Estimation of Strong ground motion

Ground motion estimation for the purpose of seismic hazard and loading analysis may be eventually carried out in some empirical manner or on the basis of theoretical models. The empirical approaches consist essentially of the use of observed relations of strong ground motion parameters derived from various earthquakes zones. In a more refined manner the analyst tries to cull from a vast data set of strong ground motion records those accelerograms which duplicate the source, travel path and local site conditions. The difficulties coming along with this strategy are quite well-known: The empirical relations derived from records of other earthquake zones with differing seismotectonic setting, characteristics of wave propagation effects and subsurface site conditions can hardly be adjusted to the situation of actual interest. Given the large variety of possible earthquake scenarios with respect to source parameters, effects of wave propagation and site conditions, the choice of “suitable” example seismograms poses severe questions as to the significance of these approaches. The drawbacks of the use of some empirical strategy is certainly exacerbated in earthquake zones with long quiescent periods as, e. g., Italy where the return periods of large damaging earthquakes range from tens to hundreds of years. A possible way out of these problems, which we have adopted in SHAKYGROUND, is the estimation of strong ground motion by means of synthetic simulation of acceleration seismograms. As shown for various earthquake zones and sites synthetic simulation according to the concepts in SHAKYGROUND gives indeed a reasonable match of observed data, provided there is a sufficient knowledge of the source and geotechnical parameters (see, e. g., Langer et al., 1999; Gresta et al., 2004).

Global source parameters are accounted for by applying a band-pass filter to the Gaussian white noise, i.e. C M0 S(f,f0) P(f, fmax) where C is a constant for geometrical spreading and radiation pattern, M0 the seismic moment of the event, f0 the corner frequency, S(f,f0)=f2/(1+(f/f0)2), P(f,fmax)=(1+(f/fmax)2q)-1/2, q the parameter of the steepness of the high frequency decay (here q=4). The corner frequency f0 can be related to the size of the source (its radius r0) after Brune (1970) by: f0=0.372 c/r0, where c is the shear-wave velocity. Finally the seismic stress drop is computed as 7M0/(16r03).

The strength of the synthetic approach resides in the possibility to account for the specific geological site conditions, the effects of wave propagation and the characteristics of the seismotectonic zone. According to the strategy chosen in SHAKYGROUND the user’s experience is exploited for establishing the model parameters rather than for searching of a suitable, experimental example seismogram. Given the stochastic nature of the source model, The program performs a number of simulations and calculates average, standard deviations and peak hold values of engineering seismological parameters.
Furthermore, it allows to randomly change the geotechnical and source parameters, thus enabling the statistical evaluation of the stability of the results. Finally SHAKYGROUND produces a report of relevant simulated engineering parameters of seismological signal together with response spectra which can be directly compared to standard seismic codes.