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...

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

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The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.

Hydrogeologic properties of fault zones are critical to faulting processes; however, they are not well understood and difficult to measure in situ, particularly in low‐permeability fractured bedrock formations. Analysis of continuous water level response to Earth tides in monitoring wells provides a method to measure the in situ hydrogeologic properties. We utilize four monitoring wells within the San Andreas Fault zone near Logan Quarry to study the fault zone hydrogeologic architecture by measuring the water level tidal response. The specific storage and permeability inferred from the tidal response suggest that there is a difference in properties at different distances from the fault. The sites closer to the fault have higher specific storage and higher permeability than farther from the fault. This difference of properties might be related to the fault zone fracture distribution decreasing away from the fault. Although permeability channels near faults have been documented before, the difference in specific storage near the fault is a new observation. The inferred compliance contrast is consistent with prior estimates of elastic moduli in the near‐fault environment, but the direct measurements are new. The combination of measured permeability and storage yields a diffusivity of about 10−2 m2/s at all the sites both near and far from the fault as a result of the competing effects of permeability and specific storage. This uniform diffusivity structure suggests that the permeability contrast might not efficiently trap fluids during the interseismic period.