Tephra2 FAQ

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1.      What are the inputs?

2.      What are the outputs?

3.      Is there an explanation for the KEY-NAMES in the configuration file?

4.      Why does the plume always blow toward the northeast?

5.      How is TEPHRA2 verified?

6.      How does TEPHRA2 use a DEM?

7.      How is TEPHRA2 validated?

8.      Is Tephra2 freely available?

 

1.  What are the inputs?

TEPHRA2 requires three input files. A configuration file, a grid file, and a wind file.

The configuration file is a text file consisting of KEY-NAME VALUE pairs, one pair per line, separated by a space. The KEY-NAME must not be changed. Only the VALUE should be changed by the user. Comment lines in the configuration file must begin with the symbol:  #

The KEY-NAMES for the configuration file are as follows:

(note: the VALUES are just examples)

PLUME_HEIGHT 8000

ERUPTION_MASS 5.69e9

MAX_GRAINSIZE -4.0

MIN_GRAINSIZE 4.0

MEDIAN_GRAINSIZE 1.0

STD_GRAINSIZE 1.0

VENT_EASTING 220016

VENT_NORTHING 1627311

VENT_ELEVATION 1287

EDDY_CONST 0.04

DIFFUSION_COEFFICIENT 10.0

FALL_TIME_THRESHOLD 288

LITHIC_DENSITY 2600.0

PUMICE_DENSITY 1000.0

COL_STEPS 100

PLUME_MODEL 0

PLUME_RATIO 0.1

 

The grid file is a text file consisting of three values per line, values separated by a space.

These 3-tuples are determined by the user and represent locations around the volcano where mass per unit area values of tephra are calculated by the program. Easting and northing are specified in meters (usually UTM coordinates with respect to a particular zone) and the elevation is specified as meters above sea level. Zero can be specified for the elevation, meaning at sea level. If elevations are not known, a consistent average elevation value can also be used. The format being:

EASTING(m)  NORTHING(m)  ELEVATION(m)

 

The wind file is a text file consisting of three values per line (again separated by spaces), height above sea level, wind speed at this height, and direction the wind is blowing toward at this height. The file format is:

HEIGHT(m) WINDSPEED(m/s) DIRECTION(+/- degrees)

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2.  What are the outputs?

TEPHRA2 outputs mass per unit area and the associated weight percent of each phi size at each grid point specified in the input grid file. The range of phi sizes is specified in the input configuration file.

The file format is:

EASTING NORTHING ELEVATION MASS WT.%-MIN_PHI ….. WT.%-MAX_PHI

 

TEPHRA2 does not create an output file automatically. The output must be redirected to a file on the command line using the the unix redirection symbol:  >

An example:  tephra2 tephra2.conf grid.in  wind.in > tephra2.out

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3.  Is there an explanation for the KEY-NAMES in the configuration file?

PLUME_HEIGHT is the maximum height of the tephra cloud in meters above sea level.

ERUPTION_MASS is the total mass of tephra erupted from the volcano in kilograms.

MAX_GRAINSIZE is the maximum particle size of tephra erupted from the volcano in phi units.

MIN_GRAINSIZE is the minimum particle size of tephra erupted from the volcano in phi units.

MEDIAN _GRAINSIZE is the median particle size of tephra erupted from the volcano in phi units (the mean of a Gaussian distribution).

STD_GRAINSIZE is the standard deviation in particle size of tephra erupted from the volcano in phi units (one standard deviation of a gaussian distribution).

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4.  Why does the plume always blow toward the northeast?

 

It does not if the input files are formatted correctly. Check the input wind file. The file format is:

HEIGHT(m) WINDSPEED(m/s) DIRECTION(+/- degrees)

If these are reversed (windspeed and direction are swapped) then the plume often appears to blow to the north or to the northeast.

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5.  How is TEPHRA2 verified?

 

TEPHRA2 is a highly modular code. Each function has been tested for correct computation. An additional test to verify the function of the code has been to compare the output mass – by integrating the isomass source of the tephra deposit – with the input mass specified in the configuration file. These two masses should agree.

We find that the masses always agree within 1 percent, usually within 0.1 percent, as long as several conditions are met. These are: (1) the grid file covers sufficient area to contain the entire deposit (usually we perform tests in which the 99.99% of the deposit falls within the grid area), (2) the grid spacing is sufficiently dense so that the near-vent mass is well-defined by the output grid (this usually means that the test must be performed with a sufficiently dispersive column), (3) as described further in the following, the isomass falls on to a “flat” digital elevation model (DEM).

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6.  How does TEPHRA2 use a DEM?

The grid file for TEPHRA2 requires grid point location and elevation for each point where the tephra accumulation is calculated, the format of the grid file being:

EASTING(m)  NORTHING(m)  ELEVATION(m)

 

The elevation may be constant for all grid points. For example, a grid point file of UTM coordinates with constant elevation might be:

534000 4208000 100

535000 4208000 100

536000 4208000 100

537000 4208000 100

 

…the grid point elevation is a constant (100 m above sea level). In this case the volcano vent height may be any elevation, but the tephra falls to a surface of uniform elevation. For many volcanoes and volcanic eruptions this is a reasonable approximation. For grid files with constant elevation, the limits of integration in the analytical solution to the advection-diffusion equation are satisfied and the eruption mass will be conserved.

For some eruptions, particularly eruptions with weak plumes and low column height on large volcanoes, the assumption of a constant elevation of grid points is a poor one. In this case grid point elevations should vary, usually using elevations from a digital elevation model (DEM). In this case the grid file might have entries like:

534000 4208000 100

535000 4208000 220

536000 4208000 339

537000 4208000 530

 

where the elevations are in meters above sea level. For small eruptions, accounting for the elevation difference results in realistic looking isomass maps, especially in the near-vent region. For example, there is often a much lower tendency for a significant fraction of tephra to accumulate upwind of the vent, because the total diffusion time of particles is limited by the higher elevations (short particle fall times) near the vent. Unfortunately, mass is not conserved in this calculation because the limits of integration are not the same for each grid point. Mass will be slightly over-estimated at low elevations.

For highly dispersive eruptions (high columns, high column ratio) this effect is magnified, and mass can be overestimated by up to 20%. For these large, highly dispersive eruptions, the “flat” DEM solution should be used. Note that for such eruptions terrain effects on deposition are very minimal anyway. Any elevation can be used – for example sea level or an average elevation across the region.

For eruptions that are not highly dispersive (low column, low column ratio), a better solution might be obtained by using a DEM (varying elevations between grid points). Our tests show that for such low columns the error in mass introduced by using the DEM is less than 1%.

In order to correct for the over-estimate of mass at lower elevations a “terrain correction” is necessary, similar to the terrain correction used in modeling gravity data, This terrain correction arises because particles assumed by the code to reach “low” elevations, have already been deposited at “high” elevations. This terrain correction is not yet implemented in TEPHRA2.

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7.  How is TEPHRA2 validated?

 

TEPHRA2 is not currently validated. Its predecessor, TEPHRA was validated by comparing output from the code to actual eruptions by modeling deposits. The details of this validation are found in the papers:

Connor, L.J., and C.B. Connor, 2006, Inversion is the solution to dispersion: understanding eruption dynamics by inverting tephra fallout, in: H. M. Mader, S. Coles, C.B. Connor and L.J. Connor (eds), Statistics in Volcanology, IAVCEI 1, Geological Society of London, 231-242.

Scollo Simona, Tarantola Stefano, Bonadonna Costanza, Coltelli Mauro, Saltelli

Andrea, Sensitivity analysis and uncertainty estimation for tephra dispersal models, in press in JGR.

Bonadonna, C., C.B. Connor, B.F. Houghton, L. Connor, M. Byrne, A. Laing, T.K. Hincks, 2005, Probabilistic modeling of tephra-fall dispersal: hazard assessment of a multiphase rhyolitic eruption at Tarawera, New Zealand, Journal of Geophysical Research, Vol. 110, No. B3, B0320310.1029/2003JB002896.

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8. Is Tephra2 freely available?

 

Yes, Tephra2 is a freely available open-source code.

As defined on the GNU Project webpage: Free software is a matter of the users' freedom to run, copy, distribute, study, change and improve the software. More precisely, it refers to four kinds of freedom, for the users of the software:

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