North Island (known as subduction thrust earthquakes), and the Alpine Fault with its large earthquakes represent the largest seismic hazards for the country. The amount of shaking they can cause, and their close proximity to large population centres and key national infrastructure is a major cause for concern.

The Marsden Fund has just announced a $44 million investment in 93 scientific projects, two of which will focus on different aspects of the Alpine Fault, and at different locations. The two projects have received funding of $1½ million spread over three years – one will be led by GNS Science, the other by Victoria University of Wellington.

The 450 km-long earthquake fault is thought to have ruptured four times during the past 900 years, each time triggering magnitude 8 earthquakes. Research indicates that the most recent ruptures on the fault occurred in about 1720, 1620, 1450 and 1100 A.D. This suggests that big quakes could occur every 100-300 years, and it is generally accepted that the next event on the fault could occur in the near future. However, it is not currently possible to determine whether the “near future” is measured in months, years or decades.

GNS Science geologist Dr. Kelvin Berryman will lead a project to investigate whether the Alpine Fault ruptures at regular intervals or triggers clusters of large earthquakes. This follows on from debate over whether past events have been single magnitude 8 quakes, or a cluster of magnitude 7 events as various sections of the fault ruptured, combining to leave evidence of a magnitude 8 event. The clustered quakes could occur within minutes, weeks, months or years of each other.

Dr Berryman’s team will examine evidence of as many as 25 earthquakes which have occurred during the past 7,000 years. They will study lake sediments and fossils in Fjordland to determine, through radiocarbon dating, when and how frequently the earthquakes occurred. His project has been allocated $690,000 in funding spread evenly over 3 years.

It is expected that the GNS Science project will dramatically improve earthquake hazard estimates for the Alpine Fault, as well as contribute to international research through the study of the rare geological records.

Professor Tim Stern of Victoria University of Wellington will lead a project to better understand the fault’s structure by installing and monitoring a network of ten borehole seismographs along the central part of the Alpine Fault. This section, which runs between Fox Glacier and Whataroa, is steep and subject to high erosion rates which obscures a lot of its history. Eight of the bores will take instruments down to 30 metres to study very small earthquakes, with another two taking different instruments down to 250 metres to study heat flow and other characteristics of the fault zone.

Current thinking is that most earthquakes along the central section of the Alpine Fault are shallow because the sliding-past movement of the two plates creates a steep temperature gradient in the rock, and only the top 10 km or so is brittle enough to snap in an earthquake. However, there is evidence that small earthquakes are occurring at greater depths of 20 km or more, and the structure of the collision zone may be more complex than originally thought.

Geonet’s seismographs are accurate at detecting earthquakes above about magnitude 2.5, but smaller earthquakes require different instruments to be studied effectively. The borehole seismographs will study these smaller earthquakes, known as microquakes, to determine where they occur and at what depth. This will provide a better picture of the structure of the Alpine Fault below a depth of 6 km.

Prof. Stern’s study has been allocated $810,000 spread across three years. It is being run in conjunction with Prof. P.E. Malin of the University of Auckland, and has two distinct phases. During the first year, $300,000 will be spent on installing the borehole seismographs and establishing the network. Funding of $280,000 during the second year and $230,000 during the third will be used to operate the network and analyse the results.

Two other geological projects have received Marsden funding of $770,000 in the latest round.

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