Effects of Near‐Surface Complexities on Differential Travel Times and Amplitude Ratios Between PP and Its Precursors

Abstract

AbstractThe characteristics of mantle discontinuities at depths of 410 and 660 km are important to study the mantle dynamics. With the advantage of global samplings, the differential travel times and the amplitude ratios of PP and its precursors (PdP) have been extensively used to estimate the topography and the impedance contrast of mantle discontinuities. According to previous studies, effects of large‐scale structures on the PP phase cannot be neglected. Theoretically, the complex near‐surface structure, including ocean layer and crustal heterogeneity, may complicate the PP phase. It is necessary to assess the uncertainties of waveform by taking into account the near‐surface complexities. Through the usage of an efficient algorithm for constructing synthetic seismograms, the effects of near‐surface structure on the differential travel times and the amplitude ratios between the PP and PdP are explored. We demonstrate that the crustal structure mainly affects PP‐PdP differential travel time with minor effects on the amplitude, while the ocean depth governs PP/PdP amplitude ratio rather than the travel time. Through incorporating the crust model, the anomalies of PP‐PdP travel time and PdP/PP amplitude ratio are globally mapped. A clear continent‐ocean pattern was observed in the measurements of differential times and amplitude ratios. Moreover, time shift caused by the near‐surface complexities may be wrongly attributed to the topography of mantle discontinuity and our tests show that the offset could be up to 15 km, which should be taken into account in future PP precursor studies.