A Compositional Component to the Samoa Ultralow-Velocity Zone Revealed Through 2-and 3-D Waveform Modeling of SKS and SKKS Differential Travel-Times and Amplitudes

We analyzed 4,754 broadband seismic recordings of the SKS, SKKS, and SPdKS wavefield from 13 high quality events sampling the Samoa ultralow-velocity zone (ULVZ). We measured differential travel-times and amplitudes between the SKKS and SKS arrivals, which are highly sensitive to the emergence of the SPdKS seismic phase, which is in turn highly sensitive to lowermost mantle velocity perturbations such as generated by ULVZs. We modeled these data using a 2-D axi-symmetric waveform modeling approach and are able to explain these data with a single ULVZ. In order to predict both travel-time and amplitude perturbations we found that a large ULVZ length in the great circle arc direction on the order of 10 degrees or larger is required. The large ULVZ length limits acceptable ULVZ elastic parameters. Here we find that delta V-S and delta V-P reductions from 20% to 22% and 15% to 17% respectively gives us the best fit, with a thickness of 26 km. Initial 3-D modeling efforts do not recover the extremes in the differential measurements, demonstrating that 3-D effects are important and must be considered in the future. However, the 3-D modeling is generally consistent with the velocity reductions recovered with the 2-D modeling. These velocity reductions are compatible with a compositional component to the ULVZ. Furthermore, geodynamic predictions for a compositional ULVZ that is moving predict a long linear shape similar to the shape of the Samoa ULVZ we confirm in this study.

Automated summary

The study analyzed and modeled seismic data from the Samoa ultralow-velocity zone, finding the need for a large single ULVZ to explain the data and inferring the elastic parameters of the ULVZ. Velocity reductions are associated with the composition of the ULVZ, and geodynamic predictions for a mobile compositional ULVZ support the findings of the study.