Low velocity normal fault structures associated with intermediate depth seismicity - insights from body wave dispersion

Low velocity structures in the upper slab have been inferred from guided wave studies (e.g Abers 2000) and subsequently resolved by a number of geophysical methods including receiver function analyses and high resolution seismic tomography . These low velocity structures are potentially explained by the presence of hydrous mineral assemblages in the subducted oceanic crust. This supports the long proposed theory that mineral dehydration during subduction may weaken the crust and allow for intermediate depth seismicity. Recently it has been proposed that normal faulting in the slab may provide a mechanism for hydration deep in the slab, as stress changes due to slab bending promotes the downward pumping of fluids (Faccenda et al. 2009). At greater depth, the reactivation of outer rise normal faults has been postulated by Ranero et al. (2005) as a cause Wadati-Benioff zone seismicity. However there is no direct seismological evidence of normal faults at intermediate depths. Here we present seismic evidence that these faults are in fact associated with low velocities, possibly caused by hydrous minerals or the presence of fluids. We analyse dispersive P-wave arrivals from intermediate depth events in Northern Japan. Finite difference wave propagation models are used to constrain the structure related to this dispersion. Dispersion of deeper events (>150 km) can be explained by a low velocity layer (LVL) at the top of the slab acting as a waveguide. This LVL has a thickness of ~8 km, and a 5 - 12% reduction in velocity. Shallower events (70 - 150 km depth) that occur well below the top of the slab also show dispersion. This cannot be attributed to the upper LVL as the event must be on or near to the waveguide for dispersion to occur. Numerical modeling shows that these shallower dispersive events can be explained by very low velocity structures dipping with a normal fault geometry. We propose that these dipping planes might indicate hydrated material along normal faults. This observation has significant implications for understanding where and how intermediate depth seismicity occurs.