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A clearer picture of the alpine fault's earthquake history - 10/09/1998

Scientists have reached agreement on the dates for the last four large earthquakes on the Alpine Fault, which runs from Milford Sound up the spine of the South Island to Blenheim, 650km to the north.

Sept98 alpinefault

For several years there has been debate about how many earthquakes have occurred on parts of the fault over the past 1000 years, and when they occurred.

Agreement on the dates and sizes of the historical earthquakes came at a two-day workshop on the Alpine Fault in Wellington at which 20 scientists reviewed all the latest research.

Workshop organiser Kelvin Berryman, a geologist with the Institute of Geological & Nuclear Sciences Limited, said the workshop had consolidated research done by many groups during the past five years.

The workshop marked the first time that all the main players in the research had gathered to discuss their results.

" Our aim was the synthesise the available research to see if we could reach a consensus on when the last major ruptures occurred. The improved confidence we now have will form the basis of evaluating the future hazard of this fault,'' Dr Berryman said.

Dr Berryman said that the best estimate was that there was a high likelihood of an earthquake on the Alpine Fault within a few decades. It would cause severe ground-shaking in many parts of the South Island.

To evaluate the probability of a future earthquake on a major fault, scientists need to establish the dates and size of past earthquakes. In the case of the Alpine Fault, this had been done by a combination of fault trenching, radiocarbon dating, tree-ring analysis, the study of lichens and contemporary earthquakes, statistical analysis, and highly accuarte measurements of the deformation at the Earth's surface using global positioning satellite technology.

Results of all these studies have confirmed that the previous four ruptures along the fault occurred at intervals of 100 to at least 300 years. The last happened about 1717AD.

It appears to have involved a rupture of nearly 400km of the southern two thirds of the fault and generated an earthquake of about magnitude 8.

About 100 years earlier, at around 1620AD, another earthquake occurred in the central and northern section of the fault. It produced a similar sized earthquake.

Prior to this there were earthquakes at about 1450AD and about 1100AD. Their magnitudes were also about 8.

The most active part of the fault is the central section between Milford and Inchbonnie. Further north the fault becomes progressively less active as movement is transferred onto the numerous faults in Marlborough.

Dr Berryman said when the next rupture occurred on the Alpine Fault ground-shaking intensities were expected to be less on the South Island's east coast, but throughout most of the South Island the next earthquake on this fault would be stronger than any jolt experienced there in the past 100 years.

Direct effects will include landslides and liquefaction - where some soils behave like liquid during the earthquake causing structures to sink, tilt or topple. Landslides would be most severe in and around the Southern Alps.

Some roads, bridges and services may not be fully restored for months, or even years.

Dr Berryman said one of the more profound longterm impacts would be landslides that caused major changes to rivers that drain the Southern Alps. This had implications for river control, bridges, and hydro-electricity generation.

Scientists did not want to be "doomsayers", but it was important to keep communities and local and national authorities informed about the latest research. This would allow time to prepare for such an event.

Regional and national planners could initiate a number of actions that would help to reduce the impact of a large earthquake. Dr Berryman said because New Zealand was such a geologically active country, life was all about accommodating potential natural hazards.

He added that because the Richter scale was logarithmic, each step up represented 30 times more energy released during an earthquake. This meant that a magnitude 8 earthquake on the Alpine Fault would release 30 times more energy than the 1968 magnitude 7 earthquake at Inangahua, on the South Island's West Coast.

Seismologists regarded the past 30 to 40 years as a period of quiescence in New Zealand as there had been no big on-land earthquakes. This was not unusual as large earthquakes sometimes came in "clusters" lasting 10 to 20 years. Clusters were sometimes followed by a period of quiescence.

The "busiest" period since European settlement was between 1929 and 1934 when there were at least five large on-land earthquakes in New Zealand.

Scientists and engineers are continuing to work with regional councils and local communities to help develop strategies to cope with earthquakes, landslides, volcanic eruptions, and tsunamis.