There are now in operation many national, regional and international seismic networks, which provide information about the location, size and motion of earthquakes anywhere in the world. However, shallow- focus tremors of lower magnitude, may not be detected by these means, and must be monitored more closely, on a local basis. Seismic hazard maps can be constructed to identify areas at varying risk from earthquake damage. Earthquakes constitute one of the greatest natural hazards to human society. Damage to buildings, roads, sewers, gas and water lines, power and telephone systems, and other built structures commonly occurs.
Earthquakes are predominantly natural events.
Underground explosions, particularly for nuclear testing, can also generate seismic events. Any area of active tectonics or weakness in old cratons, or where human activities change subsurface rock pressures. Remote, away from obvious sources of ground shaking, such as traffic, mines, quarries, and heavy industry. For heavily populated areas in seismically- active areas, a dense array of seismographs is recommended. The effects of increases in crustal stress, which can be released through earthquakes, is becoming increasingly important as a tool for estimating seismic hazard.