3차원 P파와 S파의 속도모델을 이용한 지진위치결정 알고리듬-미국중부지역과 대만에서의 적용사례

Abstract

Traditional local earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3-dimensional complexity of the real earth. During traditional 3-D seismic tomography, simultaneous earthquake relocation using the resultant 3-D velocity model has produced reliable earthquake locations; however only a small subset of events are typically used and thus relocated in the inversion. The rest of the events in a catalog must then be relocated using the 3-D models. The repeated calculation of travel times across 3-D Vp and Vs models is also not efficient and not practical for a routine network earthquake location when the very time-consuming exact 3-D ray tracing is used. Because high-resolution earthquake data are now available from many modern seismic networks, representative high-resolution 3-D Vp and Vs models for a region can be better determined. By taking advantage of recently available high-speed computer technology and large disk space, we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3-D Vp and Vs models. Once the Vp and Vs information for all cubic cells in a 3-D grid model are determined, P and S travel times from each grid point to all seismic stations can be calculated and stored on disk files for a later usage. During the iteration process for earthquake location, travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent 8 grid points available in the previously stored disk files without the need for ray tracing. The iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel time residual, or the difference between the observed and the calculated travel times, is a minimum. Therefore, any local earthquake can be efficiently and reliably located