Potter, S.H.; Scott, B.J.; Jolly, G.E. 2012 Caldera unrest management sourcebook. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/12 66 p.
Abstract: This report summarises the current understanding of the eruption histories of New Zealand’s 11 most recently active calderas – Raoul Island, Macauley Island, Mayor Island, Okataina, Rotorua, Kapenga, Reporoa, Ohakuri, Mangakino, Whakamaru and Taupo Calderas. Due to the large range of eruption sizes and styles from each of the calderas in the past, especially Taupo, it is extremely difficult to predict the size and style of the next eruption. While eruptions at the calderas are relatively infrequent, volcanic unrest caused by magma and fluids moving underground and regional stress adjustment occurs more frequently. Most unrest episodes at calderas do not result in an eruption (Newhall & Dzurisin, 1988). Due to their frequency unrest episodes have been documented globally, including in New Zealand. A summary of the known unrest episodes which have occurred in New Zealand is included in section 3 of this report for each caldera. Although unrest episodes are relatively frequent, they usually do not leave any trace in the geological record (except the occasional surface fault rupture, and large hydrothermal eruption deposits), so the knowledge is largely restricted to areas and times of human occupation. In New Zealand’s case, this is reasonably limited, therefore to gain an understanding of what the unrest indicators may look like before future caldera eruptions in New Zealand, we must look overseas to countries with similar volcanoes and longer histories of settlement. This report provides descriptions of eruptions and unrest at rhyolitic calderas similar to New Zealand’s including at Campi Flegrei (Italy), Long Valley (U.S.A.), Rabaul (Papua New Guinea), Chaitén (Chile), Aira (Japan), Taal (Indonesia), Novarupta (Alaska, U.S.A.) and Yellowstone Volcanic Centre (U.S.A.). Yellowstone is similar to the Taupo Volcanic Zone as both have had a similar discharge rate of magma in the past 2.2 million years and are a similar size, however the TVZ has had more frequent and smaller eruptions than Yellowstone (Houghton et al., 1995a). Unrest phenomena may include seismicity, ground deformation and changes in the hydrothermal systems. These have the potential to be hazardous, damage buildings and infrastructure, and can result in psychosocial and economic impacts, all of which have occurred at Taupo Caldera in the past 160 years, as well as overseas (for example at Long Valley and Campi Flegrei Calderas). Unrest episodes can last for hours to decades. They need to be carefully managed by the CDEM sector, responding agencies, local and regional government, media, the public and scientists, even if there is no resulting eruption. Calderas with long periods of quiescence are particularly difficult to manage due to the public, media and public officials not fully recognising the hazards of the volcano, and the potential size and style range of any future eruption. There will be high levels of uncertainty for all groups, particularly as to the outcome of the unre st episode. This highlights the need for developing excellent pre-event interagency communication and cooperation as well as with the public and media. In this report, we will discuss the physical, social and economic impacts of unrest and outline some of the implications for management of an unrest episode. (auth)