Long-period Seismic Data Research Articles

Viscous phase in rocks and the low-velocity zone

The presence of partial melt in the low-velocity zone (LVZ) explains the occurrence of basaltic volcanism and serves as a soft layer for plate tectonics. If it is assumed that the melt occurs in inclusions, theoretical and experimental results show that the effective bulk modulus depends mainly on the bulk modulus of the inclusions and their aspect ratio. The effective shear modulus and attenuation in shear depend on the viscosity, aspect ratio, and frequency of applied stress waves. Only a very small amount of melt in narrow inclusions is necessary to cause a large reduction in shear modulus. The results are consistent with observed variations with depth in the LVZ of compressional and shear waves. However, both effective shear modulus and attenuation in shear are dependent on viscosity and frequency. Consequently, if there is partial melt in the LVZ, velocity and attenuation in, and the shape of, the LVZ are frequency dependent. When short-period and long-period seismic velocity data are combined, the ratio of relaxed to unrelaxed shear moduli in the LVZ is ∼0.83, the viscosity of the melt in the LVZ is found to be approximately 107 poises, and the period at which attenuation in shear has a peak is around 20 sec.