Regional waveform modeling for 1D crustal velocity model based on the full-grid search and its application to the southern Korean Peninsula

In this study, we propose a procedure of waveform modeling for the one-dimensional (1D) crustal velocity model using the full grid-search technique. To construct four-layer crustal velocity model, we should determine at least 11 model parameters, such as four P-wave velocities, four S-wave velocities and three layer thicknesses. Here attenuation parameters are fixed from other studies and densities can be determined by empirical relationships. Even though the multiple CPUs are used, the full grid-search for 11 model parameters is still very difficult to be attained because the number of prescribed models is exponentially increasing with the number of model parameters. Thus we propose a three-step modeling procedure consisting of the preliminary, P-wave, and S-wave modeling steps. In the preliminary and P-wave modeling steps, S-wave velocities are fixed from prescribed P-wave velocities by the empirical relationship. By doing this, we can significantly reduce the number of prescribed models and the full grid-search is practicable. The preliminary and P-wave modeling steps are identical but we use coarser and finer grids to search the broader and narrower ranges of model parameters, respectively. The final best-fitting model is obtained in the S-wave modeling step in which we searched only S-wave velocities from the selected models in the previous step. 1D synthetic waveforms are computed by using frequency-wavenumber integration method and the variance reduction is used as a quantitative measure of waveform fitting. The proposed method is applied to the southern Korean Peninsula. We used the broadband waveforms from the two inland earthquakes occurred in Jan. 20, 2007 (Mw 4.6) and Apr. 26, 2004 (Mw 3.6). Our best fitting model significantly increases the variance reduction compared with other velocity models obtained from previous studies for the same region. The separated 1D models determined for the different geologic regions even more increases the variance reductions. Our 1D velocity models can be used as reference models for the development of 2D or 3D velocity models and we will attempt to understand the tectonic evolution of the Korean Peninsula based on our results.