Dec 12 2008 - Monitoring activity at Taupo

Taupo Volcano is a large caldera that has had highly explosive and voluminous eruptions, including the Taupo eruption in around 181 AD and the Oruanui eruption about 26,500 years ago. There have also been more than 20 much smaller eruptions in the last 25,000 years. The most recent eruptive activity was about 1,700 years ago.

Volcanic unrest at Taupo

Deformation at TGWH mark.

Deformation of over 5 mm at TGWH mark on the northern shore of Lake Taupo during September/October 2007.

Lake Taupo sits in a complex tectonic setting: regional east-west rifting of the Taupo Volcanic Zone is overprinted by two calderas. Additionally, there is a vigorous hydrothermal system active above the main vent area for the Taupo eruption.

Over the past 100 years there have been several periods of unrest associated with the caldera:

  • 1895: “Tremors” over a period of 3-6 weeks.
  • 1922: Felt earthquakes over 7 months. 2-3 m of deformation north of the lake, although offsets could have been partly due to landslips.
  • 1964-5: > 1,100 earthquakes with magnitude > 2.7 in two months. No observed surface deformation.
  • 1983: 50 mm of uplift over 3 months followed by equivalent subsidence, with accompanying earthquake swarms (10-30 events per day).
  • 1996-9: 40 mm of uplift in centre of lake over three years.

Earthquake activity at Taupo.

Earthquake activity at Taupo from April to October 2008.

Recent GeoNet data

Since the start of GeoNet in 2001, the New Zealand volcano monitoring networks have been upgraded significantly. This has included installation of 6 continuous Global Positioning System (cGPS) stations and 8 seismic stations around Lake Taupo. Previously there was a single cGPS station at Taupo Airport. Prior to 1985 seismic monitoring was undertaken by a single seismometer augmented by temporary networks. Later a small local network was used, though before 1994 those data were not integrated into the New Zealand earthquake catalogue. Deformation monitoring around Lake Taupo has been ongoing for over 30 years by using lake levelling at a number of sites.

Over the last 12 months, changes in the seismicity and deformation under Lake Taupo have been monitored. In September-October 2007, deformation recorded on the new cGPS network showed changes of up to 5-10 mm. This has been modelled as an inflating shallow source under the lake. Associated earthquake swarms (up to 100 events per month) occurred from April to October 2008 associated with another broader uplift signal, and were distributed over a wide area in and around the caldera. This magnitude of deformation and seismicity is not uncommon at Taupo Volcano, and can now be better resolved with the enhanced network coverage.

In comparison to previous periods of unrest at Taupo, the intensity of seismicity and magnitude of deformation during 2008 are relatively low. Indeed, by looking at the longest cGPS record and long term seismicity, it appears that the recent activity is similar, but slightly higher than pulses of activity over the last 10 years. This style of activity may indeed be commonplace.

The enhanced capability of GeoNet monitoring to detect lower magnitude earthquakes and smaller offsets in deformation is giving us the ability to model and understand the complex interaction between regional rifting episodes and caldera inflation.

Currently the level of monitored activity is not high, but the dataset is providing us with enhanced knowledge of baseline variation against which future unrest can be measured.