Oct. 28, 2020,
noon - 1 p.m.
Tong Zhou: Array-based seismic waveform coherency measurement, simulation, and application in evaluating back-projections Array-based seismic source imaging methods, e.g., back-projections, are routinely applied to the large earthquake investigation and earthquake hazard assessment. The back projection (BP) method locates the high frequency energy radiators in the fault region with the alignment and stacking of the coherent seismic phases, which represents the fault rupturing process. However, what the BP radiators actually images is still under debate, along with the accuracy and resolution of the BPs, especially when waveform complexity presents. Two candidates that are sensitive to BPs include the slip acceleration and the rupture speed change. To test the BP methods and distinguish what fault properties the BP most sensitive to, we need to simulate the real incoherency waveforms. In this work, we first measure the coherence fluctuation with time, frequency and interstation distance in an array with moderate size earthquakes which are supposed to be treated as point sources at teleseismic distance. Then we propose two methods: (1) multiple plane waves and (2) multiple Born scatterers to fit the fluctuation of coherence statistically. Such semi-analytical methods capture the statistical features of the incoherent coda waves, and save significant computation time compared to numerical simulations. With this method, we are able to test the resolvability of various issues including earthquake nucleation, bilateral propagation, fault jumping, supershear transition, etc. and evolve the methods for improving BP images, e.g., slowness calibration, artifacts mitigation, and even to build possible relationships between asperity sizes and coherence fluctuations. /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////// Erik Weidner: Radial Anisotropy in the Indian Ocean from Higher Mode Surface Waves and a Hierarchical Transdismensional Approach A fully non-linear transdimensional hierarchical Markov Chain Monte Carlo approach was developed and applied to fundamental and higher mode Love and Rayleigh wave dispersion data to constrain radial anisotropy in the Indian Ocean. We obtained three-dimensional tomographic models of shear-wave velocity and anisotropy with quantitative uncertainties down to the mantle transition zone. We compared these models to results from regularized linear inversions of the same data set obtained with and without prior constraints on 410- and 660-discontinuities topography. We found that the undulations of these discontinuities had little effect on the resulting models. We also compared results from non-linear joint and separate inversions of Love and Rayleigh waves and tested the effect of depth parametrization on the models.