Computer Programs in Seismology Tutorial

Introduction

This lesson creates some synthetic seismograms by surface wave model superposition. The user is requested to determine the phase velocities through the use of the interactive program do_pom. The observed dispersion is then compared to the theoretical values from the model.

I create the directory LessonB  and place the scripts  DOIT,  and DOCLEAN in that directory.

For testing everything is in the tarball  LessonB.tgz . Unpack this with the command gunzip -c LessonB.tgz | tar xvf -    This will create the directory LessonB and place the file 00README and the scripts DOIT and DOCLEAN in that directory.

DOIT

When you run this script, surface-modal superposition is used to create a synthetic seismograms at a distances of 2500-2850  km in 50 km increments for a source with a depth of 10 km and with a faulting model of strike=45, rake=45 and dip=45. For this source depth mechanism, the Rayleigh wave signal is simple in that the fundamental model spectrum is smooth.

Phase Velocity Analysis

After the synthetics are computed the program do_pom is started using the command

	> do_pom B*Z00.sac

to study the Rayleigh wave dispersion. You will see the following graphical menu:



Now clock "SelectALL" and then click "Do POM".

The next page sets the processing parameters. Click on the "Nray" button to select 250 phase velocities between the Vmin and Vmax values, click on the "Shade" to have a color contour display and click on the "Type" to select Rayleigh since you are using vertical component traces. I set "Length" to 2 to get more frequency domain resolution for the plots.
 
When you are done, click on the "Do POM" button to go to the next page.



 At this point the FORTRAN program sacpom96 is executed to create the dispersion information which is the next page displayed.



When sacpom96 is done, you will be presented with a graphical menu. Phase velocity analysis consists of time shifting signals and stacking the spectra. The maximum of the amplitude spectrum will give dispersion values reflecting the true values as well as the artifact of spatial and temporal aliasing. You use this screen to select the dispersion curve.

Clicking on the "Auto" button will ask you to define the mode, here Fundamental. Auto means that clicking on the dispersion window will initiate a "rubber band" for selecting the dispersion. A second mouse click will select the dispersion values nearest the rubber band line.  When you are done, click on "Exit" and then select "Yes" to save the dispersion values.

If we start using the B*T00.sac traces, we will get Love wave dispersion. The graphical results and selected dispersion are shown in the next figure.

Comparison with Theory

The shell script now compares the selected dispersion to the theoretical dispersion for the model. This is a very useful exercise since we can learn something about the imperfections of the multiple filter analysis used to get the group velocities:

Comparison of theoretical and observed phase velocities
Love Wave Comparison	Rayleigh Wave Comparison

We see that the phase velocities are well determined at all periods.

Cleanup

After you are done testing these programs, enter DOCLEAN to clean up the directory. You will be left only with the 00README, DOIT and DOCLEAN files.

Other Tests

Modify the script so that the source depth is 30 km, the faulting mechanism has strike 0, dip 90 and rake 0.  Then run the script. In this case you will notice that the Rayleigh wave signal on the vertical and radial components has a spectral hole. See if this affects the phase velocity determination.