STRUCTURAL AND FLUID-CHEMICAL PROPERTIES OF SEISMOGENIC NORMAL FAULTS

Structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines. Important classes include the segment bend, segment termination, segment branch, cross-fault intersection, and segment offset. The effect of these structures on rupture history is not necessarily consistent, neither between individual earthquakes nor between different fault zones. Rupture behavior is dependent on several other factors including loading conditions (regional and localized rupture tip stress fields), fluid-mechanical processes, and chemical processes. Fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments. Greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility. An internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments. The dimensions and structure of this fracture network may also partly control rupture propagation. Presumably, activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other. The geometry of a segment boundary may change with depth and the three-dimensional nature of the structure may be important in controlling rupture history. Fluids influence rupturing via fluid-pressure effects and time-dependent chemical processes. Fracture propagation by stress corrosion may favor instability, and chemical alteration may produce minerals of lower strengths, allowing time-dependent creep. Sealing and healing of fractures, however, may remove damage and increase strength. Elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions, well within the recurrence intervals of most large earthquakes. Time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure. © 1990.