23.5 Fault plane solution program to use and some advice

The different programs all have advantages and disadvantages. FOCMEC uses the most data since it can use more amplitude ratios than HASH but it might be difficult to find the 'correct' solution since a small change in input limits might make a large change in output. If a few of the amplitude ratios are very wrong, an unrealistic high ratio limit must be used and many errors allowed. This problem is avoided by using HASH since the limit is the average amplitude ratio error and not the number of errors. If only working with polarities, all 4 programs can be used. PINV gives a very quick solution which can be used as an indication of a possible best solution, however for final results one of the other 3 programs should be used. It is often a good idea to compare the results from the different programs. Ideally they should all give the same result, but there will be difference due to different methods used and different data, however if solutions are very different, the solution might not be very stable.

Polarities are often not very clear so in general, when using FOCMEC, one should allow for polarity errors. Even if polarities are very clear, there might be errors since the theortical ray calculated by HYP does not correspond to the observed ray. This can be due to a wrong model or a wrong hypocentral depth. If e.g. the program calculates that the first arrival is Pg, but the arrival is very emergent, the first arrival might be a refracted arrival due to wrong model or depth. In some cases a clear Pg arrival might be seen a bit later and can be used instead.

In some cases due to incorrect model, the hypocenter must be fixed to a depth different from what it locates to naturally in order to get a solution.

Note that in order to compare FOCMEC and HASH, amplitudes should only be used on Z and T since HASH cannot use amplitude on Z. In anay case, these amplitudes are the most reliable. In HASH, the number of solutions will depend mainly on the error limit given for the amplitude ratios. If very few solution are found, the solution might be less reliable so increase the limit then. A few hundred solutions is a good number. The results from two solutions can be compared with EEV command 'fd' which calcultes the difference it P and T orientations. It is easy to compare the solutions. Run each program in EEV, then plot using command fo. Each solutions will be plotted in a different color, see Figure 23.8. If doing composite solutions, use program FOC with input from fps.out.