As seismologists gained more experience from earthquake records, it became obvious that the problem could not be reduced to a single peak acceleration. In fact, a full frequency of vibrations occurs.
Emphasis was usually put on the horizontal acceleration factor, for the simple reason that ordinary structures have a built-in safety factor for the vertical component; that is, gravity.
Nothing is less predictable than the development of an active scientific field.
This is very similar to astronomy where different magnitudes are assigned to the brightness of an astronomical object, depending on the range of wavelengths being measured.
Intensity like signal strength will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point.