Intense ground shaking during large earthquakes can damage or even cause failure of engineered structures such as buildings, bridges, highways, and dams. Sustained strong shaking can also trigger ground failures, such as rock falls, landslides, earth flows and liquefaction. Strong motion seismology uses special sensors, called accelerometers, to record these large-amplitude ground motions and the response of engineered structures to these motions. This information is used to upgrade building codes, to design earthquake-resistant structures, and to predict the patterns of strong shaking from future large earthquakes. Rapid reporting of shaking levels also helps to focus emergency response efforts in areas where damage is likely to be the greatest. Recordings of large-amplitude seismic waves near the earthquake source can be used to investigate the fault motions that produced the earthquake.