Non-linear Finite-Frequency Waveform Inversion for 1-D Structures

One-dimensional velocity models are representative of regional tectonic units. They are important in determining the locations and focal mechanisms of earthquakes, and provide initial models for tomographic studies. We develop a new approach to the non-linear inversion of finite-frequency traveltimes and amplitudes for 1-D models. Frequency-dependent traveltime and amplitude anomalies are measured by cross-correlation of three-component synthetic and recorded waveforms windowed around body and surface waves. Sensitivity kernels to parameters involved in the 1-D model, such as P- and S-wave speeds and depths of seismic discontinuities, are computed numerically by perturbing the reference model and measuring the resulting traveltime and amplitude perturbations, thus avoiding the invocation of Born approximation. An iterative inversion is carried out with updates of traveltime and amplitude measurements and sensitivity kernels following each iteration. We apply this new approach to the inversion of 1D structures around the source region of the May 12, 2008, Wenchuan earthquake. Numerous moderate aftershocks (Mw=5-6) and densely deployed broadband stations provide plenty of records for obtaining 1-D models along a variety of source-receiver path, revealing lateral structural variations in both the Tibetan Plateau and Sichuan Basin.