hydraulic fracturing has been sometimes linked to induced seismicity or earthquakes. the magnitude of these events is usually too small to be detected at the surface, although tremors attributed to fluid injection into disposal wells have been large enough to have often been felt by people, and to have caused property damage and possibly injuries. a u.s. geological survey reported that up to 7.9 million people in several states have a similar earthquake risk to that of california with hydraulic fracturing and similar practices being a prime contributing factor.