S Sample Rate: number of times data are collected in a given period of time (i.e. a 30-second sample rate means that data are collected every 30 seconds). Scalar: a quantity that only has a magnitude, and is represented by a single number, such as time and mass. Seismic Moment: a parameter related to the angular leverage of the forces that produce slip on a fault; determined from the seismic waves and field measurements that describe the fault area. Seismicity: relative frequency and distribution of earthquakes. Seismogram: real-time record, made of metal tape, of seismic waves. Seismograph: an instrument used to record seismic waves. Seismology: science that deals with earthquakes and attendant phenomenon including the study of artificially produced elastic waves in the Earth's material. Shear: a type of strain in which the shape of a material is displaced laterally with no corresponding change in volume. Signal: detectable physical quantity. Silicate: insoluble metal salt with silicon and oxygen in its anion; silicates with the addition of quartz describe the chemical group of minerals which make up most of the Earth's crust. Slip Fault: the relative motion of one face of a fault relative to the other. Solution: the final position of the GPS station determined after data are collected and processed. Solution interval: amount of time between GPS data solutions (i.e. daily, monthly, etc.) Strain: change in the shape or volume of a material, often recorded in three-dimensions. Stress: a measure of forces acting on a body. Surface Wave: waves that move close to or on the outside surface of the Earth. S Waves: secondary body waves that shear, or cut the rock they travel through sideways at right angles to the direction of motion; cannot travel through liquid; produce vertical and horizontal motion in the ground surface. System: a group of devices forming a network or used for a common purpose. Systematic Errors: errors in accuracy that make results differ from their "true" value, but are reproducible; usually a result of operator error.