Monitoring

Auckland volcano seismic monitoring network

What is it for?

The Auckland Volcano-Seismic Monitoring Network (AVSN) is designed to monitor seismic activity associated with the possible onset of a volcanic eruption.

Much of Auckland is built on a potentially active volcanic field. Between Manurewa (South), and Takapuna (North), there are about 50 volcanoes. None of these existing volcanoes is expected to erupt again. The next eruption will be in a new, unknown location.

By monitoring the seismicity of the volcanic field, an early warning of an impending volcanic eruption can be given.

Knowing when and where the next eruption will occur will help Civil Defence and other emergency services decide where help and support is required.

Where is it, and how does it work?

The AVSN currently consists of five sites at which seismic activity is monitored. The data collected is radioed to a central recording site at the Regional Council.

1. As magma rises towards the surface, the earth vibrates.
2. These vibrations (or pressure waves) travel through the ground, and are measured by sensitive motion detectors called seismometer. Each seismometer will detect the arrival time and strength of each wave.
3. Seismometers send the data they record via radio to a central recording unit at the Auckland Regional Council. If there is no direct line of sight between the seismometer location and the Auckland Regional Council, a radio repeater must be used.
4. The data is received, digitally recorded, and visually displayed on drum recorders.
5. The data is analysed, and the location of any earthquakes and tremors is calculated. The analysis will also determine whether or not the earthquake is of volcanic origin. 'Volcanic earthquakes and tremors' look different to 'tectonic earthquakes' associated with the release of strain in the earth's crust (often along faults).

There are other ways of monitoring volcanoes, such as deformation monitoring (measuring any bulges in the earth's surface caused by magma rising), and measuring the amount and chemistry of volcanic gases (associated more with monitoring frequently active volcanoes). Predicting what might happen based on changes in the pattern of seismicity, deformation, or chemistry of gases is difficult and there is always a lack of certainty in predicting hazard activity.

How was it achieved?

The AVSN was established in the mid-1980's after a working party recommended that the most suitable pre-eruption warning system would be a seismic monitoring network. In 1993 the Auckland Regional Council made a commitment to maintain an effective operational network and a major upgrade commenced.

The network initially consisted of two sites, Nihotupu/Waiatarua in the Waitakere Ranges to the west of Auckland, and Moumoukai in the Hunua Ranges south-east of Auckland. Motutapu was added in February 1995, Otara in September 1996, and Kauri Point in June 1998.

Digital recording commenced in December 1994. While ARC staff carry out daily visual assessments of the data, the Institute of Geological and Nuclear Sciences (GNS) - on behalf of the Auckland Regional Council - carry out analysis of the digital data.

The Auckland Regional Council would like to acknowledge the technical support provided by the Institute of Geological and Nuclear Sciences Ltd and the University of Auckland.

References and further reading:

• Cox, G.J. (1989) Fountains of Fire: The Story of Auckland's Volcanoes. William Collins Publishers Ltd, Auckland
• Johnston, D.M., Nairn, I.A., Thordarson, T., Daly, M. (1997) Volcanic Impact Assessment for the Auckland Volcanic Field. Auckland Regional Council Technical Publication No. 79, April 1997
• Smith, I.E.M. & Allen, S.R. (1993) Volcanic Hazards at the Auckland Volcanic Field. Civil Defence Volcanic Hazards Information Series, No. 5, CD 304
• Hazardfacts in the volcano series: H01 (Auckland's Volcanic Field), H02 (How do Auckland Volcanoes Form?), H03 (Rangitoto: Auckland's Youngest Volcano), H05 (Auckland Volcano-Seismic Monitoring Network), H10 (Volcanic Eruption in Auckland - Civil Defence advice).