In contrast to the prevalent notion that fault breakage is quite rare, every earthquake since 1906 with M > 6 1/2 and with epicenter on land in northern California or Nevada has been accompanied by some degree of fault breakage at the surface of the earth. The smallest shock for which such effects have been observed was of M = 5¾.
Earthquake energy in ergs (E), length of surface in km. (l), and maximum ground displacement at the fault in cm. (D) are related through the empirical equation
in the range 6¼ < M < 8¼. This suggests that the energy released in these shocks is proportional to the area at the earth's surface of the zone of initially strained rock, and, as a corollary, that the volume of the initially strained rock is no thicker for the very large shocks than for the smallest shock with a surface fault trace. Strain energy for large, shallow-focus shocks in California and Nevada is stored in surface slabs of rock which are bounded below by a zone in which the rock can deform plastically at a rate that keeps up with the elastic deformation of the overlying slab.