An upper mantle discontinuity topography map of southern Nazca Plate and western South America Plate (120°W?60°W, 30°S?5°N) was obtained by using common mid-point stacking in bins of 5 degree radius over ten thousand USArray teleseismic records. The resolution was higher than that of previous studies because the records more densely sampled the region so that stacking bins were smaller. Discontinuity topography corresponding to high temperature anomalies in deep mantle was observed beneath Galapagos, Easter and San Felix hotspots. Other findings include a depression of 410 km discontinuity beneath Southern East Pacific Rise and depressions of both 410 km and 660 km discontinuities beneath the subduction zone.

Because the most used finite difference method to calculate wave propagation cannot handle irregular boundaries easily, a hybrid boundary element and finite difference method is proposed. Firstly, the boundary element method is used to obtain the wavefileds along a horizontal boundary which is going to be the boundary condition of the finite difference method in the next stop. Then the finite difference method is applied to obtain the wavefields under the interface. The wavefields are separated to vector P-wave and S-wave constituents, and considering the coherence of P-wave and S-wave in a conversion point, the image condition of cross-correlation between P- and S-waves is applied to obtain discontinuity image underground. Synthetic tests show the effectiveness of the proposed method.

Based on a 3-D global velocity structure model, the authors used teleseismic P wave data from Asian and South American array to image the rupture process of 2016 Mw 7.8 New Zealand earthquake via backprojection analysis. The results show that the rupture is a unilateral one with northeast direction, extending to the ocean. The rupture speed is about 1.65 km/s. There are two phases dominated by high frequency power radiation, occurring during 20−40 s and 60−80 s, respectively. The second phase is the major one, whose distribution of low frequency power radiation is consistent with the centroid location of the event. The high frequency back projection result of the South American data is better correlated with the peak ground acceleration result. According to the comparison and analysis of the Asian and South American results, it could be inferred that in order to obtain more detailed rupture information of high frequency, the data of array deployed in the region with lower 3-D heterogeneity should be adopted in back projection analysis to enhance the coherency of waveforms.

First, SS precursor boundary sensitivity kernel is calculated based on finite-frequency theory and the sensitivity of SS precursor traveltime perturbation to the topography perturbation implemented on mantle discontinuity is analysed. Next, SS precursor waveform with topography perturbation implemented on mantle discontinuity is simulated using SPECFEM and its traveltime perturbation is measured and compared with the traveltime perturbation predicted by finite-frequency theory. It is found that finite-frequency theory can well explain the wavefront healing effect of SS precursor. At last, an inversion scheme is built based on boundary sensitivity kernel, and more reliable topography of the mantle discontinuity can be obtained after considering the finite-frequency effect of SS precursor. This research provides some preliminary knowledge for inversion of the topography of mantle discontinuities using SS precursor.