Gisborne moves to the east again - 12/04/2010

Gisborne has moved to the east at a rate of several millimetres a week during the past three months, the fifth such episode of slow tectonic movement for Poverty Bay in the past nine years, scientists say.

The movement has been detected on more than a dozen GPS instruments between Hawke Bay and Poverty Bay and is due to slippage on the boundary between the Pacific and Australian tectonic plates between 10km and 15km under Poverty Bay.

Diagram of Gisborne on the move.

Normally Poverty Bay moves westward at a rate of several millimetres a year, because it is being dragged along by the subducting Pacific plate which is moving inexorably to the west. But about every two years, Poverty Bay ‘springs’ back slowly to the east cancelling out almost all of the previous westward motion.

The phenomenon has also been observed under the Kapiti and Manawatu regions, where it occurs deeper and more slowly than under Poverty Bay and Hawke Bay.

This land movement, or deformation, is being detected by a network of 150 continuous GPS instruments around New Zealand, operated by the GeoNet project within GNS Science. The instruments take position recordings every 30 seconds and send an hourly ‘package’ of data to GNS Science where researchers analyse it.

The phenomenon is sometimes called a ‘slow earthquake’ or a ‘slow-slip event’. Worldwide these events have been observed at a number of subduction zones since the mid 1990s, with two of the best studied examples being in Japan and in the Cascadia region of western Canada and the US Pacific Northwest.

Slow-slip events are important because they relieve accumulated stress between the rocks on either side of a geological fault. But they do it slowly over a period of days or weeks rather than rapidly - over seconds - as in a normal earthquake.

During slow-slip events an area on the interface between the two tectonic plates that is normally locked is temporarily released, allowing the overlying crust to move back eastwards at rates of a few millimetres per day.

The strong shaking and damage that occurs in a normal earthquake is due to the fact that movement between the two sides of the fault occurs at the huge rate of several kilometres per second.

GNS Science geophysicist John Beavan says the exact mechanism for the Gisborne slow-slip events is not known.

“The most likely explanation involves water that is expelled from the rocks of the subducting Pacific plate - including marine sediments that are saturated with water - as the plate sinks deeper in the Earth and experiences higher pressures and temperatures. The released water is probably playing a major role in reducing friction between the locked tectonic plates,” Dr Beavan said.

The recent easterly movement had removed earthquake stress from the plate boundary between 10km and 15km deep below Poverty Bay. However, at shallower depths in the Earth’s crust, earthquake stresses continued to accumulate and remained a threat to the East Cape region, Dr Beavan said.

“So it’s a partial let-off for Poverty Bay. The 2010 slow-slip episode has occurred in four separate, but related, movements. It’s probably more complex than the previous slow-slip events we’ve observed at Poverty Bay, though with the additional GPS instruments installed over the past couple of years this is the first time we’ve been able to track one of the Poverty Bay slow-slip events with such precision.”

Dr Beavan said in mid January 2010 Tolaga Bay was first off the mark with steady easterly movement. It was followed a couple of weeks later by movement south of Mahia Peninsula. Then in mid-March Gisborne joined in with the largest slow-slip event yet seen there. The movement appeared to be dying away, but then another smaller pulse of movement occurred, again near Gisborne.

People living in Poverty Bay would not be aware of the slow-slip events because the movement was so gradual. At its fastest, the slow-slip involved only about 30mm of ground movement over a few days. Large parts of Poverty Bay were moving at roughly the same rate and in the same direction, Dr Beavan said.

He added that it was likely these slow-slip events had been occurring for as long as there had been a subduction zone off the North Island east coast – several million years. However, scientists have only been able to observe them since 2002 when GPS instruments were installed along the East Coast of the North Island.

“The advent of very accurate GPS equipment has enabled scientists to see this movement occurring in real-time.”