FRACTURING AND HYDROTHERMAL ALTERATION IN NORMAL-FAULT ZONES

Large normal fault zones are characterized by intense fracturing and h ydrothermal alteration. Displacement is localized in a slip zone of ca taclasite, breccia and phyllonite surrounding corrugated and striated fault surfaces. Slip zone rock grades into fractured, but less comminu ted and hydrothermally altered rock in the transition zone, which in t urn grades abruptly into the wall rock. Fracturing and fluid flow is e pisodic, because permeability generated during earthquakes is destroye d by hydrothermal processes during the time between earthquakes. Fract ure networks are described by a fracture fabric tensor (F). The permea bility tensor (k) is used to estimate fluid transport properties if th e trace of F is sufficiently large. Variations in elastic moduli and s eismic velocities between fault zone and wall rock are estimated as a function of fracture density (epsilon). Fracturing decreases elastic m oduli in the transition zone by 50 100% relative to the country rock, and similar or even greater changes presumably occur in the slip zone. P- and S-wave velocity decrease, and V(p)/V(s) increases in the fault zone relative to the wall rock. Fracture permeability is highly varia ble, ranging between 10(-13) m2 and 10(-19) m2 at depths near 10 km. C hanges in permeability arise from variations in effective stress and f racture sealing and healing. Hydrothermal alteration of quartzo-feldsp athic rock at T > 300-degrees-C creates mica, chlorite, epidote and al ters the quartz content. Alteration changes elastic moduli, but the ch anges are much less than those caused by fracturing. P- and S-wave vel ocities also decrease in the hydrothermally altered fault rock relativ e to the country rock, and there is a slight decrease in V(p)/V(s), wh ich partially offsets the increase in V(p)/V(s) caused by fracturing. Fracturing and hydrothermal alteration affect fault mechanics. Low mod ulus rock surrounding fault surfaces increases the probability of exce eding the critical slip distance required for the onset of unstable sl ip during rupture initiation. Boundaries between low modulus fault roc k and higher modulus wall rock also act as rupture guides and enhance rupture acceleration to dynamic velocity. Hydrothermal alteration at t emperatures in excess of 300-degrees-C weakens the deeper parts of the fault zone by producing-phyllitic mineral assemblages. Sealing of fra cture in time periods between large earthquakes generates pods of abno rmally pressured fluid which may play a fundamental role in the initia tion of large earthquakes.