The geologist infers that the once-distant rocks have been brought close together by vertical or horizontal movement on a fault. East of the Rockies, most earthquakes probably occur by new slip on existing faults. The earthquake is generated when the two sides of the slipping part of the fault grind against each other and send out shock waves or vibrations. The vibrations travel through the Earth and, if they are strong enough, they reach ground level, and can shake and damage any man-made structures there. In general, the bigger the patch of the fault that slips, and the more it slips, the bigger the earthquake.
Big faults can include big slipping patches and make big earthquakes.
For definitions and examples of magnitude and intensity, see our site index . Most earthquakes east of the Rockies are too small to be felt or to cause damage, and most earthquakes large enough to be felt are still too small to cause damage. Damaging earthquakes east of the Rockies are rare. Of those that do damage buildings or other man-made structures, most cause only slight, localized damage with few injuries. However, the U.S. east of the Rockies is such an enormous region that, every few years, even one of the rare damaging earthquakes can occur somewhere.