Fracture (or failure) phenomena are observed over a very broad range of size scales, from atomistic-scale to microscopic-scale to macroscopic-scale fractures. A shear failure (or rupture) of laboratory-scale, whether the shear failure of intact rock or frictional slip failure on a precut rock interface, would be of the order of 10−3 to 1 m. In contrast, shear rupture phenomena occurring in the Earth's interior, including microearthquakes and huge earthquakes, encompass a much broader range of size scales from 10−1 to 106 m. Rupture phenomena over such a broad scale range covering both laboratory-scale and field-scale are encompassed by continuum mechanics. This book deals with shear failures (or ruptures) of a scale range of laboratory-scale and field-scale within the framework of continuum mechanics.

It has been established that the source of shallow focus earthquakes at crustal depths is shear rupture instability along a fault embedded in the Earth's crust, which is composed of rock. At the same time, laboratory experiments have demonstrated that a rock specimen fails (or ruptures) in shear mode under combined compressive stress environments, and that the shear failure (or rupture) of rock is governed by constitutive law. These facts physically mean that earthquake rupture processes are governed by the constitutive law. This enables a deeper understanding of the process of earthquake generation in terms of the underlying physics, if the constitutive law for earthquake ruptures is properly formulated by taking into account the real situation of seismogenic fault zone properties such as fault heterogeneities.