Body wave tomography – Northern Japan Arc

Nakajima et al. [2001]•Highest resolution tomography•Shows inclined zone of slow velocities•But does it represent temperature,

melt, or fluids?

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]

Melt Geometry affects melt/velocity relation as well as porosity/permeability

Tubule

Node

S velocity

derivative

wrt melt

% change Vs

--------------

% change Vp

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

Faul et al., 2004

•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

Melt and seismic attenuation

•Seismic velocity reduction occurs through both “melt squirt” and grain boundary sliding

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

P velocity calculatedfrom temperature model

S velocity calculatedfrom temperature model

Temperature Model

Model From J. Conder

RelationsFrom Faul et al2005

TomographyFrom Conder &Wiens 2006

Modeling Attenuation StructureCalculated 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