WELLINGTON — Researchers have discovered New Zealand’s earthquake-prone
landscape is even more unstable than previously thought, recording deep tremors lasting up to 30 minutes on its biggest fault line.
Scientists measured the so-called “creeping earthquakes” when they investigated a puzzling lack of major seismic jolts along a section of the Alpine Fault, which runs the length of the South Island.
The quakes, which caused no surface damage, occurred 20-45 kilometres (12-28 miles) beneath the Earth’s crust and continued for as long as half an hour, much longer than ordinary earthquakes.
In contrast, the 6.3-magnitude quake that killed 185 people in the South island city of Christchurch in February last year lasted just 37 seconds and struck at a depth of about five kilometres.
The quakes could not be measured by regular seismic monitoring devices and researchers from Wellington’s Victoria University had to place sensors in boreholes 100 metres deep to pick them up.
Seismologist Aaron Wech said the research showed the Alpine Fault, regarded as New Zealand’s most hazardous, did not remain still between major earthquakes but was constantly shifting.