Seismologists’ explanation

Press, 5 March 1987, Page 4

Seismologists’ explanation

PA Wellington Part of the Earth’s crust which is being stretched by up to two centimetres a year finally tore on Monday afternoon.

That is the seismologists’ simple explanation for the Bay of Plenty earthquake. The acting superintendent of the national Seismological Observatory in Wellington, Dr Euan Smith, said the east coast of the North Island was being pulled east by the Pacific crust plate, which is sinking under its own weight. . >

The North Island sits on the Australian Plate, and the boundary of the two runs under the sea off the east coast. As the Australian Plate is pulled east the upper crust of the Earth stretches at its

weakest point — on the North Island’s volcanic plateau. The initial sharp jolt at 1.35 p.m. on Monday probably caused when the weakened upper crust started to tear about 15 kilometres off the coast, said Dr Smith. The second, main shock at 1.43 p.m. was the tear developing into a significant rip, and the aftershocks were the final pieces of the crust giving way, probably very close to the surface. Mr Smith said it was thought tears usually started at the bottom of the crust, • probably 15 kilometres deep in this case. He estimated the size of the final rip at 15 kilometres deep, and 15 kilometres long. Surface cracking indicated total crust move-

ments of about a metre along the rip — “which is about what we would expect from an earthquake of this magnitude.” Since the volcanic plateau had been stretching at an average rate of a centimetre a year, this meant 100 years of strain had been relieved.

But, in this ' case, seismologists did not know how long the strain had been building because they had no record of the earthquakes which last relieved the particular fault. “So there may be some strain yet to be relieved,” said' Dr Smith. “We do know, though, that about half the strain is often dissipated by other means.”

It was probable the strain had been relieved, but earthquakes could continue for some time.