Anatomy of the Kaikoura quakes

Anatomy of the Kaikoura quakes

Seismologists now believe four separate fault lines ruptured in Monday morning's twin major earthquakes on the South Island's east coast.

Three of them, part of the Marlborough Fault Zone were known to seismologists, but a fourth, near Waipapa Bay was previously undiscovered.

The Marlborough cluster of fault lines are connected to the South Island's main fault, the Alpine Fault.

"Once the Alpine fault gets to the top of the South Island it branches off into a number of different faults," Dr John Ristau from GNS Science told Newshub.

Dr Ristau believes two separate earthquakes made up the initial magnitude 7.8 tremor just after midnight.

"Our data shows that the initial earthquake was at the southern end of the Fault Zone around Kaikoura and that was a reverse-fault type earthquake. That's when one side of the ground lifts up over the side of another.

"But after that the rupture continued, and it initiated another earthquake that was strike-slip, a side to side type motion.

"What we don't know for sure yet is what the relative strength of these two earthquakes were, if they were roughly the same size or if one was quite a bit bigger than the other."

The twin earthquakes appeared to start just northeast of Culverden on the Kekerengu Fault which vibrated some 10 metres, before rupturing the newly discovered Waipapa Bay Fault, the Hundalee Fault, and ending its massive vibration at the western end of the 230km-long Hope Fault, which connects to the South Island's main Alpine Fault.

While there have been thousands of aftershocks since the initial 'big two', only six have been classed as severe by GeoNet, with the last really big one a magnitude 6.3 near Cheviot on Monday afternoon.

GNS Science seismologist Bill Fry says: "Aftershock activity does tend to spread out over time. The very first few hours after a big earthquake we see aftershocks clustering around the original fault plains and then the days and weeks after we tend to see a migration outward of the activity."

The South Island's main Alpine Fault is due for a major shake, with many seismologists predicting there could be up to a magnitude 8 earthquake somewhere along its 600km length sometime this century.

GNS Science says there is a 30 percent chance of the Alpine Fault rupturing in the next 50 years.


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