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...

Large coseismic slip was thought to be unlikely to occur on the shallow portions of plate-boundary thrusts, but the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (Mw) = 9.0] produced huge displacements of ~50 meters near the Japan Trench with a resultant devastating tsunami. To investigate the mechanisms of the very large fault movements, we conducted high-velocity (1.3 meters per second) friction experiments on samples retrieved from the plate-boundary thrust associated with the earthquake. The results show a small stress drop with very low peak and steady-state shear stress. The very low shear stress can be attributed to the abundance of weak clay (smectite) and thermal pressurization effects, which can facilitate fault slip. This behavior provides an explanation for the huge shallow slip that occurred during the earthquake.

The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

International audienceA 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism

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.

Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪  Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪  Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪  Dynamic friction during the earthquake was lower than static friction. ▪  Complete stress drop is possible, and stress reversal is plausible.

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