As a gravitational wave passes an observer the observer will find spacetime distorted by the effects of strain. In principle gravitational waves could exist at any frequency. However waves at very low frequencies would be impossible to detect and there is no credible source for detectable waves of very high frequency. Although gravitational waves are constantly passing through Earth even the strongest waves have a minuscule effect on Earth and their sources are generally at a great distance, which further reduces their impact.
This text is adapted from a Wikipedia article Gravitational wave. This text is licensed under the Creative Commons Attribution-Share-Alike License 3.0.