Fewer Landslides Than Expected In Fiordland Quake

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MEDIA RELEASE, 12 AUGUST 2009

Fewer Landslides Than Expected From Fiordland Quake

The recent magnitude 7.8 earthquake in Fiordland caused fewer than expected landslides in Fiordland National Park, geologists said today.

The July 15 earthquake produced roughly half the number of landslides that occurred during the magnitude 7.2 earthquake of August 2003 – 187 against 422 observed in 2003.

Last month’s landslides in Fiordland occurred across an area of about 5600km2 on steep-sided fiords, compared to about 11,000km2 in 2003. And last month’s landslides were mainly superficial and small in size.

The ground motions produced during last month’s earthquake were smaller than would normally be expected for a magnitude 7.8 earthquake, said engineering geologist Graham Hancox of GNS Science.

“The low level of landsliding caused by the 2009 earthquake was probably due to the lack of high frequency shaking associated with this earthquake,” Mr Hancox said.

“This combined with the dominant fault rupture motion away from land and towards the sea were the main factors resulting in fewer landslides than in August 2003.

Lower than average rainfall in the two months prior to the 2009 quake may also have had an influence.

“Overall, the landsliding caused by last month’s earthquake was scientifically interesting, but was much less than expected for a 7.8 magnitude earthquake. “

The next Alpine Fault earthquake in Fiordland, however, is expected to produce much stronger high frequency shaking and cause more and larger landslides. It would also cause liquefaction effects and tsunamis, with the potential to seriously damage roads and disrupt tourism in the region.

Research by GNS Science is aimed at determining the extent and effects of earthquake-induced landslides caused by previous large earthquakes in Fiordland, including those on the Alpine Fault.

“This will help us understand what might happen in the region during the next Alpine Fault earthquake,” Mr Hancox said.

ENDS

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