Structured regularization based local earthquake tomography for the adaptation to velocity discontinuities
share
Here we propose a local earthquake tomography method that applies a structured regularization technique to determine sharp changes in the Earth's seismic velocity structure with travel time data of direct waves. Our approach focuses on the ability to better image two common features that are observed the Earth's seismic velocity structure: velocity jumps that correspond to material boundaries, such as the Conrad and Moho discontinuities, and gradual velocity changes that are associated with the pressure and temperature distributions in the crust and mantle. We employ different penalty terms in the vertical and horizontal directions to refine the imaging process. We utilize a vertical-direction (depth) penalty term that takes the form of the l1-sum of the l2-norm of the second-order differences of the horizontal units in the vertical direction. This penalty is intended to represent sharp velocity jumps due to discontinuities by creating a piecewise linear depth profile of the average velocity structure. We set a horizontal-direction penalty term on the basis of the l2-norm to express gradual velocity tendencies in the horizontal direction. We use a synthetic dataset to demonstrate that our method provides significant improvements over the estimated velocity structures from conventional methods by obtaining stable estimates of both the velocity jumps and gradual velocity changes. We also demonstrate that our proposed method is relatively robust against variations in the amplitude of the velocity jump, initial velocity model, and the number of observed travel times. Furthermore, we present a considerable potential for detecting a velocity discontinuity using the observed travel times from only a small number of direct-wave observations.
READ FULL TEXT