New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large‐displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault‐normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid‐assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross‐cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time‐varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault‐valve behavior to recharge the fault with fluid, post‐seismic self‐sealing of fracture networks to reduce permeability and trap fluids, and time‐dependent compaction of the core to generate high pore pressure. The localized slip and fluid‐saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mech...

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Fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening. Fracture energy has been estimated from seismological and experimental rock deformation data, yet its magnitude, mechanisms of rupture surface formation and processes leading to slip weakening are not well defined. Here we quantify structural observations of the Punchbowl fault, a large-displacement exhumed fault in the San Andreas fault system, and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake. If fracture energy is attributed entirely to the production of fracture surfaces, then all of the fracture surface area in the Punchbowl fault could have been produced by earthquake displacements totalling <1 km. But this would only account for a small fraction of the total energy budget, and therefore additional processes probably contributed to slip weakening during earthquake rupture.

2 Materials and Methods Temperature DataTemperature data were collected with 55 miniature temperature loggers (MTLs): 10 TDR-2050s and 15 TR-1050s manufactured by RBR Ltd. (Canada; www.rbrglobal.com/) and 30 Antares 1357 high-pressure data loggers manufactured by Antares Datensysteme GmbH (Germany; www.antares-geo.de/). Each of the MTLs has an autonomous data logger and a temperature sensor enclosed within a titanium casing pressure rated for up to 10,000 m water depth. The TDR-2050s also have a pressure sensor that effectively records the sensor's water depth inside the cased borehole. The MTLs were attached to spectra rope and wrapped with a rubber protective covering. The sensor string was attached to a hanger and hung within 4.5" steel tube casing with a check-valve at the bottom that prohibited fluids from flowing into the casing from below. Spacing between sensors varied from 1.5 m at the bottom near the fault zone to 3 m, 6 m and greater intervals higher up. The sensors recorded every 10s, 20s or 10 minutes depending upon the model. The RBR temperature sensors have precision of <0.00005°C and the Antares 0.001°C. In addition to factory calibration constants, each temperature sensor was calibrated using a Hart Scientific water bath containing a mixture of ethylene glycol and water and an NIST reference temperature probe over 8 or more different temperatures from 0 -30 o C and spanning the range recorded during the JFAST experiment. The resulting sensor corrections permit accuracy for all temperature sensors to within ~0.001 o C. Reliable corrections could not be obtained for sensors at 744.77 and 805.17 mbsf. The absolute temperatures for these two sensors may be off by a few 10 -3 o C , although their residual temperatures appear consistent with neighboring data. Additional details regarding the sensors and observatory are described in (13). Thermal PropertiesKnowledge of thermal-physical rock properties is important for interpreting the temperature data. Differences in thermal conductivity may lead to steady-state perturbations in the background geothermal gradient. Estimates of the thermal diffusivity are important for interpreting an observed temperature anomaly from frictional heating, and volumetric heat capacity controls the relationship between heat and temperature. We utilize thermal property measurements taken on core material from borehole C0019E that cover lithologic and depth intervals that correspond to the regions covered by sensors in the observatory. Thermal conductivity values consist of 45 shipboard measurements on split cores using a TEKA thermal conductivity half-space probe (13). An additional 38 discrete samples were also measured using a divided bar system revealing similar results. Four large samples were also measured using the transient plane heat source method revealing very little anisotropy in thermal conductivity. Thermal diffusivity and heat capacity measurements were also determined for these four samples. The lowermost three samples are most representative of the intervals...