Additional constraints on the Samoa ULVZ from SKKS and SKS differential travel-times and amplitudes

Of the known ultralow-velocity zones (ULVZs) the Samoan ULVZ is inferred to be one of the largest in physical size. The Samoan ULVZ has been directly linked by tomography to a whole mantle plume, giving rise to active Samoan hot spot volcanism. Yet, little is known about its physical properties and relationship to other large-scale ULVZs such as those beneath Hawaii or Iceland. Here, we further investigated the physical properties of the Samoa ULVZ using differential travel-time and amplitude measurements of SKS and SKKS arrivals. We measured differential travel-times and amplitudes for 13 high-quality events occurring in the Kermadec trench region between 2002 and 2018, sampling the Samoan ULVZ with receivers in the Americas. In order to minimize interference from depth phases, we only considered events with depths greater than 300 km. From these high-quality events we analyzed a total of 4,754 broadband seismic recordings. We then made comparisons of our observations to 2.5D synthetic predictions using the PSVaxi code. We performed a line minimization to find the best fit ULVZ models for the best events in our study. Our results indicated the Samoan ULVZ is well explained by a ULVZ model with a height of 26 km, a width of 10.75⁰ in the great circle arc direction (~650 km on the CMB), and a P-wave and S-wave velocity reduction of 17% and 20% respectively. We show that the large width we inferred here is necessary to explain the SKKS/SKS amplitude ratios observed. In addition, we performed full 3D modeling to assess the results of the 2.5D line minimization. Finally, we show that ULVZ shape in 3D has a large impact on seismic waveforms in the time window around SKS.