A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary

Abstract

Abstract. A P-SV hybrid method is developed for calculating synthetic seismograms involving two-dimensional localized heterogeneous structures. The finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process. The seismic motions, after interacting with the heterogeneous structures, are propagated back to the Earth's surface analytically with the aid of the Kirchhoff method. Anomalous long-period SKS-SPdKS observations, sampling a region near the core-mantle boundary beneath the southwest Pacific, are modeled with the hybrid method. Localized structures just above the core-mantle boundary, with lateral dimensions of 250 to 400 km, can explain even the most anomalous data observed to date if S velocity drops up to 30% are allowed for a P velocity drop of 10%. Structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core-mantle boundary. Some important issues, such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle, however, remain unresolved due to the nature of the data.

Irregular topography at the Earth’s inner core boundary

“…The observed PKiKP amplitude variations and rapid changes of PKiKP travel time and amplitude across the seismic array further indicate that the ICB topography is irregular and varies at a small length scale. We tested a series of ICB models to place quantitative bounds on the characteristics of the ICB topography, using a hybrid method (Materials and Methods) (25). We tested ICB topographic models with various geometries (Gaussian-shaped, dome-shaped, step), heights, horizontal scales, and spatially repeated patterns.…”

Sound velocity and density of magnesiowüstites: Implications for ultralow‐velocity zone topography

“…Combined with previous work, details such as interior fine-scale structure [Rost et al, 2006], aspect ratio [Wen and Helmberger, 1998a], and interface concavity [Helmberger et al, 2000] paint an increasingly diverse and complicated picture of the lowermost mantle. The wide variety in ULVZ character and further complexity at the core-mantle boundary permit an equally diverse set of mineralogical conditions and could reflect large amounts of heterogeneity in chemistry (such as Fe enrichment, Si depletion), mineralogy (presence of postperovskite, hydrated minerals), and phase (solid, partial melt).…”

Approximate 3D Body-Wave Synthetics for Tomographic Models

“…Note that the w function is nearly the linesource solution except for the inclusion of the ( ) factor. p Ί This feature is exploited in developing 2D numerical simulation of earthquake (dislocation) sources (Helmberger and Vidale, 1988;Wen and Helmberger, 1998). To produce a point-source solution, we simply perform the operations in equation (2) which involves a convolution with .…”

“…Some recent applications involve 2D deep-earth models (Wen and Helmberger, 1998), and mapping abrupt jumps in crustal thickness (Zhu and Helmberger, 1998). Although there are many possible modifications (Frazer and Sen, 1985), the approach followed here is commonly referred to as the KirchhoffHelmholtz integral equation (Born and Wolf, 1964), with p the potential at position r.…”