1. Laboratory and Field Evidence for the Involvement of Fluids in Earthquake Faulting

It is widely accepted that pore fluids in fault zones have a significant influence on crustal faulting processes. Investigating the geological evidence for fluid involvement in earthquake faulting over a range of scales should provide valuable constraints on fault modeling and consequently lead to a better understanding of the diversity of earthquake occurrences. A quantitative understanding of fault rupture processes in the presence of fluid relies largely on geological data derived from detailed structural, petrological, and geochemical analyses as well as experimentally derived results. The main purpose of this chapter is to introduce the geological and experimental settings in which the involvement of fluids in the generation of earthquakes is likely to be recorded. In Sect. 1.1, field evidence recorded by fault-hosted veins is introduced as a possible indication of cyclic fluctuations in fluid pressure associated with faulting. Recent advances in our understanding of the geochemical evidence for the involvement of high-temperature fluids in earthquake faulting are also introduced. In Sect. 1.2, fluid sources within shallow part of subduction zones are introduced briefly. Finally, experimental studies relevant to the role of fluids in faulting processes are summarized in Sect. 1.3.