Buildings are designed to support vertical loads, but are usually built to face only the modest amounts of side-to-side motion resulting from winds. In an earthquake that lateral motion can cause massive structural stress, often resulting in catastrophic failure. Engineers can reduce the damage by various means, including minimization of weight in the upper stories, securing the walls adequately to the framing or providing strong diagonal bracing for the structural elements of the building.
The ground underneath buildings vibrates in an earthquake, causing it to behave like water or quicksand. In those conditions the footings of most buildings can move independently of each other, causing a building to collapse. Many buildings also have especially tall ceilings at ground level, meaning the columns that require the most strength tend to be the longest and the worst-supported. Earthquake engineers compensate for these problems by securing a building's feet to each other with structural steel, so they can't move independently. Flexible joins between the foundation and the main structural columns can also help prevent column failure.
One of the major causes of collapse is resonance, the unfortunate tendency of some buildings to vibrate at the same wavelength as the quake. This can cause them to fail spectacularly, while taller and shorter buildings on either side do not. This may be dealt with by deliberately designing some flexibility into the building's frame. In the event of an earthquake these areas tend to absorb vibration and change the building's resonance, in much the same way one child may spoil another's bounce on a trampoline.
There are some design considerations in earthquake prevention that have little to do with materials or construction methods. Buildings in earthquake-prone areas should not be unusually thin in cross-section, as this makes them unnecessarily fragile. Buildings should not be designed with self-supporting cantilevered sections or areas of large overhang, as these are especially vulnerable in a quake. Another source of quake damage is physical battering, as nearby structures crash into each other at higher levels. This may be avoided by calculating appropriate clearances, or making buildings taper near the top.