Page 427 - Caldera Volcanism Analysis, Modelling and Response

402 Micol Todesco

elemental sulphur may also occur upon cooling (heating) (Giggenbach, 1996;
Delmelle and Stix, 2000; Symonds et al., 2001; Oppenheimer, 2003, and references
therein). Surface hydrothermal features will therefore depend on the relative
proportion of magmatic volatiles with respect to shallower fluids, and on whether
or not thermal and chemical equilibrium among different components has been
achieved. This in turn may depend on rock permeability, which controls fluid
mobility and determines the extent and duration of the interaction between
different fluid components. Temperature, discharge rate and composition of surface
hydrothermal features may change as any element of these complex systems
changes, either at the magma chamber level, in the groundwater system or in rocks
hosting them both. As a consequence, interpretation of geochemical data can be
highly controversial. Sophisticated theoretical tools are needed to estimate the
conditions at which chemical equilibrium was attained, or to quantify the departure
from such equilibrium conditions (Giggenbach, 1996; Chiodini and Marini, 1998;
Oppenheimer, 2003).

4.2. Geophysical monitoring
The role of hydrothermal fluids in volcanic surveillance goes beyond the informa-
tion we can obtain through geochemistry. The presence of hot fluids alters rock
properties and affects the response of the entire volcanic edifice to thermal and
mechanical changes. Seismic wave velocity is known to depend on the presence of
pore fluids, and different degrees of seismic attenuation are expected in gas- and
liquid-dominated regions. Anisotropy associated with fluid-filled microcracks
is known to generate shear wave splitting. Shear wave splitting parameters
(polarisation and time delay between split waves) change with stress distribution,
and these are increasingly adopted as a tool in volcano monitoring (Miller and
Savage, 2001; Crampin and Chastin, 2003; Bianco et al., 2004). Although
volcanological applications mostly focus on stress changes induced by magma
intrusion, changes in polarisation of the faster split shear wave may also arise
as a consequence of increased pore pressure, as observed during injections in hot,
dry-rock geothermal reservoirs or in oil fields (Cramping and Booth, 1989;
Angerer et al., 2002; Crampin and Chastin, 2003).
Hydrothermal fluids can also trigger shallow seismicity. Elevated pore pressure
reduces the effective normal stress, favouring focused stress release. Where rocks are
close to failure, pore pressure perturbations may drive seismicity, even in non-
volcanic areas (Shapiro et al., 2003; Miller et al., 2004). Microseismicity is
commonly observed at geothermal fields during fluid injection in boreholes
(Maillot et al., 1999). In volcanic areas, hydrothermal fluids can trigger, or
participate to the generation of, long- and very-long-period seismic events and
volcanic tremor (Newhall and Dzurisin, 1988; Chouet, 1996; Hellweg, 2000;
Konstantinou and Schlindwein, 2002; Bianco et al., 2004; De Angelis and McNutt,
2005). Banded tremor correlated with hydrothermal and geyser activity has been
observed at Yellowstone and at other locations (Newhall and Dzurisin, 1988, and
reference therein). Pressurisation of hydrothermal fluids, possibly associated with
shallow magma intrusion, has been invoked as a possible source for long-period