Earthquakes are far reaching natural disasters that may be barely felt or devastate entire kingdoms. Earthquakes are unpredictable with our current technology, and will remain unpredictable for the foreseeable future. We know that there have been this many earthquakes of this type in this time frame, so it will probably happen again in a similar way. Earthquakes are caused by natural forces such a volcanic activity, magma movement, and tectonic plate movement. Human based activities that have a strong correlation to earthquakes is “fracking” during oil and gas recovery and waste water injection wells. In games that have a mystical or religious component, earthquakes from small to large can be used to justify that the gods are listening or punishing various people depending on the timing. Games with magic may have people who can control the earth and unleash pent up stored energy in the rocks to trigger a major earthquake or cause their own smaller more directed ones.
Earthquakes can efficient devastate large areas, particularly in modern and futuristic cities and states. Some places like Japan and California that experience many medium to large size earthquakes take precautions in their building codes and emergency responders are prepared for the potential devastation. But even the prepared have a hard time dealing with damages to infrastructure. Most damage from earthquakes in modern times are from fires from busted natural gas lines that can’t be put out because there is no running water to fill the fire hoses. Big cities also have big buildings and bridges that bring the risk of being crushed if they collapse.
In more historical settings large earthquakes still topple buildings, but buildings were shorter, in less population dense areas, and there weren’t natural gas lines to catch a city on fire. Some cultures, such as the Inca, were so familiar with earthquakes, that they designed their buildings and cities to withstand most of them. One simple way this was accomplished was by building important structures with a rigid foundation on top of a subgrade of rounded stones. This allowed the building to move as one unit and slide instead of ripping itself apart. Also, windows (and sometimes doorways) would be built as trapezoids that were taller than wide and were narrower at the top than the bottom. This reduces the stress cracking where doors and windows weaken the walls of the building.
Liquifaction, tsunamis, and landslides can be started by earthquakes. Liquifaction occurs when wet, loosely packed soils (usually sandy) are shaken and temporarily acts like a liquid that can cause things on the ground surface to sink up to a few feet with associated damage based on what is sinking. Typically, liquifaction will also cause sand boils (mini sand volcanoes pushed up by the force of water escaping the ground) and can be seen in open areas such as farm fields. Sand boils can cause farm fields to be unusable until they are removed, but are more of a nuisance than a health threat. Tsunamis are caused by earthquakes that occur out at sea. They are large tidal waves that can flood coastal areas from 10-50 feet deep with enough power to wipe out everything in their path. Tsunamis typically occur in a chain of waves from as few as a couple to dozens of them. As discussed in Natural Hazards: Landslides, earthquakes can trigger landslides of various sizes miles away from the epicenter or origin of the earthquake.
Though earthquakes are defined as occurring on earth, quakes can occur on any planetary body. Gravitational forces, volcanic eruptions, and meteor strikes could cause quakes in outer space.
Larger earthquakes don’t happen very often, except in very active zones, so use them appropriately to your world. If earthquakes are common, people are going to adjust and be prepared for them, so you can’t assume everyone is caught unaware in these types of areas.
Earthquakes can be used in games as gentle nudges from gods, to disrupt a man hunt through a city, to engage the characters in humanitarian efforts, or change the face of an entire continent’s power balance.