It will be observed that most of the methods described above require one to know how the strength of shaking attenuates with distance. The amount of shaking at any point will vary with the magnitude of the earthquake, its depth, and the distance from the point to the epicentre or closest point on the rupturing fault.
Calculating this for intensity is usually not too hard as there are plenty of observational data available. But since strong ground motion recording instruments are a fairly recent development and still not widely used, finding empirical data for calculating acceleration or spectral attenuation can be difficult. This is particularly the case for low-seismicity areas, where significant earthquakes are rare anyway.
Acceleration attenuation data are plentiful for California, the Mediterranean, and Japan, but not elsewhere. In places like Eastern North America and Northern Europe seismologists are looking to theoretical techniques to estimate attenuation, in order to circumvent the lack of hard data.