A blind thrust earthquake occurs along a thrust fault that does not show signs on the Earth's surface, hence the designation "blind". Such faults, being invisible at the surface, have not been mapped by standard surface geological mapping. Sometimes they are discovered as a by-product of oil exploration seismology; in other cases their existence is not suspected. Although such earthquakes are not amongst the most energetic, they are sometimes the most destructive, as conditions combine to form an urban earthquake which greatly affects urban seismic risk.