The program can be used to determine double couple earthquake focal mechanisms using polarities and/or amplitude ratios for both local and global earthquakes. The program also provides an interactive graphical display. The existing solution can be plotted without any station data or location being available, however if existing polarities should be plotted, the event must be locatable in order to calculate angles of incidence. Several solutions can be plotted on the same figure in order to compare solutions.
The SEISAN program FOCMEC provides the interface between the database and the program that determines focal mechanisms, which in SEISAN is the program FOCMEC_EXE. This program is written by Arthur Snoke [Snoke et al., 1984] and distributed as part of the FOCMEC package (http://www.geol.vt.edu/outreach/vtso/focmec). FOCMEC_EXE is identical to FOCMEC in Snoke's package and can be easily upgraded (unless formats are changed). Generally the user will use FOCMEC when working with SEISAN data, however, it is also possible to run the original version (see documentation by Snoke: INF/focmec.pdf). Before FOCMEC_EXE is started the user can optionally change the inputfile focmec.run.
The program works with polarities and amplitude ratios. See the MULPLT section on how to read polarities and amplitudes. Note that since amplitude ratios are used, there is no need to correct for instrument response provided the response is the same for the different components (within 5-10 %).