AFTERSHOCKS following last week’s large earthquake are likely to continue for two years.
Eastern Bay residents rattled by the 7.1 magnitude earthquake that shook them from their beds at 4.39am on Friday morning can look forward to aftershocks that will continue for an extended period of time, a quake expert says.
GNS Science seismologist John Ristau said the area along the east coast of the North Island and north of East Cape was one of the more seismically-active regions of the country.
“Large earthquakes occur regularly in this area,” he said.
“These include a February 6, 1995, magnitude 7.2 earthquake off the east coast of East Cape; the December 20, 2007, magnitude 6.7 Gisborne earthquake; and a 6.8-magnitude earthquake on November 17, 2014.
“The seismicity is associated with the Pacific Plate pushing beneath the Australian Plate, which the North Island is part of.”
With the tectonic plates typically being between 10 kilometres and 20km thick, the location of the earthquake in this subduction zone was a significant factor in how much energy was being released.
Dr Ristau said as the September 2 earthquake was located below the boundary of the Australian and the Pacific tectonic plates, it had not generated a significant tsunami.
“But it did create enough displacement to generate a 25 centimetre tsunami at East Cape.
“Typically, tsunamis are produced by very large earthquakes that occur on the boundary between two tectonic plates, where one plate is subducting beneath the other.”
Examples of on-the-boundary quakes included the 2004 Sumatra earthquake that caused the tsunami in the Indian Ocean, and the 2011 Japanese earthquake, with its subsequent tsunami.
Dr Ristau said to produce a tsunami an earthquake had to cause a large displacement in the sea floor, which in turn displaced a large amount of water.
“Without these conditions, a significant tsunami does not occur.”
Dr Ristau said in 1947, there were two magnitude 7 earthquakes off the Gisborne coast that generated tsunamis with heights of several metres.
They were a special type called slow-rupture tsunami earthquakes.
Dr Ristau said the high-frequency energy generated by a normal earthquake was what people normally felt.
“This means the 1947, quakes were only weakly felt, and that no one was aware that a large tsunami was approaching,” he said.
The other method by which tsunami can be produced was by an undersea landslide, but no such landslide appeared to have resulted from Friday’s earthquake.
He said that on February 6, 1995, a magnitude 7.2 earthquake occurred in the same area and aftershocks continued for two years.
“Since this earthquake is similar in size, it will likely have a similar aftershock sequence,” he said.
“There will be hundreds of aftershocks, but given that the earthquake and the aftershocks are offshore it’s unlikely that aftershocks with magnitudes below 4 will be widely felt,” Dr Ristau said.
Aftershocks expected to continue
ANOTHER cluster of earthquakes rocked the East Cape around Te Araroa early yesterday, the strongest rousing people from their sleep at 3.15am.
Twenty-seven other aftershocks occurred within 12 hours of the magnitude 5.7 tremor, centred 95 kilometres northeast of Te Araroa and 21 kilometres deep. The largest of these was magnitude 5.3 and took place at 5.02am at the same location at a depth of 19 km.
The remaining quakes registered between magnitudes 2.1 and 4.2.
The United States Geological Survey says the east of the North Island has a history of large quakes and seismic activity was particularly high in the Kermadec region.
Within a 250km radius of the 7.1 magnitude shake, there had been 28 earthquakes of magnitude 6 or greater during the 20th century.
