Select now the “General” sheet with your mouse. Your screen should look like Fig. below. This sheet is organized in 5 logical units, which are the parameters for the synthetic “Seismogram Information”, the “Simulation Parameters”, the “Statistics [of] Source” parameters, “Statistics [of] Layer” parameters, and the “Absorption mode”.

**The General Sheet with the parameters controlling the nmerical calculus.**

The “Seismogram Information” concerns the “Length” of the synthetic seismograms given in points. For internal reasons of this number must be a power of 2. The second item of interest is the “Digitization frequency” given in Hz. If you select for example 200 Hz the spacing of points in the synthetic seismograms will be 0.005 s. With a length of 4096 points the seismogram length expressed in seconds or, in other words, the time window will be 4096/200 = 20.48 s. Limits imposed by SHAKYGROUND are 16384 points for the seismogram length and 1000 Hz for the digitization frequency.

In the field “Simulation parameters” you have two boxes where you select the “Number of Simulations” to perform during a run, and the “Seed Value..” for the “..Random Gen[erator]”. The seed value should be an integer. It initializes the random generator at the beginning of a SHAKYGROUND session. The number of simulations are important for the degree of statistical stability of your simulations. A good compromise between the needs of short computing times and statistical stability of SHAKYGROUNDs output parameters can be 50 simulations. In this case SHAKYGROUND will perform 50 simulations with the model parameters specified by the user,varying them according to the choices explained above, then produce a statistics of a number of output parameters and of the response spectra. For testing purposes you may select a small number of simulations, such as 3 which is the default. A choice of less than 2 simulations is blocked by SHAKYGROUND, since otherwise the standard deviation would not be defined any longer.The next two fields “Statistics Source” and “Statistics Layers” concern the manner of how to perform the random parameter variation. You may choose a “Uniform Distribution” where all values within a given range have the same probability. The uniform distribution has finite limits, i. e., certain values cannot be exceeded. In the gaussian distribution the average values have the highest probability of occrurence, however, in theory there is no upper or lower limit of possible values. For the sake of numerical stability SHAKYGROUND limits in any case the parameter variation with respect to its lower boundary in the sense that a value less than 5% of the average is not permitted.

The last field “Absorption mode” you have the choice between an “acausal” or “causal”, absorption model. The acausal absorption model causes the appearance of a little amount of signal energy before the seismic signals physical arrival time. The reason for this phenomenon resides in the fact that the acausal absorption model is zero-phase, in other words there is no phase shift since the seismic velocities are assumed independent of frequency. In the causal mode a velocity dispersion is assumed according to a model developed by Futterman (1962). Even though appearing more reasonable from a theoretical viewpoint one could argue the causal absorption model objecting that the true phase shifts cannot be described correctly neither by a zero-phase nor by Futterman’s model. After all, as experience has shown the results in most cases are affected only to a minor degree by the choice of the absoprtion mode.