Avoiding numerical trash

The choice of the seismogram length and the digitization frequency requires some care. From the viewpoint of computing time and numerical exactness it is desirable to work with short seismogram length and, on the other hand, with high digitization frequencies. The attempt to match both demands at the same time, however, may cause problems due to an effect known as “wrap around effect” or “alias in the time domain”. The “wrap around effect” resides in the periodicity of the Fast Fourier Transform. It happens if the duration of the signal is longer than the time window given by the seismogram length divided by the digitization frequency. The parts of the signal, which would fall outside the time window, appear at its beginning, interfering with the signals occurring there for true physical reasons.The effect of wrap around may occur essentially for two reasons: First, the duration of the Green’s function is longer than the selected time window. This typically happens if the sources are distant from the receiver, i. e., the traveltime of the direct signal becomes longer than the time window. You can backtrace this effect from the “eventcode” produced by SHAKYGROUNDs numerical kernel. The eventcode is reported in the file “simul.log”. A further reason for wrap around may be the choice of seismic source moment and global stress drop yielding a large source with a low corner frequency and a long source duration. Actually this phenomenon is not reported by SHAKYGROUNDs eventcode, but can be easily identified by comparing the “Strong motion duration” to the seismogram length. If the strong motion duration is close to the seismogram length then you should indeed suspect the presence of “wrap around effect”. You can convince yourself about the validity of your choice writing the single synthetic seismograms to disk and visualizing them one by one.