Tectonic plates move horizontally past one another at transform boundaries in either strike-slip or aseismic motions. Earthquakes and tsunamis occur when the pressure locking plates in a strike-slip boundary releases suddenly. Volcanic activity, while common at convergent and divergent plate boundaries, is rare at transform borders.
Most transform boundaries exist on the ocean floor as short cracks perpendicular to and connecting two segments of a separating ridge. These transform boundaries are the site of shallow earthquakes. Transform boundaries between continental masses and oceanic plates tend to be much longer and produce more severe earthquakes. The San Andreas Fault is a transform boundary in a network of faults at the interface of the North American Plate and Pacific Plate. It connects two divergent boundary ridges, the South Gorda - Juan de Fuca - Explorer Ridge system and the East Pacific Rise.
Transform boundaries exhibit fault creep, the slow movement of plates against one another in the absence of locking. Fault creep occurs near the surface due to the lower pressures between the plates and lubrication from clays and rainwater. Generally aseismic, areas of fault creep develop earthquakes when locking exists in deeper surfaces. The stress on the locked rocks from the movement above breaks the locks and causes an abrupt slip, shifting the relative position of the plates at the boundary.