Aso volcano

Hydrothermal cracks, filled with water, steam, or a combination of nonmagmatic fluids, may also be the source of VLP events. Unsteady flow of pressurized hydrothermal fluids through cracks (Nakamichi et al. 2009 Ohminato 2006) can produce VLP events that are very similar to those produced in magmatic systems. The low viscosity of water and steam relative to magma inhibits measureable fiow-related frictional single forces, so mechanisms tend to be dominated by volume changes in cracks. At Aso volcano, nearly continuous VLP tremor is attributed to resonance in a horizontal crack (Kaneshima et al. 1996). 

Kaneshima S, Kawakatsu H, Matsubayashi H, Oikawa J, Takeo M, lidaka T, Yamasato H (1996) Mechanism of phreatic eruptions at Aso volcano infrared from near-field broadband seismic observations. Science 273(5275) 642-645. doi 10.1126/science. 273.5275.643... 

Fig. 15.3 Upper figure Raw data showing 1 Hz time series of instrument output for electrochemical sensors S02-AF, H2S-A1, N02-A1, CO-AX, CO-AF and HCl-A1 at Aso volcano for...

Fig. 15.4 ScatlCT plots of the gas mixing ratios derived from the electrochemical sensor data of Fig. 15.3, shown as X vctsus SO2 where X is H2, CO or H2S. The estimate of H2S from N02-A1 sensor is preferred over that of H2S-A1 sensor which exhibits enhanced scatter see text for explanation. Linear regression is used to determine the characteristic gas ratios in the Aso volcano fumarole emission...

Nevertheless, sensor cross-sensitivities can also bring new opportunities for gas measurement. Roberts et al. deployed an NO2 sensor in the plumes of Japanese volcanoes Aso and Miyakejima, and, noting that NO2 concentrations were much less than H2S, used the sensor s cross-sensitivity to measure H2S abundance. The sensor presented advantages over traditional H2S sensors because it did not exhibit a cross-sensitivity to SO2, and also because the time-response of the NO2 sensor (to H2S) is more similar to the SO2 sensor (see further discussion of measurement uncertainties below). In the same study Roberts et al. also co-deployed two CO sensors, both of which exhibited cross-sensitivities to H2. By co-analysing the two sensor outputs (which contrasted in their degree of cross-sensitivity), both CO and H2 abundances could be determined in the volcanic plume. 

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