2. Seismological Implications of Fluid Effect on Earthquake Occurrence

It is becoming increasingly clear that pore fluids play an important role in a wide range of phenomena related to the generation of earthquakes and crustal deformation. Such view has been formed by combining results across diverse geophysical observations. Pore fluids will mainly fulfill a twofold role. One is purely mechanical one that occurs through fluid pressure change and the other is mechano-chemical effect such as stress corrosion cracking, pressure solution, and others. We put a focus on the effect of fluid pressure change here. In Sect. 2.1, we introduce the concept of Coulomb’s law of friction coupled with the effective normal stress, which forms a basis for understanding mechanical effects of pore fluids on earthquake occurrence. In Sects. 2.2–2.9, we mention a variety of examples of seismological and geodetic observations that suggest the involvement of pore fluids in the generation of earthquakes. Finally in Sect. 2.10, we discuss a possibility to estimate hydraulic diffusivity of rocks forming fault zones on the basis of observation of seismic activity.