We have already discussed the fact that earthquakes occur on faults . Many active faults have already been discovered but some questions remain about the potential size of earthquakes on faults which have no associated historical earthquakes. For such faults, scientists attempt to estimate future earthquake magnitudes from fault size. In contrast the magnitude 6.7 Northridge earthquake was caused by displacement on a 14 km long fault segment and the duration of shaking was just 7 seconds. Displacement on faults is related to crustal deformation associated with plate tectonics and is concentrated in relatively narrow zones along plate margins.
Stresses build up in rocks where plates interact.
Faults exhibit movement when stresses reach sufficient levels. Rocks adjacent to the fault may be deformed prior to fault movement. Stick-slip fault deformation associated with earthquakes. Strain can be measured in the vicinity of active faults using a variety of instruments including creepmeters , strainmeters , and satellite positioning systems . Creepmeters survey displacement between two points on opposite sides of a fault.