Rheology of hexagonal close-packed(hcp) iron

The viscosity of hexagonal close-packed (hcp) Fe is a fundamental property controlling the dynamics of the Earth’s inner core. We studied the rheology of hcp-Fe using high-pressure and -temperature deformation experiments with in situ stress and strain measurements. Experiments were conducted using D111-type and deformation-DIA apparatuses at pressures of 16.3–22.6 GPa, temperatures of 423–923 K, and uniaxial strain rates of 1.52 × 10−6 to 8.81 × 10−5 s−1 in conjunction with synchrotron radiation. Experimental results showed that power-law dislocation creep with a stress exponent of n = 4.0 ± 0.3, activation energy of E* = 240 ± 20 kJ/mol, and activation volume of V* = 1.4 ± 0.2 cm3/mol is dominant deformation mechanism at >∼800 K, whereas a mechanism with power-law breakdown prevails at lower temperatures. An extrapolation of the power-law dislocation creep flow law based on homologous temperature scaling suggests the viscosity of hcp-Fe under inner core conditions is ≥∼1019 Pa s. If this power-law dislocation creep mechanism is assumed to be the dominant mechanism in the Earth’s inner core, the equatorial growth or translation mode mechanism may be the dominant geodynamical mechanism causing the observed inner core structure

Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir

We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (σ1) axis was stably oriented approximately W20°N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the σ1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone