Quake scientists mind The Gap

The mysteries of "The Gap" and its earthquake-generating potential remain unresolved two years from the start of Canterbury's violent rumblings.

Research is continuing in the area around Prebbleton and Lincoln, with some scientists warning that careful monitoring is crucial to discover whether hidden faults there could generate a large quake.

However, research funding is lacking for what could be a critical study for Christchurch.

"The Gap" is both a physical void - an area of land between the eastern end of the Greendale Fault and the western end of the Port Hills Fault - and a zone where scientists have calculated there is a shortfall in the amount of energy released by quakes compared with that released close to the two big faults on either side.

The area has generated two quakes of magnitude 5.0 or higher and more than 30 between 4.0 and 5.0 in the past two years.

A special edition of the New Zealand Journal of Geology and Geophysics has been compiled to mark the second anniversary of the first big quake and analyses the characteristics and impacts of the Canterbury earthquake sequence.

Co-editor Mark Quigley, of Canterbury University's geological sciences department, said lessons from the long-lived and complex aftershock sequence, and their effects on structures, were relevant to quake-prone areas around the world.

One of the volume's key findings related to "the gap".

"Studies highlight an area west of Christchurch that has not produced substantial moment [energy] release in the current earthquake sequence," he said.

"The significance of this can only be resolved by more studies of the fault structure in this area."

GNS Science geophysicist John Beavan has co-authored a paper measuring fault slip across Canterbury since the quakes, using satellite radar images and GPS (global positioning system).

He and his co-authors have looked at "the gap" and why it has not released as much energy as might have been expected.

"There are possible reasons why this region will not fail in an earthquake or earthquakes during the present sequence. For example, faults in this region may have failed relatively recently compared to the faults that have broken in the present sequence, so even the increases in stress due to the recent earthquakes are insufficient to cause failure," the paper said.

"But the truth is that we do not know whether or not this region is likely to fail in the near future, nor, if it did, whether it would be as a single earthquake or as several smaller earthquakes.

"We think it is of high importance to continue and expand ongoing . . . monitoring of this region in case this will provide clues to its future behaviour."

Beavan told The Press the research group was trying to get funding from the Government's natural hazards platform to carry out this work.

"In order to see what is happening, we have started to do some GPS surveys in that ‘gap' area. We started in February, took some measurements in April and finished a third survey [last] week."

It would be up to a year before any initial conclusions could be drawn from the approximately four-monthly surveys of 30 points, he said.

Quigley said other findings outlined in the bulletin included:

❏ Evidence in the geologic record of fault systems very similar to the Greendale Fault system elsewhere in Canterbury, with "blind thrusts" linked to major strike-slip faults.

❏ Geophysical and geologic evidence for other active faults around Canterbury, including some with minimal evidence for surface rupturing.

❏ Major earthquakes that occurred in 1869 and 1870, less than 15 years after Christchurch became a city in 1856, generated strong local ground accelerations. The earlier quake could have caused large enough ground accelerations to initiate liquefaction and disturbance to the Avon River.

The Press