Anomalous Love-wave phase velocities in the Pacific: sequential pure-path and spherical harmonic inversion

Summary. We have determined the lateral distribution of Love-wave phase velocities in the Pacific for the periods 40, 67, 91 and 125 s. Application of the pure-path and spherical harmonic representation methods indicates that the velocities are primarily a function of the age of the seafloor. A comparison of the results from these two techniques indicates inherent modelling constraints in both methods. The pure-path method is limited by its a priori nature while the spherical harmonic approach is unsuitable in describing sharp lateral velocity gradients. To circumvent these limitations, we propose the sequential application of the pure-path and spherical harmonic methods. The sequential inversion separates the velocity distribution into two separate components; velocity as a function of the age of the oceanic plate and variations superimposed on this relationship. Application of this method demonstrates the presence of velocity anomalies which cannot be modelled by an age–velocity relationship. These anomalies are tentatively correlated with regions of anomalous seafloor depths and/or the presence of active hot-spots. In the central south Pacific, an area with numerous active hot-spots coincides roughly with a region of anomalously slow Love wave velocities. A method for determining the errors associated with the slowness distributions calculated by the spherical harmonic method is presented and provides a means for determining the resolvability of these features.