R Random Errors: errors in precision that effect the reproducibility of results from test to test. These are usually a result of errors that occur in the instrument itself and the experiment surroundings. Rate: an expression that describes a change in position or velocity with respect to time. Rayleigh Waves: surface waves that move in an elliptical motion, producing both a vertical and horizontal component of motion in the direction of wave propagation. Receiver: a piece of equipment that receives information, such as a radio signal. Recurrence Interval: the average period of time between earthquakes in a seismic region. Repeatability:the average change in the position over the total time. Rigid Body: objects whose constituent particles remain in fixed orientations relative to each other, as well as systems whose constituent parts may move relative to one another.