The inner core (IC) east-west hemispheric dichotomy is widely recognized, but the reported position of the hemispheric boundary varies among studies due to uneven sampling coverage and the data analyzed. This study investigates the sharpness of the western hemispheric boundary (WHB) near 180°W by analyzing differential time residuals of PKiKP-PKPdf and PKP(bc-df) for PKPdf phases that sample 155°E-130°W in various azimuthal directions. Using PKiKP-PKPdf observations, the WHB is located at 175°E-180°W in the southern hemisphere, based mainly on the lateral isotropy-anisotropy transition. However, based on the lateral isotropic velocity contrast and this isotropy-anisotropy transition between the two hemispheres, its location is 170-160°W in the northern hemisphere. These findings indicate that the WHB is sharp and exhibits a latitudinal dependence with a 10°-20° kink, as well as 1.75% anisotropy in the uppermost IC across the 180-155°W range of the western hemisphere. As suggested by PKP(bc-df), the WHB could remain at 160°W at depth. The isotropic velocity contrast near the WHB (160°W) between the eastern and western hemispheres is lower than previous estimates using PKPdf phases sampling the bulk part of each hemisphere.

Temporal changes in the subsurface seismic velocity structure reflect the physical processes that modulate the properties of the media through which seismic waves propagate. These processes, such as healing of the surface damage zone and deep crustal deformation, are described by similar functional forms and operate on similar timescales, making it difficult to determine which process drives the observed changes. We examine earthquake-induced velocity changes using the measured lag-time time series τ(t) of the repeating earthquake sequences since the 2004 M w 9.2 Sumatra and 2005 M w 8.6 Nias earthquakes. The S coda velocity changes (δV S , equivalent to −τ S) recover steadily during the 2005-2015 period. The Rayleigh wave velocity changes (δV LR , or −τ LR) undergo transient recovery, followed by a strong δV LR reduction in late 2007. δV S recovery is most likely driven by deep processes, whereas the temporal breaks in δV LR recovery in 2007 mostly reflect surface damage and healing induced by the strong ground

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Spatial sampling, finite bandwidth, and overlying-strata shielding are three key issues to affect the spatial resolution of seismic imaging for deep targets. Some factors have a great impact on the horizontal resolution, whereas others influence the vertical resolution. How to quantify these effects remains controversial for complex media. Most previous studies on seismic acquisition geometries focus on the horizontal resolution for layered media but neglecting to measure the vertical resolution especially in complex media. Conventional criteria for vertical resolution are based on the theory of geometric seismology with the assumption of a simple medium. As a practical alternative for resolution estimation in complex media, numerical methods with wavefield extrapolation for focal-beam analysis can provide comprehensive insight into the combined effect of acquisition geometries, bandlimited frequencies, and complex media on the horizontal and vertical spatial resolutions of acquisition geometries. We incorporate some classic criteria into the focal-beam numerical analysis to measure the spatial resolutions. Four parameters are used to quantify the performance of acquisition geometries. The horizontal (vertical) resolution is defined as the main-lobe width of a focal beam along the horizontal (vertical) direction, whereas the square root of the peak-to-total ratio of energies is referred to as the horizontal (vertical) sharpness. These parameters describe the horizontal and vertical spatial resolution and sharpness to image the target. Numerical examples with typical acquisition geometries demonstrate the performance of numerical resolution analyses in complex media.