2013 code change for processing S/P ratios

In December 2013, I modified the procedure for processing S/P amplitude ratio data, so runs made now with the same data sets as used for the earlier versions of focmec may have slightly different answers.

Prior to 2013, I used the same procedure as that used by Kisslinger and coauthors (1980, 1981, 1982) of using the same take-off angle for both the S and the P. If the Poisson's ratio is constant for the velocities along the raypaths, the take-off angles are the same. Otherwise they will be different. Although I had used both angles in program focmec_prep when processing the observed amplitudes several years earlier, it was not until December 2013 that I used both angles in program focmec when calculating the theoretical amplitudes.

To see the effect of this modification, I took an input file from the 2009 distribution of the FOCMEC package and, with the same run parameters, ran it using the current program. (I used program convert_input to convert the 2009 input file to the current format.) The output focmec.lst and focmec.out files for the two runs are in sub-directory ./focmec/doc/2013_code_change in both 2009 and 2017 format:

• focmec_8staR.lst (date created Jun 17 2009)
• focmec_8staR.out (date created Jun 17 2009)
• focmec_2009inp_2017.lst (date created Oct 15 2017)
• focmec_2009inp_2017.out (date created Oct 15 2017)

The .lst files include the input data, and one sees that the TOA differ for P and S by about one degree for the closest station (KEV) to about 2.5 degrees for the farthest station (TOL) for this teleseismically-recorded event. This results in typical changes in “Difference” of about 0.05 to 0.1, and the solutions found are identical except there is one additional one for the 2017 run. Hence, for this example at least, the modification results in changes in solutions that are non-zero, but not significant.