23.1 FOCMEC

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.

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Changes to Focmec

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SEISAN used the 2001 version of Focmec and has now updated to the 2017 version and this has implied some significant changes. Polarity weight can now be used. When using polarity weight, each wrong polarity does not count as one but as the normalized (to 1.0) amplitude. This is done in order to give less weight to polarities near the nodal planes. The number of wrong polarities to use for an accepted solution is therefore a real number and calculated as the sum of the weights. In an extreme case, all polarities could be wrong but the sum of the polarities less than 1.0 but generally, according to Snoke, polarity weight improves the reliability of a solution. Amplitude ratios are calculated as before but counted differently in case of observations near a nodal plane where the calculated ratios could be extreme as indicated by N&D in the output (nominator and denominator both below a given threshold, now hardwired to 0.1 for both P and S, before it was 0.05 and 0.15, repsectively). In the old version of Focmec, these ratios were simply discarded so. E.g. if 8 out of 10 ratios were indicated by N&D, and one error was accepted, only one of the 2 remaining rations had to be ok for the solution to be accepted. In the new version, every N&D ratio is counted as an error so it is more difficult to get an accepted solution, however unrealistic ratios are not accepted.

NOTE: The old version of FOCMEC is included. Running from EEV, the command is f_old and running from the prompt it is focmec_old.

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.iris.edu/pub/programs/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 %).