Static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature. Given the relative simplicity of the protocol for grain growth experiments, measurements of grain boundary mobility show surprising variations. Boundary mobilities during grain growth are affected by solute and impurity chemistry, chemical fugacity of trace and major elements, pore size and number, pore fluid chemistry, the presence of melts, and the presence of solid second phases, as well as temperature and pressure. All of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments. Provided that the necessary kinetics parameters are known, bounds may be placed on the interface mobility when pores, partial melts, or solutes are present. To predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature.

Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.

[1] When the power law equation, _ e / s n exp(ÀQ/RT ), which relates strain rate (_ e), stress (s), gas constant (R), and temperature (T ), is used to describe thermally activated dislocation creep of calcite rocks, the stress sensitivity (n) and temperature sensitivity or apparent activation energy (Q) differ greatly from rock to rock. To better constrain parameters of a mechanical equation of state, we performed triaxial deformation experiments on dense synthetic aggregates of polycrystalline calcite at temperatures of 873-1073 K and strain rates between 5 Â 10 À7 and 3 Â 10 À3 s À1 to strains <0.20. The strength of the marbles decreases with increasing temperature or decreasing strain rate. Combining microstructure analysis with mechanical data indicates that strength decreases with increasing grain size (d) following a Hall-Petch relation. A detailed analysis of the data revealed systematic dependence of n and Q on stress, grain size, and temperature. The variations in n and Q can be accommodated by using a Peierls law, _ e P = A P s 2 exp(s/ s P )exp(ÀQ P /RT). The resistance to glide, s P , is composed of an intrinsic Peierls stress and a grain size dependent back stress and is given by s P = (AE P,0 + Kd À0.5 )(T m À T ), where T m denotes the melting temperature. The following parameters seem to apply to all calcite rocks: A P % 10 ±0.5 MPa À2 s À1 , Q P % 200 kJ/mol, AE P,0 % 7.8 MPa kK À1 , and K % 115 MPa kK À1 mm 0.5 . Under some laboratory conditions, dislocation creep may operate simultaneously with grain size sensitive diffusion creep, complicating the quantification of the individual flow laws. More accurate flow laws will need to include the evolution of microstructure in composite flow laws, perhaps requiring a statistical description of a microstructure variable yet to be specified exactly.

S U M M A R YWe performed pumping tests with a periodic succession of injection and production intervals in a field of three shallow boreholes penetrating a jointed sandstone formation. Tests were conducted at periods ranging from 10 to 5400 s and pumping rates between 5 and 20 l min −1 . Two types of analysis for the periodic pumping tests are presented. Injectivity analysis rests on the determination of the amplitude ratio and phase shift between the periodic flow rate and pressure records from the pumping well. Interference analysis is based on an evaluation of attenuation and phase shift between the periodic pressure signals at the pumping and monitoring wells. A periodic excitation permits to employ standard signal processing tools, such as fast Fourier transformation, facilitating the evaluation of weak signals. The observed amplitude ratio and phase shift values are compared to analytical predictions of 1-D and radial flow models. Results of interference tests are particularly diagnostic for the applicability of a subsurface model. Our test results do not fully agree with any of the considered analytical models suggesting a heterogeneous subsurface. We suppose that the two analysis methods yield averaged hydraulic properties of different volumes of a heterogeneous subsurface. Periodic injectivity tests screen the subsurface to increasing penetration depth if pumping period is systematically increased, while the subsurface volume between pumping and monitoring well dominates the outcome of an interference test at all periods. Periodic pumping provides several operational advantages compared to conventional testing, such as drawdown, pulse or shut-in tests. Periodic testing can be superposed on a long-term operation reducing operational conflicts and permitting continuous monitoring of changes in subsurface properties. Processing of periodic signals provides results even in a field with strong transient changes in fluid pressure related, for example, to operations in nearby wells. Periodic pumping can be performed in a closed loop minimizing the total fluid volume involved.

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[1] A combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a~100 m wide damage zone exists to the northeast with a~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P-wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter-to decimeter-sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro-scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory-derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault-parallel orientation of microfractures, resulting in the lowest P-wave velocity orientated in fault-perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge-like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro-scale damage. Fault-parallel damage fabrics are consistent with existing models for pulverized-rock generation that predict strong dynamic reductions in fault-normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods.

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