From Introduction:
Earthquakes are common on Thraxis, because of Arril, the massive moon that looms in the sky. Characters may frequently find that the earth shakes at inopportune moments. In an earthquake, a character is safest in two places, out in the open, or within a structure. Smart characters stay away from loose, heavy objects that can fall on them. The GM must determine the strength of the earthquake in terms of how much damage it does to structures. All buildings and structures (such as roads) are dealt this amount of damage every round, for the duration of the quake. If damage overcomes a structure’s hardness, pieces fall from it, possibly damaging anyone below. Quaking buildings threaten an area around themselves equaling their height. Anyone within these squares must make a reflex save or take damage from falling objects. Keep in mind as well that the earth itself is taking damage, for the duration of the earthquake, and if its hardness is overcome, fissures might open, possibly adding lava to the threats.