Improvement of the Ray‐Tracing Based Method for Calculating Hypocentral Loci for Earthquake Location

AbstractHypocentral loci are useful for reliable and visual earthquake location, but they can hardly be analytically expressed when the velocity model is complex. There is a ray‐tracing based numerical method to calculate them, in which a focal locus is represented in terms of ray paths from the minimum point (namely initial point) to low residual points (referred as hypocentral locus reference points, HLRPs) in its residual field. It has no restrictions on the complexity of the velocity model and can produce quite fine results. However, this method is incapable of addressing multi‐segment loci and inadequate for processing large quantity of data. Additionally, it is rather laborious and difficult to set its controlling parameters for obtaining both fine and complete hypocentral loci. In this study, we improve the ray‐tracing based numerical method to overcome its disadvantages: (1) HLRPs are selected from nodes of the model cells that the hypocentral locus goes through, by means of a so‐called peeling method. (2) The calculation domain of a hypocentral locus is defined as such a low residual area that its connected regions each include one segment of the locus and then all the focal locus segments are respectively calculated with the minimum traveltime tree algorithm for tracing rays by repeatedly assigning the minimal point among those HLRPs that have not been traced as an initial point. (3) Short ray paths without branching are removed to make the calculated locus finer. Numerical tests of virtual and real events show that the improved method is capable of efficiently calculating hypocentral loci with good completeness and fineness for earthquakes in a complex model.