23.1.7 Making synthetic amplitudes and polarities to test FOCMEC and HASH

A synthetic input can be generated for FOCMEC. The purpose is, for a given fault plane solution with a given set of stations, to test how well 'correct amplitudes and polarities' can be inverted to the known fault plane solution. The procedure is: 1: Select an event with a set of observations and a fault plane solution.

2: Write SYNTET in the F-line in S-file where method is given, using e.g. FOCMEC, this is FOCMEC, replace with SYNTET. 3: Run FOCMEC from EEV. The synthetic polarities and amplitudes will then be generated for the same observations as given in the S-file. These observations are normally written out to the file focmec.dat as polarities and corrected amplitudes. The focmec.dat is then input to the original FOCMEC. The synthetic observations are now made by correcting the focmec.dat file to the theoretical polarities and amplitude ratios corresponding to the SYNTET fault plane solutions. The theoretical values are therefore at the source and not affected by Q and free surface. The original observations in the S-file are not changed. 4: Do the inversion in the normal way.

Since the theoretical values are generated by FOCMEC routines, FOCMEC should exactly return the SYNTET solution. However depending on the number and types of observations, this might not be the case. Before making the solution with FOCMEC, it is possible to edit the focmec.dat file to introduce errors or remove observations in order to test the sensitivity of the solution to errors in input or insufficient input.

All promt line options for FOCMEC:

  a: Calculate angles
  o: Plot saved solution, no questions
  p: Just run foc preparation
  d: Use auto amp as default, use velocity
  pd: Options p and d above
  s: Calculate thoretical amplitudes from fps
  None: Fault plane solution after using HYP

Theroretical amplitudes: Used to calculate amplitudes on the focal sphere for a given fault plane solution, example:

c:\Seismo\PRO>focmec s

30 40 50
 Enter az and ain, enter to stop
22 33
p-amp=  0.0301 sv-amp=  0.6091 sh-amp= -0.3259
log ratios, sv/p, sh/p and sh/sv  2.0272  1.7556  0.2716
P-polarity is: C

 Enter az and ain, enter to stop

For this option, vpvs is hardwired to 1.74. There is no Q-correction or free surface correction, it is just the amplitude on the focal sphere.

Computer limitations: Total number of polarities must be less than the dimension of array DATA (parameter max_data) for Nordic data (see file seidim.inc in INC directory).

Figure 23.1 shows an example of a fault plane solution calculated with FOCMEC.

Figure 23.1: Top: An example of a fault plane solution plot. Symbols are explained in the text. Bottom: A fault plane solution also showing the stations with corresponding polarities.