Crustal and Upper Mantle Structure across Northeastern Tibet

Recently, a large-scale dense seismic array consisting of 800 broadband seismographs with an averaging spacing of 30-40 km was deployed in the northeastern margin of the Tibetan plateau by the Institute of geophysics, China Earthquake Administration (CEA) between 2013 and 2106. This array facilitated collection of more high-quality broadband data, thus providing us an opportunity to investigate crustal and uppermost mantle structure of northeastern Tibet with high resolution by joint inversion of receiver function and surface wave dispersion. We use the neighborhood algorithm (NA) to invert receiver function data and surface wave dispersion. In the model parameterization, we choose smoothed B-splines to represent velocity models. In our model searches, we give about 60% misfit from the receiver functions, about 30% misfit from the dispersion, and 10% for damping and smooth constraint in the final total residual. Our result shows a weak low-velocity anomaly at depths of 20-40 km beneath the central Qilian orogeny. The crustal anomaly is also accompanied by significant low velocities of the uppermost mantle. The coincident anomalies in both the mid-lower crust and the uppermost may suggest an initial localized asthenospheric upwelling after delamination under the Qilian orogeny.

Acknowledgments. The waveform data were provided by the China Seismic Array Data Management Center at the Institute of Geophysics, China Earthquake Administration. This research was supported by the NSF of China (grant 41474089) and the China National Special Fund for Earthquake Scientific Research in Public Interest (201308011).