TA sub-array measurements of SmKS ray parameters to determine lower mantle influence

Abstract:

Velocity variations within the Earth can deflect seismic waves from the paths predicted by 1-D velocity models. This manifests as perturbations in the travel-time and incoming wave direction of observed waves. While the presence of velocity heterogeneity in Earth’s mantle has long been known, its affects on waves that travel into Earth’s core are less well documented. The radial velocity structure in the outermost core continues to receive attention, as structure there likely relates to heat and chemical exchange between the core and mantle, as well as core convection and thus the geodynamo. Here we investigate the wave group SmKS, which are SV waves in the mantle that convert to P waves in the core, bouncing on the underside of the core-mantle boundary (CMB) with m legs in the core (for example, S2KS has two path segments in the core with one underside CMB reflection). We specifically investigate the relationship between lowermost mantle velocity heterogeneity and perturbations in SmKS path geometry and travel-times. We use sub-arrays of EarthScope’s Transportable Array (TA) and high-resolution array processing methods to resolve SmKS slowness, back-azimuth, and travel-time information. Regional patterns of both time and incident direction anomalies relative to 1-D reference model predictions are determined, which relate to heterogeneity along the wave path. SmKS travel-times and incident direction anomalies are used in conjunction with information on 3-D heterogeneity from tomography to identify the source of the observed anomalies. By identifying the lower mantle contribution to the times and paths of SmKS waves, the efficacy and robustness of SmKS waves to resolve fine-scale outermost core structure can be better established.