Body wave tomography – Northern Japan Arc Highest resolution tomography Shows inclined zone of slow velocities But does it represent temperature, melt, or fluids? Nakajima et al. [2001]

Temperature and grain size dependence of dV/dT Studies linking seismic velocities and temperature often use a single value of dV/dT However, dV/dT has strong temperature dependence due to anelastic contribution So we can get very low velocities at high temperatures, small grain sizes  = dlnVs/dlnVp = % change Vs/ %change Vp > 1.6 are often said to indicate melt Experimental results show we can also get large  values without melt dVs/dT vs Temperature dlnVs/dlnVp vs Temperature Anharmonic Calculated using results from Faul et al. [2005]

 is ratio of solid bulk modulus Melt Geometry affects melt/velocity relation as well as porosity/permeability S velocity derivative wrt melt q < 60 Node q % change Vs -------------- % change Vp Tubule  is ratio of solid bulk modulus to liquid bulk modulus After Takei [2002]

Shear Velocity Reduction and Attenuation for Olivine containing Melt Modulus Reduction and Attenuation Mechanism Melt and seismic attenuation Line thickness gives melt content; line color gives grain size For a given grainsize, 1% melt gives nearly an order of magnitude increase at 1 Hz Seismic velocity reduction occurs through both “melt squirt” and grain boundary sliding Faul et al., 2004

P,S, and Q Tomography - Tonga Arc Velocity tomography from Conder and Wiens [2005]; Q tomography from new tomographic Inversion of data from Roth et al [1999]

Geodynamic Modeling of Tomographic Velocities Temperature Model Model From J. Conder P velocity calculated from temperature model S velocity calculated from temperature model Relations From Faul et al 2005 Tomography From Conder & Wiens 2006

Modeling Attenuation Structure Calculated Q model (temperature effect only) Temperature model Q tomography

Thoughts Low velocity regions in arcs, spreading centers do represent melt production and transport regions It is difficult to assign porosity due to lack of experimental results on the seismic properties of partially molten peridotites It is probably difficult to obtain upper mantle low velocities without SOME porosity but can we really rule out very low porosity (< 0.1%)? It is essential to understand melt geometries as a function of melt fraction Beware of circular reasoning -- seismologists interpreting results to be consistent with models -- modelers assigning porosity to be consistent with seismological results