Volcanic unrest can be hazardous even if no eruption occurs. Injuries and fatalities have occurred at calderas around the world during unrest episodes, as well as during periods of quiescence. The physical hazards described below are no different to those seen at other types of volcanoes during unrest. Information relating to caldera unrest hazards needs to be
communicated before and during unrest episodes. A combination of the following unrest phenomena may occur at varying levels of severity and frequency.
Ground shaking
Earthquakes are the most common expression of volcanic unrest and eruptive activity (Newhall & Dzurisin, 1988). Volcanic processes generate a wide variety of seismicity. These may be reflecting sub-surface processes such as the movement of magma and/or related fluids and gases, eruptive activity, or post-eruption readjustment (McNutt, 2000). Earthquakes near volcanoes are termed ‘volcanic earthquakes’ and can have the same impacts on society as tectonic earthquakes. In some cases seismicity is only detected if monitoring is adequate, while in other cases it will be felt locally and may cause alarm. Earthquakes can occur in swarms, which are defined by McNutt (2000, p. 1095) as “a group of many earthquakes of similar size occurring closely clustered in space and time with no dominant main shock”; or they can be isolated events, affecting localised areas.
Volcanogenic earthquakes are thought to predominantly occur at a magnitude of two to three, and rarely exceed magnitude five (Richter scale). Larger earthquakes can also occur during unrest; for example, multiple magnitude six earthquakes occurred during a caldera unrest episode at Long Valley Caldera, California, U.S., in 1980 (Mader & Blair, 1987). Long Valley Caldera contains the small ski resort town of Mammoth Lakes, which is the tourism base for the popular ski field on Mammoth Mountain. Mammoth Lakes experienced three magnitude
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six earthquakes within one day in May 1980, causing scientific concern of volcanic unrest (Hill, 1998). Following years of continued unrest and perceived impacts on the business and tourist industry (discussed further below), a particularly intense seismic swarm caused power outages in 1983. While there were no reported casualties, these earthquakes prompted volcano- related emergency plans to be made and an alternative road to be built for potential future evacuations (Mader & Blair, 1987; Hill, 1998).
Earthquakes during eruptions have caused deaths at volcanoes due to building collapse (or partial collapse; Blong, 1984), and they can cause structural and infrastructure damage (Johnston, 1997; Zobin, 2001). Collapsing brick chimneys can fall through building roofs; the rupturing of gas lines and electrical circuits may lead to fire; and broken water pipes can cause flooding (Blong, 1984). Liquefaction can occur in areas with sand and gravel substrates, especially near low gradient waterways, if the earthquakes are of sufficient magnitude. Fissures can be formed on the ground surface, potentially causing damage to roads and destruction of buildings and underground services.
Ground deformation
Ground deformation at volcanic centres can take place as a result of magmatic processes, such as subterranean magma movements, occurring before, during, and after eruptions (e.g., Murray et al., 2000). As volcanoes, particularly large caldera systems, tend to lie in active tectonic environments they may also be influenced by regional deformation, such as rifting. Horizontal and vertical deformation can cause damage to buildings and infrastructure but are not usually directly life threatening. The deformation can range from millimetres to metres, can affect a wide area, and may cause fissures (e.g., Murray et al., 2000). Uplift and subsidence can cause flooding through altered water courses, or from ground subsidence below the water level (as seen at TVC in 1922 (Morgan, 1923), and in Pozzuoli, Campi Flegrei over a number of centuries (e.g., Dvorak & Mastrolorenzo, 1991; Bellucci et al., 2006)).
Uplift was observed during unrest at Rabaul Caldera, as described by McKee et al. (1985), when the rate increased from a background level of 8 mm per month in the 1970's to an average rate of 50 mm per month from November 1983 until May 1984. The maximum amount of uplift during an individual crisis was 100 mm, while the total amount of uplift between 1971 and 1984 was 3.5 m (McKee et al., 1985). Occasional episodes of increased seismicity and deformation were observed over the next decade, and the 1994 eruption was preceded by 6 m of uplift in a space of a few hours (Nairn & Scott, 1995). This example
demonstrates the potential rate of deformation and total uplift which may be seen during unrest at calderas.
Poisonous gas emissions
Volcanic gases are commonly emitted at volcanoes and geothermal areas through fumaroles, hot springs, and other areas of the ground surface (Stix & Gaonac’h, 2000). Volcanic gases include carbon monoxide (CO), carbon dioxide (CO2), sulphur dioxide (SO2), hydrochloric acid
(HCl), hydrofluoric acid (HF), hydrogen sulphide (H2S), and radon (Rn), as well as heavy metals
and water (H2O) (Stix & Gaonac’h, 2000). Interpretation of geochemical data from monitoring
volcanic fluids and gases can indicate the source and movement of magma, interactions with hydrothermal and meteorological fluids, and types of potential eruptions (Stix & Gaonac’h, 2000).
Documented health effects of volcanic gases include discomfort and/or asphyxiation due to the accumulation of gases in topographic lows; respiratory effects (and occasionally deaths) from exposure to acidic sulphate aerosols formed from sulphur dioxide, primarily in
geothermal areas; as well as long-term health effects (e.g., Blong, 1984; Hansell &
Oppenheimer, 2004). Many casualties caused by volcanic gas exposure were effected during volcanic unrest, rather than eruptions (Blong, 1984).
High levels of gas in soil during unrest can cause areas of vegetation to die, and can impact animal life. This has been observed during unrest at Mammoth Mountain (e.g., Sorey et al., 2000; Hill, 2006); Rotorua Caldera (Durand, 2007); Furnas Volcano (Azores, Fructuoso, 1583, cited in Baxter et al., 1999); and Rabaul Caldera (Fisher, 1939, cited in McKee et al., 1985).
Approximately 70,000 people reside on an active geothermal field at Rotorua Caldera,
New Zealand. Potentially dangerous and damaging levels of gas are emitted in parts of Rotorua city, discharged from cracks in paving, waste water drains, in low and narrow spaces, and inside buildings (Durand & Scott, 2005). 14 people have been killed in Rotorua by H2S and CO2
gas poisoning or asphyxiation in the past century (Durand & Scott, 2005). These casualties have occurred in small, low, contained spaces such as natural hot spa baths.
In Cameroon, 1986, approximately 1700 people were killed by volcanic gas when a build-up of CO2 was released suddenly from the waters of Lake Nyos (at the summit of a volcano) and
flowed down the slopes to a nearby town (Baxter et al., 1989). While there is a Crater Lake at Ruapehu, based on data from 1991, Christenson (1994) found that a Lake Nyos-type gas
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release is unlikely at that volcano. At Rabaul in 1990, CO2 gas killed six people who were
collecting eggs in a small depression (Rabaul Volcano Observatory, 1990). At Mammoth Mountain, three members of a ski patrol died from toxic levels of CO2 in 2006 after falling into
a snow cave melted by a fumarole (Cantrell & Young, 2009).
Further information on health hazards resulting from volcanic gases is provided by Hansell and Oppenheimer (2004) and the International Volcanic Health Hazard Network (www.ivhhn.org).
Geothermal system changes
A magma body may provide additional heat, gas, and fluids, which interact with the overlying geothermal system during volcanic unrest. This can result in changes to the flow, temperature, and/or chemistry of fumaroles and springs. Ground shaking (from tectonic or volcanic
processes) can also alter underground cracks and pressure systems, resulting in hydrothermal changes (Vandemeulebrouck et al., 2008). As the temperature and/or pressure of the
geothermal system increases, activity at surface features and gas emissions can increase, and potentially result in hydrothermal explosions (Browne & Lawless, 2001). No magma is erupted in a hydrothermal eruption, by definition (Nairn, 2002; Nairn et al., 2005), however they can still be very dangerous, as demonstrated by the prehistoric hydrothermal eruption from Rotokawa (approx. 10 km northeast of Taupo township, and TVC) 6060 ± 60 years ago, which deposited ejecta over an area with a diameter of 4 km (Browne & Lawless, 2001). Many of the casualties from the 1886 Tarawera rift eruption in OVC were caused by a large magmatic- hydrothermal eruption at Lake Rotomahana (Nairn, 1979; Simmons et al., 1993; Browne & Lawless, 2001).
As a complication for volcanic eruption forecasting, hydrothermal explosions can also occur without the influence of magma, from changes in rainfall, barometric pressure, landslides, earthquakes, or due to exploitation (e.g., drilling and fluid extraction) (Bromley & Mongillo, 1994). Additionally, inadequate borehole maintenance can cause failure of the casing and the leaking of hot fluids, resulting in hydrothermal eruptions in residential areas, as occasionally occur in Rotorua city.
2.2.2.2 Socioeconomic impacts of caldera unrest