Frequency-Dependent Nature of Pn in Western China

摘要

We are investigating Pn propagation in Western China based on regional events recorded by the Hi-CLIMB (An Integrated Study of the Himalayan- Tibetan Continental Lithosphere during Mountain Building) array in the region. Seismic attributes, including arrival times, Hilbert envelope amplitudes and instantaneous and spectral frequencies, are being used to investigate how the crustal and upper mantle velocity and attenuation structure affects the propagation of Pn arrivals in Tibet. We have constructed more than 30 high- quality regional seismic profiles, and of these, 14 events have been selected with excellent crustal and Pn arrivals for further analysis. Regional travel- times recorded by the Hi-CLIMB array are used to constrain the larger scale velocity structure in the region, with four events near the array used to constrain the crustal structure. We are also independently constraining the variation of upper mantle velocity and depth of the Moho by modeling SK-coupled shear waves at teleseismic distances. The amplitude of the SsPmP phase is particularly sensitive to upper mantle velocities when it is near critical. The initial modeling of the SsPmP phase indicates that the Moho beneath southern Lhasa is over 75 km deep with Pn velocities greater than 8 km/s. In contrast, the data sampling the Qiangtang terrene north of the Bangong-Nujiang suture shows thinner crust with Pn velocities less than 8 km/s. This is generally consistent with the results from the Pn traveltimes for regional events recorded on the Hi-CLIMB array. Nonetheless, there is additional variability of the regional travel-time data, suggesting further 3D complexities. Seismic amplitude and frequency attributes are being extracted from the crustal and Pn wave-trains, and these data are being compared with numerical and analytical results for models with upper mantle velocity gradients, which can strongly affect Pn amplitudes and frequencies.