Tremor, a type of slow fault slip, occurs along almost the entire length of Cascadia at regular intervals of 13-16 months. Tremor occurs deeper on the subduction interface than the locked area where megathrust earthquakes occur. The depth of tremor along the subduction interface in Cascadia ranges from 28 km to 45 km, and the motion is so slow that it is not felt at the surface by people or animals, but it can be measured geodetically. The highest density of tremor activity in Cascadia occurs from northern Washington into southern Vancouver Island, and in northern California. Tremor in Cascadia is monitored by the Pacific Northwest Seismic Network's semi-automatic tremor detection system.