Why do earthquakes occur? Key to understanding this is knowing the strength of faults. Strong faults can sustain large stresses; weak ones fail under much lower stresses. Because we cannot directly observe faults during earthquakes, we evaluate their strength after the fact by measuring how hot surrounding rocks became as they slid past one another (like rubbing your hands together). Such heating ‘cooks’ the fossilized remains of prehistoric plants in the fault rock, which we can measure to get a snapshot of the heating that occurred during an earthquake.