Study casts light on quake impact

The catastrophic Christchurch earthquake did not cause any significant changes in the tectonic stresses on the region as a whole. Photo / Greg Bowker

Data from Christchurch likely to help scientists understand what triggers earthquakes.

It killed 185 people and devastated a city, but the catastrophic Christchurch earthquake did not cause any significant changes in the tectonic stresses on the region as a whole, scientists have discovered.

The finding is a key outcome of the largest study to date of the tectonic stress field along the Australia-Pacific plate boundary.

The study also produced the first 3D map of tectonic stress parameters throughout New Zealand.

Using a dataset made up of more than 3400 "focal mechanisms" - indicators which show what types of earth movement have occurred in individual earthquakes, based on high-quality seismic data recorded by GeoNet - seismologists formed a comprehensive picture of tectonic stress at 100 sites across the country.

It would prove one useful baseline for scientists as they try to understand how the Earth deforms and what triggers our earthquakes, said study author and Victoria University geophysicist John Townend.

"What we are particularly interested in is the overall effect of stresses acting on faults - whether they are pushing the sides of a fault together or causing it to shear, producing an earthquake-type motion."

The Canterbury earthquake sequence of 2010 to 2012 renewed the need to understand the tectonic stress field in New Zealand, specifically in the central South Island.

According to a 2011 study, the Christchurch earthquake of February 22 involved a "stress drop" - the difference between the stress imposed on a fault before and after an earthquake - higher than that of most continental earthquakes. Researchers suggested because of that the Christchurch quake might have been expected to produce larger changes in the surrounding stress field than other earthquakes of comparable magnitudes.

But having analysed data before and after the February quake, the Victoria University and GNS Science researchers instead found the high stress drop produced by the earthquake was still not enough to alter the stress field in the surrounding region.

"The quake was absolutely devastating because it was located so close to Christchurch and caused very strong accelerations - but the effects of that are very hard to detect in terms of stress, once you go a few kilometres from the quake," Dr Townend said.

"What we have done is use a large number of earthquakes as pressure gauges to work out how tectonic stress varies throughout New Zealand."

With ongoing technical developments in seismic monitoring, scientists are developing an ever more detailed understanding of how faults work, and Dr Townend said the new study would help to build the picture.

"The actual triggers for earthquakes appear to be quite small, in comparison with the overall stresses the Earth's crust is under, which makes understanding how one earthquake triggers another such an important challenge."

What is the tectonic stress field?

In examining the stress field, geophysicists try to work out the effects of different loads on the rocks that form the outermost 30-50km of the earth, known as the crust.

In New Zealand, which sits astride the Australia-Pacific plate boundary, these "loads" are produced by plate tectonic processes such as mountain-building in the Southern Alps, subduction beneath the North Island and volcanic processes in the Taupo Volcanic Zone.

Victoria University geophysicist John Townend gives the examples of standing on a table, or bending a pencil, to the point where the load overcomes the strength of the object, causing it to break.