You have to be realistic when it comes to earthquakes, if a big one hits then kiss your tanks goodbye. In smaller quakes, it helps if your tanks are acrylic since they can take more impact than say a glass tank of comparable size. A magnitude 2 quake shouldn't bother anything and most people would feel slight movement. Taller tanks are going to be more affected than shorter tanks. Remember that the sloshing water can throw a tall tank off it's stand. In a big enough quake even 10,000 gal tanks will move and not always side to side. The type of quake also determines the damage your aquarium might incur.