This maintains that the seismic hazard at a place is constant in time, ie the expected intensity in 50 years is the same whether the 50 years in question starts in 1996 or 2006. Therefore modifications to the PSHA method have been made in which parameters describing average inter-event times, and time since last event, are introduced. Unfortunately, recent research suggests that commonsense may be wrong. The stationarity assumption has therefore gained a new lease of life. There are various ways of measuring ground motion.
Engineers usually prefer something more physically quantitative.
The design spectrum is thus shifted up or down according whether the hazard in pga terms is higher or lower. At some frequency values the hazard will be over-estimated, at others it will be underestimated. It is now recommended that the hazard should be calculated separately for a range of frequencies, using the same source zone model and a regionally-valid spectral attenuation model. From this can be built up a spectrum that reflects the real levels of hazard at the site at all frequencies. This is known as a Uniform Hazard Spectrum.