Friday, October 30, 2009

syndat.f -- sum greens functions with moment tensor and centroid time correction

click title to see Google Drive directory JParkCodes

NOTE:  This code was updated on 7/24/2012.  The title of the post points to the newer code.  The older code can be found at Google Drive JParkCodes/syndat_2009.f

This code is a version of the old sac_syndat.f that incorporates the IRIS-suppied EVRESP subroutine to compute and apply instrument response correction to convert displacement to instrument counts.

c f77 -o /Users/jjpark/bin/syndat syndat.f /Users/jjpark/Ritz/libevresp.a /Users/jjpark/Ritz/jlib.a
c xf77 -o /Users/jjpark/bin/syndat syndat.f /Users/jjpark/Ritz/libevresp.a /Users/jjpark/Plotxy/plotlib.a /Users/jjpark/Ritz/jlib.a
c program to make synthetics using green fcts
c the greenfcts are raw displacement in meters. This code applies the instrument
c response in the frequency domain with a FFT --> multiply by H(f) --> IFFT process.

program asks for the centroid time delay, which is applied with a phase-ramp in the frequency domain
The centroid time delay occurs because the start time of a large event is typically not the best time to place the stepfunction onset of the long-period course. If moment release occurs over T seconds, then a better fit to data is found by delaying the synthetic onset by T/2 seconds. (Assuming a symmetric source time function, natch).

print *,'enter time delay e.g. centroid-time correction'
read(5,*) tconst
40 print *,'do you want to use strike,dip and rake (0=no) '
read(5,*) ick
if( then
call fault(sol)
print *,'enter f(x6) : '
read(5,*) (sol(i),i=1,6)
print *,'routine expects the gfct filename to start with g'
print *,'the code substitutes s for g in the output filename'
30 print *,'green fns file name(=stop) : '

The program will read a succession of greensfct files, one per station-component, until it reads 'stop' e.g.

0 --> zero centroid time delay
1 --> read strike,dip,slip
329 8 110 --> strike dip slip
400 --> seismic moment (units of 10**27 dyne-cm/10**20 nt-m)
/Users/jjpark/Synth/gsph_ffc.lhz --> greensfct file

There are PLOTIT calls within the code (commented out for now) that allow the user to view his/her handiwork, testing the causality of the instrument response, etc.

The EVRESP routine is found in the libevresp.a library that you can create from the IRIS EVALRESP software distribution. For reasons only the DMC programmers know, the latest versions of the EVALRESP distribution hide the libevresp.a library in a subdirectory ".lib" that wont show up in a "ls" command unless you are looking for it. Just another convenience feature, I guess.

Evalresp follies

The IRIS-supplied instrument response software was not straightforward to incorporate into our codes. The key C-subroutine EVRESP is built for incorporating into Fortran codes, but seems to have developed a bug over its past revisions, and no one may have complained of it. The subroutine generates the filename of a RESP file, nominally output by rdseed. In previous versions of the subroutine I have used (circa 2005) the routine opens the file, reads the contents and generates a frequency domain instrument response for convolving synthetics or deconvolving data. The 2008-9 versions of EVRESP cant seem to find the file whose name they create (seemingly correctly) and the program fails. Diddling with the SEEDRESP environmental variable does not seem to help. The fix that I found was to feed EVRESP the RESP.* filename explicitly, by generating the filename myself from the same group of character-string variables that EVRESP uses. To wit:

print *,file
c remove blank spaces
do i=30,7,-1
if(file(i:i).eq.' ') file=file(1:i-1)//file(i+1:80)//' '
end do
print *,file
print *,'entering evresp'
print *,sta,cha,net,locid,datime,unts,nscan,npad,nha
x rtyp,vbs,start_stage,stop_stage,stdio_flag)
print *,'done evresp'

The character string "file" is passed to the routine and the code looks for it successfully, if the file is there!

Thursday, October 29, 2009

sacread.f -- subroutines to read and write SAC-format files -- adjusted to write 6-component greens functions


subroutines to read and write SAC-format files -- adjusted to write 6-component greens functions. The routines provided by SAC are fine, of course, but I use a hydrid file format for greens-function files. These files contain six displacement traces rather than one, one for each independent component of the seismic moment tensor. There is a flag in the SAC header that I took over for indicating that six traces need to be read instead of one.

if(chead(145).ne.' GRN'.and.chead(145).ne.' grn') then
print *,'reading greens functions'

The subroutines are sacread, sacout, sacread_swap and sacout_swap. The last two routines swap big-endian and little-endian data. You will be happier if you never need to swap bytes, but they are there for when you are painted into that corner.

csolve.f -- solve a system of equations with complex coefficients

click title to see Google Drive directory JParkCodes

csolve.f is a subroutine for doing Gaussian elimination with partial pivoting to solve a system of linear equations A.x=y with complex matrix A and vector y. Useful in several routines.

Wednesday, October 28, 2009

Snow Leopard transition: Fortran

Just an informational item regarding Snow Leopard's treatment of Fortran. The first comment is that programmers need to install XCode from the Snow Leapard install DVD. I bulled ahead without checking this and ran into a roadblock in trying to compile a Fortran program -- the computer could not find the unix command "as" which assembles object files. Once I loaded the XCodes package from the Install disk, this problem went away, and others cropped up.

The innovation of Snow Leopard (and Leopard, I think, Im not exactly sure when the transition occurred relative to Tiger (MacOS 10.4)) is the full utilization of 64-bit chip architecture. The Snow Leopard gcc compiler creates object files that are 64bit, but the Fortran compiler g77 does not. If you try to assemble Fortran and C routines into one archive or a single program, the computer balks. You can tell the nature of the object file with the command "file *.o". I sampled the internets for hints on solving this problem. The simplest solution, and the one Im adopting for now, is to force the gcc compiler to generate a 32-bit object file, rather than a 64bit version, with the "-m32" runtime option. Less powerful than 64bit, and Im still getting warnings from the compiler when I create large codes, but the executables seem to work OK.

The "gfortran" option, rather than "g77" for the Fortran compiler, might work with the 64bit option, but the internets are not unanimous on its success. There are compilers available for download from the HPC site ( that claim to vectorize gcc (version 4.4) in Snow Leopard. I read comments from programmers that full-optimization of the compiler has bugs, so buyer beware. I might try it later, though.

NOTE: In 2012-2013 I switched to compile codes with gfortran compiler.

Tuesday, October 20, 2009

grn_sph.f for computing greens function for synthetic seismograms using summed free oscillations for spherical earth models

click title to see Google Drive directory JParkCodes


The task of computing synthetic seismograms from summed free oscillations for an aspherical earth model is typically accomplished in two or three steps. The first step computes the hybrid free-oscillations for a chosen earth model, comprised of the "simple" spherical-earth modes whose particle motions follow single vector spherical harmonics. The second step takes those hybrid modes and computes greens functions for particle motion at a given source location (allowing six different time series, one for each component of the seismic moment tensor) and station location (for a given sensor and three time series for three components: vertical/horizontal). The last step is the computation of a synthetic seismogram by summing the six time series for each motion component by weighting with the components of the moment tensor.

Historically, I have used the greens function routine as the location for adding the instrument response, but I have reconsidered this choice. Computers are fast enough for the FFTs an instrument response to be computed on the fly in the last step. This also avoids the problem of updating the instrument response codes (notoriously unstable, owing to the various formats for instrument responses) in multiple greensfunction codes. My final code (sac_syndat.f) is used for all, and should be the place where instrument response either is (or is not) applied. The output of this routine has units of displacement in meters.

So, for making these codes more public, lets start with the simple code for synthetic seismograms on a spherical earth model.
The code is grn_sph.f, which has existed in one form or another for over 20 years (this explains the dead code inside it).

print *,'typical input for this program:'
print *,'0 20 --> freq interval to sum (mHz)'
print *,'/home/jjpark/Mineos/Models/premiss --> 1-D earth model'
print *,'/home/jjpark/Mineos/premissnl --> spheroidal info'
print *,'/home/jjpark/Mineos/premiss_eig --> spheroidal modes'
print *,'/home/jjpark/Mineos/premistnl --> toroidal info'
print *,'/home/jjpark/Mineos/premist_eig --> toroidal modes'
print *,'1000 --> # of overtone branches (here: all of them)'
print *,'0 --> # of data points to synthesize (0-> #=data: NPTS)'
print *,'/park/nobackup/Sumatra/VH/PET.VHZ --> data file'
print *,'.....'
print *,'. . . more data files'
print *,'.....'
print *,'stop'
print *,' '
print *,'The format of the greens fct files is impure SAC-format'
print *,'readable only by sac_syndat and related JPark codes'
print *,'It has a SAC header with one ASCII variable set to GRN'
print *,'and six*NPTS data values that follow the header'
print *,'NPTS for each component of the source moment tensor'
print *,'The greens-function file names are created from your input file'
print *,'by prepending gsph_ to the input filename'
print *,'and replacing the last character with z,r,t'

This code reads files full of free-oscillation displacement functions for a standard earth model, computes the strain of the mode at the desired source depth, and sums 6 traces for surface displacement at a specified distance from the source. Because the earth model is a spherical-earth model, only the source depth and source-receiver distance affect the waveforms that represent oscillatory displacement. The code, however, takes a data file to specify the source and receiver locations and creates synthetics for the geometry. A subsequent processing step (syndat.f) will take the greens function file, sum with the components of a moment tensor, and apply the instrument responses appropriate to the station's sensors and channel.

The start time for the earthquake determines where the decaying sinusoids begin. Older versions of green-fct codes used an explicit time that the user had to supply. This version assumes that the information is in the omarker time in the SAC header. There is also a bmarker time that is used in case the start time of the record does not correspond to the reference time in the data header.
c the reference time of the file could be before or after the first data point
c the ymdhmsec parameters define the reference time
c the origin quake of the quake and start of record are wrt reference time
origin=ahead(8) ! event start is the omarker in sac header
recstart=ahead(6) ! record start is the bmarker in sac header