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Super-strong crust a key factor in Canterbury quakes - 26/11/2013

Newly published research has shown that the Canterbury earthquakes were even more unusual than first thought and such a sequence is unlikely to occur anywhere else in the world.

The research, published this week in Nature Geoscience, challenges the common assumption that the strength of the Earth’s crust is constant by demonstrating that energetic quakes, such as those in Canterbury, can cause widespread weakening of the crust.

Scientists, led by seismologist Martin Reyners of GNS Science, had initially set out to determine the three-dimensional structure of the crust under Canterbury by using a technique called seismic tomography – similar to a medical CAT scan. This helps to get more accurate aftershock locations, to better define the many smaller faults that ruptured.

Instead, they found that rock properties had changed significantly over a wide area around the Greendale Fault, which ruptured on 4 September 2010 producing a magnitude 7.1 quake.

“This finding was entirely unexpected, but it explains why the main shock released so much energy,” Dr Reyners said.

Prior to this research, scientists had assumed that the strength of the Earth’s crust remains constant during an aftershock sequence.

The Canterbury quakes had their genesis 100 million years ago when very strong rocks became emplaced under Canterbury.

“It is important to realise that the Canterbury earthquake sequence was very unusual, with energetic earthquakes producing some of the strongest vertical ground accelerations ever seen in an earthquake.

“This is a result of the unusual rock structure of the region. There will be few other places in the world where a similar earthquake sequence might occur.”

Most of the quakes in the two-year-long Canterbury sequence released abnormally high levels of energy; this was consistent with the ruptures occurring on very strong faults that store energy slowly and gradually and are hard to break.

Dr Reyners said post-quake analysis such as this research was important as it helps to understand how strain builds up in the crust and how it is released during earthquakes.

“But to do that accurately, we need to understand the types of rocks that exist at depth. Strong rocks store and release strain differently to weak rocks.”

The research involved analysing the seismic waves produced by 11,500 aftershocks in Canterbury. This enabled the team to build a 3D picture of rock structure to a depth of about 35km below the surface.

Normally rocks become hot and ‘plastic’ at depths of about 10km. However, the researchers found that strong, brittle rocks continued to a depth of about 30km under Canterbury.

This unusually thick and dense slab of rock helps to explain the long and energetic aftershock sequence in Canterbury.

The researchers are continuing their work and are now focussed on determining how widespread this strong rock unit is in the lower half of the South Island.

“This is important for defining the earthquake hazard for people living between mid-Canterbury and Southland.”