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Big quakes of 2001 too deep or too distant to cause damage - 09/01/2002

New Zealand was hit by at least six potentially damaging earthquakes in 2001, but they were either too distant from population centres or too deep to cause harm, the GNS Science Limited (GNS) said today.

Earthquake summary

In what seismologists describe as an average year in terms of size, number and distribution of earthquakes, the largest jolt was a magnitude 7.0 earthquake centred offshore in the Kermadec trench 420km northeast of Gisborne on August 21.

Although distant, its large size and relatively shallow depth meant it was felt throughout much of the North Island and as far south as Christchurch. It caused minor damage and a brief electricity blackout at Tolaga Bay, 56km northeast of Gisborne.

Also of note was a magnitude 6.2 earthquake centred 20km north of Taumarunui on the morning of May 24. It was felt throughout the eastern North Island, but its impact was not severe because of its depth of 260km.

A magnitude 6.1 earthquake on the morning of December 8 located 30km southwest of Haast, and shallower than 12km, was felt throughout the southern South Island, but was away from major population centres.

Most of these larger earthquakes were followed by dozens of smaller aftershocks. In the case of the quake southwest of Haast, aftershocks occurred at the rate of up to 20 an hour during the first two days after the main shock.

GNS seismologist Peter McGinty, who flew in to the Haast area to record the aftershocks with portable seismic instruments, said they sounded like the boom of a large artillery gun.

Accurate recordings of aftershocks give seismologists valuable information about the nature of the main shock and its relation to the geology of the area. In particular, they can indicate how the main shock may have increased or decreased the stress on neighbouring faults.

About 1 percent of the 16,000 earthquakes recorded in New Zealand each year are big or shallow enough to be felt by humans. As a general guide, shallow earthquakes above magnitude 5.0 are capable of producing significant damage and casualties in built-up areas

During late September and early October a number of moderate-sized shallow earthquakes occurred in southern Hawke's Bay, off the coast at Porangahau. The largest of these, a magnitude 5.6 quake on September 24, was felt throughout the central and southern North Island.

Among other moderate-sized shallow earthquakes recorded by GNS in 2001 was a magnitude 5.0 event on April 4, located 30km southeast of Blenheim. The largest of a cluster of eight earthquakes that occurred in that area on that day, it was felt throughout Marlborough, the Wellington region, and as far north as Wanganui.

On May 18, a magnitude 5.1 earthquake located 50km west of Methven in Canterbury was felt on the West Coast and inland Canterbury.

A number of moderate-sized deep earthquakes also occurred during 2001. On October 15 a magnitude 5.8 quake with an epicentre close to Hastings was widely felt. A magnitude 5.0 quake on October 24, located 30km northwest of Taupo, was felt along the east coast of the North Island.

On November 9, a magnitude 5.3 earthquake located 30km south of Nelson was felt in the northern South Island and the Wellington region. Finally, a magnitude 5.0 event on December 8 at 6.34am, located 30km southwest of Gisborne, was felt in the East Cape and Hawke's Bay regions. There were also three earthquakes above magnitude 5.0 in the
Bay of Plenty during the first half of the year. All of these events were too deep to cause damage.

Last year marked the start of a major upgrade to the equipment that monitors earthquakes and volcanoes in New Zealand. The Earthquake Commission is providing $5 million a year over the next 10 years to fund the upgrade, with equipment being installed at new sites and existing equipment replaced.

GNS scientists and technicians installed the first of the new equipment last year and more instalments are scheduled for this year. Called GeoNet, the upgrade will provide more accurate and more rapid information about earthquakes. It will also help scientists identify the early signs of volcanic unrest. More information can be seen at www.geonet.org.nz