Earthquake's power halved by rocks
Christchurch may have experienced a much larger earthquake in September 2010 if it were not for The Gap, new research suggests.
A team of local and American scientists has uncovered evidence that the Greendale and Port Hills faults are "moderately" connected several kilometres underground.
Lead researcher Yong-Gang Li, of the University of Southern California, said that, if the two faults had ruptured together along their combined length of about 70 kilometres at 4.35am on September 4, 2010, they could have generated a quake of up to magnitude 7.3 from the Canterbury Plains under the city to the sea, rather than the magnitude-7.1 shake located further away near Darfield.
Instead, rock structures in the "step-over zone" at the eastern end of the Greendale Fault and in the area around Prebbleton and Lincoln known as The Gap, stopped the rupture in its tracks.
A magnitude-7.3 quake releases about twice as much seismic energy as a 7.1 quake.
Canterbury University scientists Mark Quigley, Greg De Pascale and Darren Gravely have worked on the study with Li.
Quigley said the Greendale and Port Hills faults had already relieved strain accumulated over many thousands of years.
"The questions we're asking pertain to areas that have contributed less total seismic energy throughout the sequence and their potential to generate future quakes," he said.
Li said yesterday that the discovery of the link between the two faults had come as a surprise.
Other scientists have discounted theories the two faults are connected.
Li said two sets of seismic equipment were deployed after the February 22, 2011, quake.
One array was placed across the central Greendale Fault where the fault had broken through to the surface, and the second was installed at Princess Margaret Hospital in Cashmere above the blind Port Hills Fault.
From May until September last year, the sets of seismometers recorded more than 1000 aftershocks of between magnitudes 1.0 and 6.0.
Analysis of the data had identified waves of energy, from aftershocks on both faults, trapped within the fault zones and travelling along them, he said.
"From the array deployed across Greendale, we observed those [Greendale Fault] aftershocks clearly, but we also found that the recorder recorded aftershocks that occurred on the Port Hills Fault," he said.
"It was a surprise.
"Our interpretation is that between the two faults there is a connection at depth - seismic depth somewhere below 3km - [showing] the faults are moderately connected."
The trapped waves suggested the Greendale Fault extended east underground another 5km to 8km.
The main rupture of the Port Hills Fault also looked like it extended further west close to the Greendale Fault.
Quigley said one of the big questions in quake science was "what stops a rupture in its tracks?".
The crust between the two faults was "somewhat complicated" and was likely to act as a "rupture barrier".
"There appears to be some buried thrusts near the eastern end of the Greendale Fault system that would have helped to accommodate much of the displacement during the Darfield earthquake," he said.
"Given the significant stress drop in the Darfield earthquake, it would seem that, if this was a crossable barrier, then it could have been breached.
"What this data tells us is that there is some ‘connectivity' between the faults in the sense that the complicated fault network between them allows for some of the seismic waves generated by earthquakes to be trapped and ‘guided'."
- © Fairfax NZ News
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