Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones

International audienceDouble Wadati-Benioff seismic zones (DSZ) with two parallel planes of seismicity separated by 15-30 km are a global feature of subduction zones in the 50-200 km depth range. Upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated. Serpentine or hydrous-phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration. High-resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization. Earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs. The contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2-3 kilometers beneath oceanic crust

3-D Kinematics of Water Masers in the W51A Region

We report proper motion measurements of water masers in the massive-star forming region W51A and the analyses of the 3-D kinematics of the masers in three maser clusters of W51A (W51 North, Main, and South). In W~51 North, we found a clear expanding flow that has an expansion velocity of ~70 km/s and indicates deceleration. The originating point of the flow coincides within 0.1 as with a silicon-monoxide maser source near the HII region W~51d. In W51 Main, no systematic motion was found in the whole velocity range (158 km/s =< V(lsr) =< -58 km/s) although a stream motion was reported previously in a limited range of the Doppler velocity (54 km/s =< V(lsr) =< 68 kms). Multiple driving sources of outflows are thought to explain the kinematics of W51 Main. In W51 South, an expansion motion like a bipolar flow was marginally visible. Analyses based on diagonalization of the variance-covariance matrix of maser velocity vectors demonstrate that the maser kinematics in W51 North and Main are significantly tri-axially asymmetric. We estimated a distance to W51 North to be 6.1 +/- 1.3 kpc on the basis of the model fitting method adopting a radially expanding flow.Comment: 20 pages, 8 figures, 8 tables, appear in the NRO report No. 564 (ftp://ftp.nro.nao.ac.jp/nroreport/PASJ-W51.pdf) and will appear in Publ. Astron. Soc. Japan, Vol. 54, No. 5 (10/25 issue

Analysis of Surface Crack Propagation Considering the Effect of Micro-separations

In the TMCP steels rolled at the finishing temperature in austenite-ferrite region by the Non-AcC type TMCP method, it becames clear that the fatigue fracture surface had a lot of micro-separations, and fatigue crack propagation was prevented by micro-separations in the case of propagation in the direction of plate thickness. In this study, the authors investigated on the behavior of surface crack propagation using three TMCP steels with different SImax, and proposed the method of crack propagation analysis considering the effect of micro-separations. The results may be summarized as follows: (1) As SImax increase, propagation pattern of surface crack become shallow. (2) The macroscopic fatigue crack propagation can be analyzed with the assumption that the fatigue crack propagation in the direction of plate thickness passes through between estimated microseparations. (3) This estimation method shows the good agreement with the experimental change of aspect ratio for surface crack