Gravity study to help plot fault lines under city

PAUL GORMAN

Last updated 05:00 12/04/2011

 

Subtle changes in gravity and the magnetic field around Christchurch after the two big earthquakes will be measured to help find out what is going on underneath the city.

Scientists from Canterbury University and Canada's Calgary University are undertaking seismic surveying to map quake-generating faults that could be hidden under hundreds of metres of river gravels and sediments.

Crown research institute GNS Science will today start a two-week survey of gravity across the city and the Canterbury Plains.

Project leader Vaughan Stagpoole said the investigation would use a sensitive briefcase-sized meter to measure small changes in gravity due to different rock densities.

"Gravity acting on the Earth's surface varies because of the change in density directly below where you are. Denser rocks have higher gravity; low-density rocks have less gravity," he said. "When you have a fault line or steps in dense rocks, you get a steady change in gravity across the fault."

Several hundred gravity measurements already existed for Canterbury, but some dated back a few decades, he said.

The spacing between the data points was uneven, which meant knowledge of the subsurface geology varied from place to place. The new work would collect more uniform readings at up to 200 locations where quake activity had occurred since last September, including on the Port Hills.

"Typically, we place the instrument beside a road or a farm track and it takes just a few minutes at each location to record the gravity," he said.

GNS is also co-ordinating an aerial survey of Christchurch to record magnetic variations.