Geophysical Research Letters
Supporting Information for
Improved detection of pre-eruptive seismic velocity drops at the Piton de La
Fournaise volcano
Diane Rivet1, Florent Brenguier2 and Frédéric Cappa1
1
Géoazur, Université Nice Sophia Antipolis, Campus Azur, 250 rue Albert Einstein, Les Lucioles 1, 06560
Sophia-Antipolis, France
2
Institut des Sciences de la Terre, Université Joseph Fourier, Maison des Géosciences, BP 53, 38041
Grenoble, France
Contents of this file
Text S1
Figures S2 to S3
Introduction
Supporting information concerns two aspects of the papers: (1) the estimation of the hydraulic
diffusivity of basalt rock at Piton de la Fournaise volcano based on hydraulic properties found in
the literature, and (2) additional figures on the process of estimating a synthetic seismic
velocity change time series produced by fluid pressure change.
Text S1. Estimation of the hydraulic diffusivity of basalt rock at Piton de la Fournaise
volcano
Specific storage
S = r g(a + fb )
ρ (density of water) = 1000 (kg/m3)
g (gravitational acceleration) = 9.81 (m/s2)
α (rock compressibility) = ~10-8 (m2/N or Pa-1) for fractured rock [Freeze and Cherry, 1979]
ϕ (porosity) = 0.24 (-) [Fontaine et al., EPSL, 2002]
β (compressibility of water) = 4.4  10-10 (m.s2/kg or Pa-1)
S = 1000  9.81  (10-8 + 0.24  4.4  10-10) = 9.9136  10-5 m-1 (a value consistent with the range
of values for fractured rock: 1  10-6 to 2.1  10-5 m-1 [Domenico and Mifflin, 1965]
Hydraulic conductivity
K=
kr g
m
μ (dynamic viscosity of water) = 10-3 (kg/m.s or Pa.s)
k (intrinsic permeability) = 4  10-11 (m2) [Fontaine et al., EPSL, 2002]
K (hydraulic conductivity) = 3.924  10-4 m/s (a value consistent with the range of values for
permeable basalt: 4  10-7 to 2  10-2 m/s [Domenico and Schwartz, 1990]
Hydraulic diffusivity
D=
K
S
D (hydraulic diffusivity) = 3.9582 m2/s
References
Domenico, P.A. and M.D. Mifflin, 1965. Water from low-permeability sediments and land subsidence,
Water Resources Research, 1(4.) :563-576.
Domenico, P.A. and F.W. Schwartz, 1990. Physical and Chemical Hydrogeology, John Wiley & Sons, New
York, 824 p.
Freeze, R.A. and J.A. Cherry, 1979. Groundwater, Prentice Hall, Englewood Cliffs, New Jersey, 604p.
Fontaine, F.J., Rabinowicz M., Boulègue J., and Jouniaux L., 2002. Constraints on hydrothermal processes
on basaltic edifices : inferences on the conditions leading to hydrovolcanic eruptions at Piton de la
Fournaise, Réunion, Island, Indian Ocean. Earth Planet. Sci. Lett., 200(2002), 1-14.
Figure S2. Two years time series of seismic velocity (blue) and fluid pressure change (red). In all
plot the fluid pressure is normalized relative to seismic velocity change. The time series are
filtered in 5 period bands: 300-120 days (b), 120-60 days (c), 60-30 days (d), 30-16 days (e) and
16-8 days (d). On top of each plot the correlation coefficient R between the filtered time series
is shown.
Figure S3. Comparison between observed seismic velocity changes (blue curve) and synthetic
velocity changes estimated from fluid-pressure change (red curves) band pass filtered between
300 and 16 days.