Numerical simulation of the tephra fallout
and plume evolution of the eruptions of the
Láscar volcano in April 1993 and July 2000
Angelo Castruccio¹; Alvaro Amigo¹ ²; Laura Gallardo²
¹ Departamento de Geología, Universidad de Chile, Plaza Ercilla #803, Casilla 13518-Correo 21, Santiago
² Centro de Modelamiento Matemático (CMM), Universidad de Chile (UMR-CNRS 2071), Casilla 203, Santiago
Introduction
The numerical simulation of volcanic tephra
fallout from an eruptive column is an important
issue in volcanology, both to understand the
eruption dynamics and as a tool to make
hazard prediction and maps. In this work, we
present preliminary results of the simulation of
tephra fallout and plume tracking, for two
eruptions of the Láscar volcano in Northern
Chile: the subplinian one of April 19th-20th
1993 and the vulcanian one of July 20th 2000.
We apply a three-dimensional chemical,
transport and deposition model, and reanalysis
winds. Emissions were taken from reported
estimates based on satellite observations and
field data.
Photo by Dr. P. W. Francis
Láscar volcano
The Láscar volcano (5592m, 23°22´S,
67°44´W) is the most active volcano of the Observed (red) and simulated (blue) limits of the ash
Andes of Northern Chile (Gardeweg and fall deposit, using ECMWF data.
Medina, 1994). It is an ESE-WNW elongated
Evolution of the ash plume for the 20 July
composite stratocone (Gardeweg et al, 1998).
2000 eruption:
Activity since 1984 displays cycles of lava dome
formation in the summit crater, lava dome
subsidence with crater collapse, vulcanian to
plinian explosive eruptions culminating in the
Simulated
major
explosive
19/20
April 1993
Column
heigth,
masseruption
flux andoftotal
mass
(Gardeweg et al, 1998). Here we address the
erupted
dispersion of tephra in connection with the
subplinian eruption observed in April 1993 and
the vulcanian eruption of July 20 2000.
Model
The MATCH model solves the continuity equation
for atmospheric tracers in a 3-D Eulerian
framework:
ci
 (vci )  ( Kci )  Qi  S i
t
where ci represents the mass mixing ratio of
the trace species of interest, v is the 3-D wind,
K is the turbulent pseudo-difussion tensor and
Qi and Si represent internal sources and sinks
(Robertson et al, 1999).
To simulate the tephra fallout, we considered
10 size categories of particles (15 um - 1.6 cm
in radius), for which a size and height
dependent removal can be applied:
Observed

Vs ci 

z
As a first approximation, we assumed a
constante Vs equal to the mean velocity of a
particle falling from a 10-20km plume heigth
(Bonadonna and Philipps, 2003). Further, only
the sedimentation from the turbulent umbrella
cloud and particles smaller than 1.6cm are
considered.
Volcanic input parameters
We use two sets of meteorological data:
ECMWF reanaysis data linearly interpolated to
Grain size distribution
0.5°, and HIRLAM fields that correspond to
Since the grain size distribution of the April 1993
dynamilcally interpolated reanalyses of 0.1°
eruption is not available, we adjusted it by trial
horizontal resolution.
and error taking into account that 1.4% of the
total emitted tephra corresponds to fine ash (112m) (Rose et al.,2000).
Satellite images of the eruptive
plume of the 20/07/2000
eruption of Láscar volcano.
Conclusions
Despite simplifications (e.g., constant settling
velocity, spreading current only, etc.) the results
presented are consistent with available
observations, especially the proximal to medial
deposition (size, shape and position), and also,
at the regional scale, the limits of the ash
deposit for the April 1993 subplinian eruption.
Results
Deposit of the 19/20 April 1993 eruption
Grain size distribution
The tracking of the plume of the July 2000
eruption is also in good agreement with
observations, especially the shape of the
plume. The temporal evolution is less well
captured, possibly due to shortcomings in the
representation of the grain size distribution and
the assumed constant settling velocity.
% (weigth)
20
15
10
distribution
Thickness v/s Distance
5
100
0
size (phi units)
Thickness (cm)
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
10
Observed
Calculated
1
0
100
200
300
0,1
Thickness v/s Isopach area1/2
Distance (km)
Thickness
100
10
Observed
Calculated
Future work considers the implementation of
vertically varying settling velocities, and the
simulation of other eruptions and Andean
volcanoes. We expect this to result in a reliable
tool for diagnostic and forecast studies and
hazard assessments.
1
0
50
100
150
References
0,1
Isopach area
1/2
(km)
Isopachs in cm for the observed (dashed red
contours) and simulated deposits (continous
blue contours), using HIRLAM fields. Also shown
observed and modeled thickness vs distance
and isopach area (Httpp…)
Bonadonna, C., Phillips, J, 2003: Sedimentation from strong volcanic plumes. Journal of
Geophysical Research. V.108 n B7, 2340
Gardeweg, M., Medina, E., 1994: La erupción subpliniana del 19-20 de Abril de 1993 del volcán
Láscar, N de Chile, 7° Congreso Geológico Chileno, Actas volumen I, p 299-304.
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Appl. Met., Vol 38, No 2, 190-210.
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from satellite remote sensors: A summary. Phil Trans R Soc Lond A358: 1585-1606
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Woods, A.W. (1997) Volcanic Plumes, John Wiley & Sons.
The mass flux for the April 1993 eruption was
obtained using :
H = 1.83Q0.259
This is based on Sparks (1997), and it considers
that the total mass erupted
was 345MtWe(Rose
et for the support provided by the staff at the Swedish Meteorology and Hydrology Institute(SMHI), FONDECYT Grant 1030809. This wo
Acknowledgements.
are grateful
al., 2000) and the column height from Gardeweg
and Medina (1994).
The same formula was applied for the July 2000
eruption, assuming a cloumn height in a range
between 5 (coarse ash) and 7 km (fine ash)