Moment Tensor, Isotropic Source querry

I am trying to run an elastic simulation using an isotropic point source using the source_type = 2 option and (Mxx=1, Mzz=1, Mxz=0). The waveform snapshots show a polarity change above and below the source and I also see an S wave generated. This isn’t what I would expect for an explosive source and I was wondering if anyone might know why this is the case?

Ive attached the waveform snapshot and have pasted the source and Parfile code below:

I appreciate any help,
Best Regards,
James Clarke,
PhD Student, University of Auckland

Source file
#source 1. The components of a moment tensor source must be given in N.m, not in as in the DATA/CMTSOLUTION source file of the 3D version of the code.
source_surf = .false. # source inside the medium or at the surface
xs = 2000. # source location x in meters
zs = 1500. # source location z in meters
source_type = 2 # elastic force or acoustic pressure = 1 or moment tensor = 2
time_function_type = 1 # Ricker = 1, first derivative = 2, Gaussian = 3, Dirac = 4, Heaviside = 5

time function_type == 8 source read from file, if time function_type == 9 : burst

If time_function_type == 8, enter below the custom source file to read (two columns file with time and amplitude) :

(For the moment dt must be equal to the dt of the simulation. File name cannot exceed 150 characters)

IMPORTANT: do NOT put quote signs around the file name, just put the file name itself otherwise the run will stop

name_of_source_file = YYYYYYYYYYYYYYYYYY # Only for option 8 : file containing the source wavelet
burst_band_width = 0. # Only for option 9 : band width of the burst
f0 = 10.0 # dominant source frequency (Hz) if not Dirac or Heaviside
tshift = 0.0 # time shift when multi sources (if one source, must be zero)
anglesource = 0.0 # angle of the source (for a force only)
Mxx = 1. # Mxx component (for a moment tensor source only)
Mzz = 1. # Mzz component (for a moment tensor source only)
Mxz = 0. # Mxz component (for a moment tensor source only)
factor = 1.d10 # amplification factor


simulation input parameters


title of job

title = Test of SPECFEM2D with curved interfaces

forward or adjoint simulation

1 = forward, 2 = adjoint, 3 = both simultaneously

note: 2 is purposely UNUSED (for compatibility with the numbering of our 3D codes)


0 = regular wave propagation simulation, 1/2/3 = noise simulation


save the last frame, needed for adjoint simulation

SAVE_FORWARD = .false.

parameters concerning partitioning

NPROC = 1 # number of processes
partitioning_method = 3 # SCOTCH = 3, ascending order (very bad idea) = 1

number of control nodes per element (4 or 9)

ngnod = 9

time step parameters

total number of time steps

NSTEP = 1500

duration of a time step (see section “How to choose the time step” of the manual for how to do this)

DT = 5d-4

time stepping

1 = Newmark (2nd order), 2 = LDDRK4-6 (4th-order 6-stage low storage Runge-Kutta), 3 = classical RK4 4th-order 4-stage Runge-Kutta

time_stepping_scheme = 1

axisymmetric (2.5D) or Cartesian planar (2D) simulation

AXISYM = .false.

set the type of calculation (P-SV or SH/membrane waves)

P_SV = .true.

set to true to use GPUs

GPU_MODE = .false.

creates/reads a binary database that allows to skip all time consuming setup steps in initialization

0 = does not read/create database

1 = creates database

2 = reads database

setup_with_binary_database = 0

available models

default - define model using nbmodels below

ascii - read model from ascii database file

binary - read model from binary databse file

binary_voigt - read Voigt model from binary database file

external - define model using define_external_model subroutine

gll - read GLL model from binary database file

legacy - read model from model_velocity.dat_input

MODEL = default

Output the model with the requested type, does not save if turn to default or .false.

(available output formats: ascii,binary,gll,legacy)

SAVE_MODEL = default




attenuation parameters

ATTENUATION_VISCOELASTIC = .false. # turn attenuation (viscoelasticity) on or off for non-poroelastic solid parts of the model
ATTENUATION_VISCOACOUSTIC = .false. # turn attenuation (viscoacousticity) on or off for non-poroelastic fluid parts of the model

for viscoelastic or viscoacoustic attenuation

N_SLS = 3 # number of standard linear solids for attenuation (3 is usually the minimum)
ATTENUATION_f0_REFERENCE = 5.196 # (Hz) relevant only if source is a Dirac or a Heaviside, otherwise it is f0 the dominant frequency of the source in the DATA/SOURCE file
READ_VELOCITIES_AT_f0 = .false. # shift velocities to account for physical dispersion (see user manual for more information)
USE_SOLVOPT = .false. # use more precise but much more expensive way of determining the Q factor relaxation times, as in

for poroelastic attenuation

ATTENUATION_PORO_FLUID_PART = .false. # turn viscous attenuation on or off for the fluid part of poroelastic parts of the model
Q0_poroelastic = 1 # quality factor for viscous attenuation (ignore it if you are not using a poroelastic material)
freq0_poroelastic = 10 # frequency for viscous attenuation (ignore it if you are not using a poroelastic material)

to undo attenuation and/or PMLs for sensitivity kernel calculations or forward runs with SAVE_FORWARD

use the flag below. It performs undoing of attenuation and/or of PMLs in an exact way for sensitivity kernel calculations

but requires disk space for temporary storage, and uses a significant amount of memory used as buffers for temporary storage.

When that option is on the second parameter indicates how often the code dumps restart files to disk (if in doubt, use something between 100 and 1000).


Instead of reconstructing the forward wavefield, this option reads it from the disk using asynchronous I/O. Outperforms conventional mode using a value of NSTEP_BETWEEN_COMPUTE_KERNELS high enough.





source parameters

NSOURCES = 1 # number of sources (source information is then read from the DATA/SOURCE file)
force_normal_to_surface = .false. # angleforce normal to surface (external mesh and curve file needed)

use an existing initial wave field as source or start from zero (medium initially at rest)

initialfield = .false.
add_Bielak_conditions_bottom = .false. # add Bielak conditions or not if initial plane wave
add_Bielak_conditions_right = .false.
add_Bielak_conditions_top = .false.
add_Bielak_conditions_left = .false.

acoustic forcing

ACOUSTIC_FORCING = .false. # acoustic forcing of an acoustic medium with a rigid interface




receiver set parameters for recording stations (i.e. recording points)

seismotype = 1 # record 1=displ 2=veloc 3=accel 4=pressure 5=curl of displ 6=the fluid potential

subsampling of the seismograms to create smaller files (but less accurately sampled in time)

subsamp_seismos = 1

so far, this option can only be used if all the receivers are in acoustic elements


every how many time steps we save the seismograms

(costly, do not use a very small value; if you use a very large value that is larger than the total number

of time steps of the run, the seismograms will automatically be saved once at the end of the run anyway)


use this t0 as earliest starting time rather than the automatically calculated one

USER_T0 = 0.0d0

seismogram formats

save_ASCII_seismograms = .true. # save seismograms in ASCII format or not
save_binary_seismograms_single = .false. # save seismograms in single precision binary format or not (can be used jointly with ASCII above to save both)
save_binary_seismograms_double = .false. # save seismograms in double precision binary format or not (can be used jointly with both flags above to save all)
SU_FORMAT = .false. # output single precision binary seismograms in Seismic Unix format (adjoint traces will be read in the same format)

use an existing STATION file found in ./DATA or create a new one from the receiver positions below in this Par_file

use_existing_STATIONS = .false.

number of receiver sets (i.e. number of receiver lines to create below)

nreceiversets = 1


anglerec = 0.d0 # angle to rotate components at receivers
rec_normal_to_surface = .false. # base anglerec normal to surface (external mesh and curve file needed)

first receiver set (repeat these 6 lines and adjust nreceiversets accordingly)

nrec = 1 # number of receivers
xdeb = 2000. # first receiver x in meters
zdeb = 1500. # first receiver z in meters
xfin = 2000. # last receiver x in meters (ignored if only one receiver)
zfin = 1500. # last receiver z in meters (ignored if only one receiver)
record_at_surface_same_vertical = .true. # receivers inside the medium or at the surface (z values are ignored if this is set to true, they are replaced with the topography height)

first receiver set (repeat these 6 lines and adjust nreceiversets accordingly)

#nrec = 11 # number of receivers
#xdeb = 2500. # first receiver x in meters
#zdeb = 2500. # first receiver z in meters
#xfin = 2500. # last receiver x in meters (ignored if only one #receiver)
#zfin = 0. # last receiver z in meters (ignored if only one #receiver)
#record_at_surface_same_vertical = .false. # receivers inside the medium or at the surface ##(z values are ignored if this is set to true, they are replaced with the topography height)


adjoint kernel outputs


save sensitivity kernels in ASCII format (much bigger files, but compatible with current GMT scripts) or in binary format

save_ASCII_kernels = .true.

since the accuracy of kernel integration may not need to respect the CFL, this option permits to save computing time, and memory with UNDO_ATTENUATION_AND_OR_PML mode



boundary conditions


Perfectly Matched Layer (PML) boundaries

absorbing boundary active or not


change the four parameters below only if you know what you are doing; they change the damping profiles inside the PMLs

K_MIN_PML = 1.0d0 # from Gedney page 8.11
K_MAX_PML = 1.0d0
damping_change_factor_acoustic = 0.5d0
damping_change_factor_elastic = 1.0d0

set the parameter below to .false. unless you know what you are doing; this implements automatic adjustment of the PML parameters for elongated models.

The goal is to improve the absorbing efficiency of PML for waves with large incidence angles, but this can lead to artefacts.

In particular, this option is efficient only when the number of sources NSOURCES is equal to one.


Stacey ABC


periodic boundaries



velocity and density models

nbmodels = 1

available material types (see user manual for more information)

acoustic: model_number 1 rho Vp 0 0 0 QKappa Qmu 0 0 0 0 0 0 (for QKappa and Qmu use 9999 to ignore them)

when viscoelasticity is turned on, the Vp and Vs values that are read here are the UNRELAXED ones i.e. the values at infinite frequency

unless the READ_VELOCITIES_AT_f0 parameter above is set to true, in which case they are the values at frequency f0.

Please also note that Qmu is always equal to Qs, but Qkappa is in general not equal to Qp.

To convert one to the other see doc/Qkappa_Qmu_versus_Qp_Qs_relationship_in_2D_plane_strain.pdf and


elastic: model_number 1 rho Vp Vs 0 0 QKappa Qmu 0 0 0 0 0 0 (for QKappa and Qmu use 9999 to ignore them)

anisotropic: model_number 2 rho c11 c13 c15 c33 c35 c55 c12 c23 c25 0 0 0

anisotropic in AXISYM: model_number 2 rho c11 c13 c15 c33 c35 c55 c12 c23 c25 c22 0 0

poroelastic: model_number 3 rhos rhof phi c kxx kxz kzz Ks Kf Kfr etaf mufr Qmu

tomo: model_number -1 0 0 A 0 0 0 0 0 0 0 0 0 0

1 1 2000.d0 3000.d0 1500.d0 0 0 9999 9999 0 0 0 0 0 0

external tomography file


use an external mesh created by an external meshing tool or use the internal mesher

read_external_mesh = .false.




data concerning mesh, when generated using third-party app (more info in README)

(see also absorbing_conditions above)

mesh_file = ./DATA/mesh_file # file containing the mesh
nodes_coords_file = ./DATA/nodes_coords_file # file containing the nodes coordinates
materials_file = ./DATA/materials_file # file containing the material number for each element
free_surface_file = ./DATA/free_surface_file # file containing the free surface
axial_elements_file = ./DATA/axial_elements_file # file containing the axial elements if AXISYM is true
absorbing_surface_file = ./DATA/absorbing_surface_file # file containing the absorbing surface
acoustic_forcing_surface_file = ./DATA/MSH/Surf_acforcing_Bottom_enforcing_mesh # file containing the acoustic forcing surface
absorbing_cpml_file = ./DATA/absorbing_cpml_file # file containing the CPML element numbers
tangential_detection_curve_file = ./DATA/courbe_eros_nodes # file containing the curve delimiting the velocity model




file containing interfaces for internal mesh

interfacesfile = ./interfaces_simple_topo_flat.dat

geometry of the model (origin lower-left corner = 0,0) and mesh description

xmin = 0.d0 # abscissa of left side of the model
xmax = 4000.d0 # abscissa of right side of the model
nx = 80 # number of elements along X

absorbing boundary parameters (see absorbing_conditions above)

absorbbottom = .true.
absorbright = .true.
absorbtop = .false.
absorbleft = .true.

define the different regions of the model in the (nx,nz) spectral-element mesh

nbregions = 1 # then set below the different regions and model number for each region

format of each line: nxmin nxmax nzmin nzmax material_number

1 80 1 60 1


display parameters


every how many time steps we display information about the simulation (costly, do not use a very small value)


meshing output

output_grid_Gnuplot = .false. # generate a GNUPLOT file containing the grid, and a script to plot it
output_grid_ASCII = .false. # dump the grid in an ASCII text file consisting of a set of X,Y,Z points or not

to plot total energy curves, for instance to monitor how CPML absorbing layers behave;

should be turned OFF in most cases because a bit expensive


every how many time steps we compute energy (which is a bit expensive to compute)


Compute the field int_0^t v^2 dt for a set of GLL points and write it to file. Use

the script utils/visualisation/ to watch. It is refreshed at the same time than the seismograms





every how many time steps we draw JPEG or PostScript pictures of the simulation

and/or we dump results of the simulation as ASCII or binary files (costly, do not use a very small value)


minimum amplitude kept in % for the JPEG and PostScript snapshots; amplitudes below that are muted

cutsnaps = 1.

for JPEG color images

output_color_image = .true. # output JPEG color image of the results every NSTEP_BETWEEN_OUTPUT_IMAGES time steps or not
imagetype_JPEG = 2 # display 1=displ_Ux 2=displ_Uz 3=displ_norm 4=veloc_Vx 5=veloc_Vz 6=veloc_norm 7=accel_Ax 8=accel_Az 9=accel_norm 10=pressure
factor_subsample_image = 1.0d0 # (double precision) factor to subsample or oversample (if set to e.g. 0.5) color images output by the code (useful for very large models, or to get nicer looking denser pictures)
USE_CONSTANT_MAX_AMPLITUDE = .false. # by default the code normalizes each image independently to its maximum; use this option to use the global maximum below instead
CONSTANT_MAX_AMPLITUDE_TO_USE = 1.17d4 # constant maximum amplitude to use for all color images if the above USE_CONSTANT_MAX_AMPLITUDE option is true
POWER_DISPLAY_COLOR = 0.30d0 # non linear display to enhance small amplitudes in JPEG color images
DRAW_SOURCES_AND_RECEIVERS = .true. # display sources as orange crosses and receivers as green squares in JPEG images or not
DRAW_WATER_IN_BLUE = .true. # display acoustic layers as constant blue in JPEG images, because they likely correspond to water in the case of ocean acoustics or in the case of offshore oil industry experiments (if off, display them as greyscale, as for elastic or poroelastic elements, for instance for acoustic-only oil industry models of solid media)
USE_SNAPSHOT_NUMBER_IN_FILENAME = .false. # use snapshot number in the file name of JPEG color snapshots instead of the time step (for instance to create movies in an easier way later)

for PostScript snapshots

output_postscript_snapshot = .true. # output Postscript snapshot of the results every NSTEP_BETWEEN_OUTPUT_IMAGES time steps or not
imagetype_postscript = 1 # display 1=displ vector 2=veloc vector 3=accel vector; small arrows are displayed for the vectors
meshvect = .true. # display mesh on PostScript plots or not
modelvect = .false. # display velocity model on PostScript plots or not
boundvect = .true. # display boundary conditions on PostScript plots or not
interpol = .true. # interpolation of the PostScript display on a regular grid inside each spectral element, or use the non-evenly spaced GLL points
pointsdisp = 6 # number of points in each direction for interpolation of PostScript snapshots (set to 1 for lower-left corner only)
subsamp_postscript = 1 # subsampling of background velocity model in PostScript snapshots
sizemax_arrows = 1.d0 # maximum size of arrows on PostScript plots in centimeters
US_LETTER = .false. # use US letter or European A4 paper for PostScript plots

for wavefield dumps

output_wavefield_dumps = .false. # output wave field to a text file (creates very big files)
imagetype_wavefield_dumps = 1 # display 1=displ vector 2=veloc vector 3=accel vector 4=pressure
use_binary_for_wavefield_dumps = .false. # use ASCII or single-precision binary format for the wave field dumps


Ability to run several calculations (several earthquakes)

in an embarrassingly-parallel fashion from within the same run;

this can be useful when using a very large supercomputer to compute

many earthquakes in a catalog, in which case it can be better from

a batch job submission point of view to start fewer and much larger jobs,

each of them computing several earthquakes in parallel.

To turn that option on, set parameter NUMBER_OF_SIMULTANEOUS_RUNS to a value greater than 1.

To implement that, we create NUMBER_OF_SIMULTANEOUS_RUNS MPI sub-communicators,

each of them being labeled “my_local_mpi_comm_world”, and we use them

in all the routines in “src/shared/parallel.f90”, except in MPI_ABORT() because in that case

we need to kill the entire run.

When that option is on, of course the number of processor cores used to start

the code in the batch system must be a multiple of NUMBER_OF_SIMULTANEOUS_RUNS,

all the individual runs must use the same number of processor cores,

which as usual is NPROC in the Par_file,

and thus the total number of processor cores to request from the batch system


All the runs to perform must be placed in directories called run0001, run0002, run0003 and so on

(with exactly four digits).

Imagine you have 10 independent calculations to do, each of them on 100 cores; you have three options:

1/ submit 10 jobs to the batch system

2/ submit a single job on 1000 cores to the batch, and in that script create a sub-array of jobs to start 10 jobs,

each running on 100 cores (see e.g. )

3/ submit a single job on 1000 cores to the batch, start SPECFEM2D on 1000 cores, create 10 sub-communicators,

cd into one of 10 subdirectories (called e.g. run0001, run0002,… run0010) depending on the sub-communicator

your MPI rank belongs to, and run normally on 100 cores using that sub-communicator.

The option below implements 3/.


if we perform simultaneous runs in parallel, if only the source and receivers vary between these runs

but not the mesh nor the model (velocity and density) then we can also read the mesh and model files

from a single run in the beginning and broadcast them to all the others; for a large number of simultaneous

runs for instance when solving inverse problems iteratively this can DRASTICALLY reduce I/Os to disk in the solver

(by a factor equal to NUMBER_OF_SIMULTANEOUS_RUNS), and reducing I/Os is crucial in the case of huge runs.

Thus, always set this option to .true. if the mesh and the model are the same for all simultaneous runs.

In that case there is no need to duplicate the mesh and model file database (the content of the DATABASES_MPI

directories) in each of the run0001, run0002,… directories, it is sufficient to have one in run0001

and the code will broadcast it to the others)


Hi James,

Polarity change: What quantity is plotted in your figure? If it’s for instance the vertical component of motion, you do expect a sign change.

Unexpected S wave front with isotropic source: This may happen when the mesh is too coarse compared to the smallest wavelength excited by the source ~cs/(2.5*f0). Have you checked this: ?

These are just hints. Others may have more ideas.


Hi Pablo,
Thanks for your reply. The figure I posted shows the displacement wave field (seismotype= 1). I guess this does show vertical displacement in the wavefield snapshots?? If this is the case then I agree that the change in polarity makes sense.

Ah, you were right about the mesh being too course. The points per S- wavelength was above the threshold value of 4.5 before, but by changing it to 6 I can see that the S- wave disappears.

Thanks for your help. I’m just trying to learn and understand the program so your comments are very helpful.


How to set moment magnitude Mw parameter in par_file ?
thank u ,

Hi Sam,

I’m not an expert but I think you set it in the source file using:

Mxx = 1. # Mxx component (for a moment tensor source only)
Mzz = 1. # Mzz component (for a moment tensor source only)
Mxz = 0. # Mxz component (for a moment tensor source only)
factor = 1.d10

I used the above for an explosive source and applied an amplification of 1e10 but I didnt really care about the Mw.

Hope this helps,