The stresses which cause seismic activity stem from several sources, including the nature of the world, its distance from its star, the size and distance of planetary satellites and so on. To compute a stress factor for a world:
F = X + P + S
F = Seismic Stress Factor for the world.
X = Roll of 1d6-3.
P = Die roll with modifier based upon core type:
Heavy Core: 1D – 2
Molten Core: 1D – 3.
Rocky Body or Icy Body: 0
S = Mass of central star in solar masses /world’s orbital distance from the star in a.u.