They enter a town and see a large crowd surrounding something that cannot quite be figured out. They soon realize that the town has gathered around a woman tied to an old wooden construction that is created to allow her neck to hang over an open a crack in the earth. A ruthless looking man is standing nearby with an ornamental looking knife. Next to him an elderly man is speaking to the crowd. Listening, they realize they are going to sacrifice this woman and let her blood spill into the earth to appease the gods of destruction and save their city.
Further questioning of those in the crowd lets them know that this town has been sacrificing people for over 50 years. The sacrifice stops the earthquakes that threaten to destroy the town. Whenever the earthquakes start, the town has a lottery to decide who will be sacrificed.
1) If the players decide to interfere they may eventually find that a demon is trapped down below in the crevasse and fresh human blood strengthens whatever power holds it at bay. The tremors are it fighting its weakening bond.
2) Perhaps an earth giant just passes by occasionally causing the earth to shake and the people believe that it is their sacrifice that stops it when it is only that the giant has left the area.