Greek concepts of winds in the earth as the cause of earthquakes persisted until the latter part of the 19th century. The proportion of earthquakes believed to be caused by volcanic processes has progressively decreased. During the 19th century, faulting gradually emerged as the principal cause, becoming firmly established as a result of studies of the 1891 Mino-Owari and 1906 San Francisco earthquakes. With improvements in instrumentation, details of the mechanics of faulting became revealed by measurements of seismograms. It is now sometimes possible to estimate fault displacement, moment, the dimensions of displaced blocks, the velocity of rupture, and the location of the initiation of faulting from seismograms.