25. Measuring amplitude ratios, AUTORATIO

The program runs on an s-file and measures P and S amplitude in both time and frequency domain. The measurements are done for a number of frequency bands and the results are obtained by averaging. The results can be used with FOCMEC and HASH to determine the fault plane solution. Data windows are selected based on the arrival time picks in the file and have a user selectable length (default 2 sec). The program adds 20% of time as pre-phase time. AUTORATIO can convert to displacement or velocity units and allows for filtering of the data. The user can set a maximum hypocentral distance, default is 100 km. While the program would work with Pg/Sg or Pn/Sn, it is probably wise to work with short distances and direct phases only. The results are written to the autoratio.nor output file in Nordic format. If called from EEV (command ar), the results are written directly to the s-file. Another output file, autoratio.out, lists all the amplitude readings.

The data processing steps are as follow, first for P, then S signal:

• Read the complete trace, for P this is the vertical channel, for S the horizontal channels are read and rotated into radial and transverse using the computed back azimuth
• Taper and remove DC, possibly filter and convert to ground motion
• Select time window
• Read amplitude of time domain data: P on Z, SV on Z and SH on T.
• Apply FFT to compute spectra
• Read spectral amplitude by fitting with Gaussian curve
• Do the above for different frequency bands and average
• Write out the results

The following amplitude names are used in the Nordic output file:

• ATPG: amplitude Pg time domain
• ATSG: amplitude Sg time domain
• ASPG: amplitude Pg spectral domain
• ASSG: amplitude Sg specrtal domain

The following amplitude names are used in the Nordic output file:

• StoPt: Sg to Pg ratio time domain
• StoPs: Sg to Pg ratio spectral domain

There are thus 2 S-amplitudes and one P-amplitude and the fault plane solution program will make 3 amplitude ratios, see FOCMEC and HASH. The least reliable S is probably S on Z. The program has a parameter s_on_z which can be set to not reading SV on Z. The program must then be recompiled.

NOTE: Each time the prgrom runs, all old automatic amplitudes are deleted. Manual amplitude pics are not deleted.