Irregular Shortest-Path Ray Tracing Method And Its Application To Earthquake Location

The purpose of this study is to introduce the multistage scheme incorporating with an irregular shortest path method (ISPM) for tracking multiple arrivals composed of any kind of combinations of transmissions, conversions and reflections in complex 2D/3D layered media. The principle is first to divide the layered model into several different computational domains (using irregular cells at the interface and the earth's undulated surface), and then to apply the multistage technique to trace the multiple arrivals. It is possible to realize the multiple arrival tracking with the multistage technique due to the multiple arrivals are the different combinations or conjugations of the simple incident, transmitted and reflected waves via the velocity discontinuities. Benchmark tests against the multistage FMM (fast marching method) and the multistage RSPM are undertaken to assess the solution accuracy and the computational efficiency. The results show that the multistage ISPM method is advantageous over FMM method and RSPM method in both solution accuracy and CPU times.Based on the above ray tracing algorithm, a new technique of matrix inversion with initial global sampling was developed to seek a global solution. Benchmark tests against two popular approaches - the direct grid search method and the oct-tree important sampling method - indicate that the hybrid global-local inversion is a practical selection for earthquake location procedure in the sense of solution accuracy, computational efficiency and non-sensitivity to noise data. The dominant feature of the new method is that a global solution can be found with matrix inversion without extra computational time and mathematic complexity required. It is a good choice to the earthquake early warning, tsunami early warning and rapid hazard assessment and emergency response after strong eqrthquake occurrence. In order to improve the solution accuracy, we also exploit the information of later arrivals.For seismic hazard mitigation, the technology of the Earthquake Early Warning (EEW) is the best choice. Because it can provide a few seconds to tens of seconds of warning time ahead before the impending ground motions. So, many countries turn to develop earthquake early warning system and earthquake emergency system. In this paper, we introduce the current algorithms used worldwide for EEW systems and discuss its benefits in the future.