Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

Abstract

[1] To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10-11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15-0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30-0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.Components: 8500 words, 15 figures.

Seismic anisotropy tomography: New insight into subduction dynamics

“…These Vp anisotropy results have been confirmed and improved by later studies using better data sets for a few individual areas in Tohoku Huang et al (2011a). The red and blue triangles denote the active and quaternary arc volcanoes, respectively.…”

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region, southwest Norway, and implications for seismic anisotropy

“…In addition, it is found that increasing the dip angle (θ) of a slab in a subduction zone highly rotates the polarization direction of the fast S-wave (Figures 13 and 14), resulting in the polarization direction of the fast S-wave subnormal to the lineation (flow direction) with increasing dip angle, which can explain trenchparallel seismic anisotropy in a high-angle subduction zone. Because chlorite has a wide stability field at high pressure and high temperature in a mantle wedge and in a subducting slab (Hacker et al 2003;Fumagalli and Poli 2004;Padrón-Navarta et al 2010;Van Keken et al 2011;Till et al 2012;Wada et al 2012), the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge (Smith et al 2001;Currie et al 2004;Park et al 2004;Pozgay et al 2007;Long and Silver 2008;Huang et al 2011b;Wagner et al 2013) as well as in subducting slabs (Wagner et al 2013;Wang and Zhao 2013) depending on the dipping angle of slab in a subduction zone where chlorite is stable. International Geology Review 665…”

“…The polarization direction of the fast S-wave is often trench-normal in the back-arc area, but changes to trench-parallel in the fore-arc area (Smith et al 2001;Huang et al 2011aHuang et al , 2011b. This anisotropy can be attributed to the B-type olivine LPO (Jung and Karato 2001a;Kneller et al 2007).…”

“…For example, the Juan de Fuca slab under the Cascadia subduction zone is dipping at low angle and the trench-normal seismic anisotropy observed under Oregon (Currie et al 2004;Park et al 2004;Wagner et al 2013) may be attributed to the LPO of chlorite. In addition, the trench-normal seismic anisotropy observed in northeast Japan with a low dipping angle of slab (θ < 45º) (Huang et al 2011b) may also be caused by the LPO of chlorite in the mantle wedge. Recently, a trench-parallel seismic anisotropy was observed in a limited area under Oregon using the Rayleigh waves (Wagner et al 2013) where the source area of the trench-parallel seismic anisotropy includes the nose of 2-D corner flow.…”

Tomography and Dynamics of Western-Pacific Subduction Zones

“…Hiramatsu et al (1997) found that the subducting slab shows a strong anisotropy resulting from phase changes in the slab from shear-wave splitting analyses of ScS waves that travel from the events in the slab down to the deep mantle and are bounced back from the core-mantle boundary. Huang et al (2011b) further studied the anisotropic structure under NE Japan by analyzing shear-wave splitting of earthquakes that occurred in the upper crust, lower crust and in the double seismic zone within the subducting Pacific slab (Fig. 48).…”