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We present a new Eulerian large-strain model for Maxwell viscoelasticity using a logarithmic co-rotational stress rate and the Hencky strain tensor. This model is compared to the small-strain model without co-rotational terms and a formulation using the Jaumann stress rate. Homogeneous isothermal simple shear is examined for Weissenberg numbers in the interval [0.1; 10]. Significant differences in shear stress and energy evolution occur at Weissenberg numbers >0.1 and shear strains >0.5. In this parameter range, the Maxwell–Jaumann model dissipates elastic energy erroneously and thus should not be used. The small-strain model ignores finite transformations, frame indifference and self-consistency. As a result, it overestimates shear stresses compared to the new model and entails significant errors in the energy budget. Our large-strain model provides an energetically consistent approach to simulating non-coaxial viscoelastic deformation at large strains and rotations.

The internal structure of the Clark fault in the trifurcation area of the San Jacinto fault zone is imaged using seismograms recorded by a dense linear array (Jackass Flat, JF) crossing the surface trace of the fault and an adjacent array (TR) to the SW. Delay times between phase arrivals associated with ∼3500 local earthquakes and nine teleseismic events are used to estimate velocity variations within the arrays. The teleseismic P waves travel faster beneath the TR than the JF array, in contrast to larger scale tomographic results. Statistical analysis of local P-wave delay times indicates that the entire JF array, with an aperture of ∼400 m, is inside a low-velocity damage zone. This low-velocity zone is bounded on the NE side by a shallow bimaterial interface generating fault zone head waves, and it contains an inner zone of more intense damage generating fault zone trapped waves. The P-wave velocity contrast across the local bounding bimaterial interface is 10-15 per cent. The trapping structure is associated with a width of ∼200 m, S-wave velocity reduction of ∼35 per cent with respect to the surrounding rock, Q-value of ∼20 and depth of ∼3.5 km. The imaging results suggest that the main seismogenic fault is near the SW end of the JF array, in agreement with a prominent geomorphologic feature. The existence of intense local damage on the crustal block with faster larger scale velocity at depth is consistent with common propagation of earthquake ruptures in the area to the NW.

Frictional heating during earthquake rupture reveals important information on earthquake mechanisms and energy dissipation. The amount of annealing varies widely and is, as yet, poorly constrained. Here we use magnetic susceptibility versus temperature measurements during cycling to increasingly elevated temperatures to constrain the maximum temperature a slip zone has experienced. The case study comprises sheared clay cored from the Japan Trench subduction plate-boundary fault zone (décollement), which accommodated the large slip of the 2011 Mw 9.0 Tohoku-oki earthquake. The décollement was cored during the Integrated Ocean Drilling Program (IODP) Expedition 343, the Japan Trench Fast Drilling Project (JFAST). Heating signatures with estimated maximum temperatures ranging from ∼300 to over 500 °C are determined close to the multiple slip surfaces within the décollement. Since it is impossible to tie a specific slip surface to a certain earthquake, thermal evidence for the cumulative effect of several earthquakes is unveiled. This as yet preliminary rock magnetic ‘geothermometer’ would be a useful tool to detect seismic heating along faults that experienced medium temperature rise, a range which is difficult to assess with other approaches.

The International Ocean Discovery Program (IODP) Expedition 348 extended the borehole of Site C0002, Nankai Trough, down to over 3000 meters below sea floor (mbsf) collecting core samples and in-situ geophysical data from the inner accretionary prism. In order to investigate the in-situ stress state within the prism, we characterized the occurrence and shapes of wellbore failures as observed by resistivity image logs and sonic caliper logs collected in the vertical well of Hole C0002P. Most wellbore failures were observed in the top 70 meters of Hole C0002P (2150 to 2218.5 mbsf), where resistivity images were acquired several days after the borehole was initially exposed by previous drilling and coring runs. Wide breakouts spanning up to 140 degrees are observed in the NW/SE to NNW/SSE direction suggesting that the maximum horizontal principal stress in the cored interval is in the direction generally consistent with those observed in the shallower sections of Site C0002 from earlier expeditions. In the remaining section of the borehole below, wellbore failures are much sparse and subtle in the resistivity image possibly due to the short exposure time between drilling and image acquisition (about 2 hours). Preliminary examination suggest that these features are breakouts aligned in the direction consistent with the upper section, but examination in combination with the sonic caliper data is required for further confirmation. These observations suggest that the occurrence of breakouts exhibit significant time-dependence due to processes such as pore pressure diffusion or time-dependent rock deformation. Moreover, sonic caliper data suggests that shapes and width of the wellbore failures in the studied dataset is influenced by the formation strength anisotropy (in the horizontal direction) enhanced by the steeply dipping bedding planes (60-90 degrees). Thus, constraints on in-situ stress magnitudes will be provided through comparison of wellbore failures, borehole cro