Geologists to keep watch after big quake

GNS Science geologists are today discussing whether to rush portable seismometers into sites around Fiordland to record the thousands of smaller aftershocks expected in the wake of last night's shallow but strong earthquake on the coast near Milford.

The region was rocked by a 7.8 magnitude quake at 9.22pm, just 12 km deep and 100km northwest of Tuatapere in western Southland.

It was the nation's biggest shake since the 7.8 Napier earthquake -- which killed at least 256 people on February 2, 1931 -- and ranks with similar-sized quakes at Murchison in 1929, and Marlborough in 1848.

The biggest quake recorded in this country was an 8.2 magnitude jolt on the Wairarapa faultline, which wrecked much of Wellington in 1855.

Last night's quake was followed by a strong aftershock 19 minutes later, at 9.41pm 150km west of Tuatapere, with a series of aftershocks declining in strength, until a 5.9 magnitude quake hit at 1.50am -- 120km northwest of Tuatapere -- between the focal points for the first two big ones. Both the big aftershocks were only 5km deep.

GNS duty seismologist Bill Fry said scientists worked into the night to assess whether there was any immediate danger to people or property.

"The more shallow an earthquake is, the more shaking there is going to be -- and these were quite shallow".

Asked whether last night's quakes were likely to trigger regulators to look at whether more robust building standards for earthquake resistance are specifically required in areas such as Te Anau, Milford Sound and Queenstown, Dr Fry said this was likely to be considered.

"If any buildings have been damaged, that's something we can expect," he told NZPA.

The previous biggest quake recorded in Fiordland, a 7.1 impact 10km northwest of Doubtful Sound in August 2003, lifted the seabed 45cm, triggered 400 landslides, and caused considerable damage in Te Anau.

The EQC received almost 3000 claims totalling $10.5 million after that quake, which was followed by a big 6.2 magnitude aftershock.

A total of 8000 aftershocks were recorded over the next two months, using a network of nine portable seismographs.

Aftershocks occur as the earth's crust adjusts to stresses caused by the main shock, and no two aftershock sequences are exactly the same. Recording aftershocks helps to define the plane of rupture and enables seismologists to determine if the earthquake has increased stress on nearby faults such as the alpine fault.

A pointer to a potential outcome of the latest quake was the way the 2003 damage prompted a fine tuning of the ground-shaking hazard estimates for big earthquakes in the lower half of the South Island.

Further "ground-truthing" of previously ground-shaking models for the south may potentially lead to "adjustments" in insurance premiums and building requirements.

Surprisingly, the greatest shake intensity of the 2003 quake was recorded on sedimentary soils in South Dunedin, nearly 400km away.

In the suburb of St Kilda, shaking was 1.6 times greater than previous quake impact models had predicted, though the intensity was much lower on nearby hilly volcanic rock sites.

This sort of evidence from the latest quake could result in engineering standards being reviewed and boosted in future for buildings on sedimentary soils.

The potential for new standards has been boosted by National Institute of Water and Atmospheric Research (Niwa) scientists, who mapped the seabed in line with the alpine fault to about 230km south from Milford Sound.

According to GNS Science principal scientist Kelvin Berryman, of Wellington, the mapping showed earthquakes originating along this line could be slightly bigger than previously thought -- magnitude 8 faults instead of the previously expected range of 7.5 - 7.8.

In the 2003 Fiordland quake, the main damage area ran 60km along the coast north and south of the epicentre, and about 40km inland, an area of about 3200 square kilometres, with landslides recorded as far north as George Sound and as far south as Breaksea Sound.

It also only caused a small tsunami -- which travelled up the South Island's west coast and into several fiords -- but was also felt moderately strongly in coastal New South Wales.