Deformation sources derived
from InSAR analysis at
volcanoes:
How realistic are the models?
Andrea Manconi, T.R. Walter
M. Motagh, J. Ruch, M. Shirzaei, R. Wang & J. Zschau
Helmholtz Centre GFZ Potsdam, Telegrafenberg, 14473 Potsdam (Germany)
Volcanic unrest:
Inflation/deflation of the
magma chamber, dyke/sill
intrusion, hydrothermal
activity, ring fault
dislocation…
Surface displacement
Geodetic data (InSAR)
Inversion
Source depth,
volume/pressure change,
position, shape, orientation,
evolution over time…
Interpretation
Model’s Assumptions
- Simplified geometries (points, spheres, rectangular planes, ellipsoidal, etc.)
- Earth’s lithosphere is an elastic, isotropic, and homogeneous half-space…
Skaergaard,
Greenland
…however, in nature
volcanic plumbing systems
and volcanic areas look
differently!
Gran Canaria, Spain
Homogeneous models and layered models may lead
to the same fitness of the data, but…
(Darwin Volcano, Galápagos, ca. 20 cm uplift between 1992-1998 Amelung et al., 2000)
L
A
Y
E
…differences in the source parameters
(Depth and Pressure) are up to 60%
depending on the mechanical contrast
Manconi et al., 2007
Pritchard et al., 2004; Trasatti et al., 2005; Newman et al., 2006; Ellis et al., 2007;
Crescentini et al., 2007; Masterlark, 2007; Amoruso et al., 2008; etc…
R
E
D
View from Vesuvius crater
CAMPI FLEGREI CALDERA
WE ARE HERE !
NAPOLI Bay
Long and accurate observation of the surface displacements…
SBAS-DInSAR (1992-2008, ERS – ENVISAT), M.Manzo, G.Zeni, P. Tizzani, E. Sansosti & R.Lanari
…Standard approach: joint inversion of Asc & Desc
data assuming as source a Mogi source.
rms=0.3 cm/year
E(km)
N(km)
d(km)
DP(MPa/yr)
min
426.3
4518.8
2.5
9.4
max
427.2
4519.1
2.9
12.5
A priori information about the sub-surface structure
Zamora et al., 1994
Judenherc et al.,2004
Chiarabba et al., 2006
Berrino et al., 1998
Chiarabba et al., 2006
Low Vp/Vs anomaly 3 km depth
Seismicity between 2-4 km depth
During unrest, gas chemistry and
gravity changes compatible with
hydrothermal reservoir
(De Natale et al., 1991, Chiodini et al., 2001;
Battaglia et al., 2006)
3-D Finite Element models
constrained by independent
geophysical observations and
constraints
Forward model considering
20 MPa overpressure(Sartoris,1990)
Our FE model
E(km)
N(km)
d(km)
DP(MPa/yr)
426.82
4519.3
3
20
Forward model on a 3D
heterogeneous
medium
Inversion using Mogi and
assuming an “equivalent”
homogeneous half-space
rms=0.3 cm/year
E(km)
N(km)
d(km)
DP(MPa/yr)
min
426.4
4519.2
2.8
23.5 (+15%)
max
426.9
4519.5
3.1
26.5 (+25%)
Synthetic displacement on 3D
Best-fit model assuming an
Finite Element heterogeneous
“equivalent”
model (3DHET)
homogeneous half-space
U ( x, y ,0)
(HHS)
geom etry& position

D
P
INVERSION
m ech. properties
DPHHS > DP3DHET
The surface displacement U(x,y,0)
is linearly
related to the source strength DP
DP3DHET
3DHET
HHS
This procedure retrieves the source location (coordinates and
depth) but OVERESTIMATES the pressure changes…
If we consider the same source
U3DHET
parameters (x, y, z, DP) in
both models…
3DHET
UHHS
<
U3DHET
U 3 DHET
K
U HHS
HHS
“K” represent the factor of amplification or reduction of
the surface displacements due to the 3-D
heterogeneities
Applying this “correction
InSAR Data
factors” to the real
data…
“corrected” for the 3D
heterogeneity
effects
“Reality”
U InSAR
 U InSAR *
K
Similarly to seismology applications, “correction factors” might be
The
inversion
of U“station”
InSAR* will retrieve pressure changes as
calculated
for every
to take into account the local effects
they
in a applied
fully 3D
model!
and occur
will be then
to heterogeneous
the real measured data
A priori constraints ?
UInSAR
3D FEM
Synthetic tests
Inversion
The source position is
not affected (like CF)
Interpretation
U InSAR
 U InSAR *
K
Standard approach
E(km)
N(km)
d(km)
DP
(MPa/yr)
“Corrected” dataset
E(km)
N(km)
d(km)
DP
(MPa/yr)
min
426.5
4519.3
2.5
5.4
(-40%)
max
427.1
4520.2
3
8.5
(- 32%)
min
426.3
4518.8
2.5
9.4
max
427.2
4519.1
2.9
12.5
Application to the complete SBAS time series
Manconi et al., in preparation
Summary and Conclusion
- The use of InSAR data alone to constrain source
parameters MAY lead to uncertainties!
- The simplified analytical models fit the
data…Numerical tests help to understand why!
-
Good news…under certain conditions we can still
use the simplified models (e.g. monitoring)!
Thanks for your
attention!