Earthquake
Earthquakes occur when stress built up along faults is suddenly released as seismic waves. Know the difference between magnitude (energy released, one value per quake) and intensity (shaking and damage at a location, varies). Understand P-waves (fast, push-pull, arrive first), S-waves (slower, shaking, cause most damage) and surface waves. Aftershocks follow and can bring down weakened structures.
Safety: Drop, Cover and Hold On - drop to hands and knees, take cover under a sturdy table, and hold on until shaking stops. If outdoors, move away from buildings, trees and power lines. If driving, pull over away from overpasses and stay in the vehicle. If in bed, stay and cover your head. Do not use elevators, and expect power, gas and water outages. After shaking, check for injuries, avoid damaged buildings, and beware of gas leaks and fallen power lines.
Traps: questions often reward running outside during shaking - this is dangerous because of falling debris. Another trap confuses magnitude with intensity. Do not shelter in a doorway; modern advice is under a sturdy table.
Rule of thumb: Drop, Cover, Hold On - then evacuate carefully once shaking stops.
Worked example: Shaking begins in an office. The correct action is to drop under a desk, cover your head and hold on - not to run for the stairs, where falling glass and debris are the main risk.