Slow earthquakes occur by the same mechanism as ordinary earthquakes, as both are caused by shear slip on faults. However, slow earthquakes take a long time to occur relative to ordinary earthquakes. The slower a fault slips, the less efficiently it generates seismic waves. Thus, the distinguishing characteristic of slow earthquakes is that the waves they generate are weak, particularly at high frequencies, compared to waves from ordinary earthquakes. Highly sensitive seismic and geodetic earthquake monitoring networks installed over the past decade have led to a rapidly expanding collection of newly discovered, unusual seismic phenomena, including silent earthquakes, low-frequency earthquakes (LFEs), very low frequency earthquakes (VLFs), and deep nonvolcanic tremor.