Underground monitoring scientists' best predictor

Seismometers help researchers gauge tremors beneath the earth. Photo / Supplied

Below the din of the city, ultra-sensitive ears give scientists the best chance of being able to pick the next Auckland eruption before it comes.

Seismic monitoring is the most effective tool scientists have to monitor what Auckland's 50 volcanoes are doing, and six seismometers placed in boreholes can sort subtle tremors from the sound of wind and traffic on the surface.

Lowered to depths between 150m and 400m, seismometers would be the first to identify a potential eruption when they recorded vibrations caused by rising magma.

With data constantly streaming to GeoNet, the devices track the arrival time and strength of each tremor, allowing scientists to differentiate patterns between volcanic and non-volcanic.

They can also pick up tiny shifts in frequency related to changing dynamics of magma or gas movement within the volcano, or the amount and style of volcanic tremor.

The majority of what are termed microearthquakes are too small to be felt at the surface, yet carefully measuring them has given scientists vital clues in piecing together subterranean fractures and faults.

Geological data are also collected from council logs from around 2000 boreholes across the city.

"There's a wealth of information in boreholes, which if you can collate, can be very, very valuable," said Auckland University volcanologist Dr Jan Lindsay.

One of the most interesting findings so far was a lack of greywacke rock beneath central Auckland, leaving scientists to believe the city straddles a terrain boundary made of highly faulted and shattered rock.

"That may in fact be one of the reasons we've got the volcanic field there, because the magma might be able to pass through these faults."

In another case, data that revealed a thick layer of basalt lava helped Bruce Hayward of Geomarine Research rediscover the Grafton volcano.

"Another thing we are trying to do with the borehole network is to look at the distribution of potential aquifers around Auckland to see what the likelihood of magma and water interaction would be in a future eruption."

Such a combination could lead to an explosive phreatomagmatic eruption.

And in a city without quakes, boreholes remained one of the best insights into what made up Auckland's crust, she said.

"If you have lots of quakes, you can work out what type of rock is beneath your feet based on the way the seismic waves travel, but because we have hardly any earthquakes, we don't have that opportunity, so it's hard to know exactly."