Fewer landslides than expected after big quake

Fiordland's big earthquake last month caused fewer than expected landslides in Fiordland National Park, according to GNS Science geologists.

The 7.8 magnitude earthquake on July 15 produced roughly half the number of landslides that occurred during the 7.2 earthquake of August 2003 - 187 against 422 observed in 2003.

Last month's landslides in Fiordland occurred across an area of about 5600 sq km on steep-sided fiords, compared to about 11,000 sq km 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 7.8 earthquake, said engineering geologist Graham Hancox.

"The low level of landsliding caused by the 2009 earthquake was probably due to the lack of high frequency shaking associated with this earthquake.

"This combined with the dominant fault rupture motion away from land and toward the sea were the main factors resulting in fewer landslides than in August 2003," Mr Hancox said.

Lower than average rainfall in the two months before the July 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, was 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.

- NZPA