1. 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]

2. 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]

3.  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]

4. 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

5. 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]

6. 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

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

8. 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