FOCMEC: Eastern Tennessee Event

Directory ./sample_runs/ETNEvent includes drivers, input, and output files for an application of focmec to a magnitude 4.2 event with a focal depth of 19 km which occurred on 27 March 1987 in eastern Tennessee. The location was based on data from regional networks recorded on short-period, vertical-component seismographs, and only P polarities were used when determining focal mechanisms. This event is number 12 in Chapman, et al. (1997).

The FOCMEC input file, 87032707.dat, includes direct-P arrival polarity picks for 37 stations. This file was produced from 87032707.arc, an "archive" output file produced by the earthquake location program Hypoellipse using program hypo2foc (included in this distribution). The results presented here are for allowing up to four fixed-weight polarity errors. Using relative weighting for the errors did not improve the solution. Plots produced by program focplt include one which shows the P nodal surfaces for all solutions with four polarity errors (4err.pdf) and one which shows one from among the set (they are all very similar) superimposed on the plot of the polarity data (Best.pdf). Circles indicate compressions and triangles indicate dilatations. The plots are both lower-hemisphere projections of the focal sphere.

The driver files — rfocmec, for program focmec, and rfocplt_data plus rfocplt_solutions, for program focplt — are Unix csh scripts which include comments. The programs can be run interactively, but using the scripts allows quick editing to fine-tune multiple runs. For focmec, the first run was done with eight polarity errors, but from an examination of the output file it became clear that four was the minimum which could give a solution. After a run of focplt, program sgfp was used to display the plot file on the (Sun Openwindows or CDE) terminal, so iterations were again very quick. The command for running sgfp is

sgfp 4err.sgf x

for file 4err.sgf. If one cannot see this on the terminal, one can print it using

plsgf 4err
for the same plot file. The script plsgf first converts the .sgf file to a postscript file using program sgf2ps, which is included with this distribution. It then prints the postscript file and erases the postscript file. If one wants the postscript file, one can simply run sgf2ps. Enter the command sgf2ps with no argument to display the options for this program.

If one wants to include the plot file in a document, one can enter

sgf2eps 4err
This program assumes one has programs epstool and ghostscript, which are not included with this distribution but can be obtained without cost from many servers. One can make PDF versions of the files using either ghostscript or the Adobe Acrobat package.

The two plot files were produced by runs of focplt using driver files rfocplt_solutions and rfocplt_data and then converting the SGF files to PDF files using program ./bin/sgf2eps. The PDF files are also in the manual, focmec.pdf.

Prior to March, 2002, program focmec input code allowed one to enter + and - for emergent polarities (compression and dilatation respectively), but it did not use them. (They could be plotted in program focplt, if the user so chose.) A user suggested they be used in program focmec. Accordingly, if any "SENSE" is + or -, the user will be prompted to choose if the emergent polarity picks should be used or ignored. To demonstrate this, a set of files has been added: a single input in which three of the P picks have been changed from "C" to "+" plus two driver files and sets of output files with results for the two options. A single acceptable solution has been chosen by confining the focal sphere search. The new files all have "emergence" in the file name.