Simple Epicenter Determination Method by Grid Search Using Arrival Time Order

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  • SEKIGUCHI Shoji
    National Research Institute for Earth Science and Disaster Prevention

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  • 地震波着信順を用いたグリッドサーチによる簡易震央推定法
  • ジシンハ チャクシンジュン オ モチイタ グリッド サーチ ニ ヨル カンイ シンオウ スイテイホウ

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Abstract

We developed a simple method to determine epicenters of earthquakes by grid search using only the arrival time order of seismic waves at several observation stations. A target area is divided into small square areas. For each epicenter in the center of the area, a series of stations ordered by the epicentral distance is calculated. When a seismic event occurs and the observed station series sorted by arrival time is obtained, we search for the calculated series of the same order. The corresponding epicenter location is adopted. Generally, the calculated data are voluminous, so the search requires much time. In order to reduce the searching time, we use a UNIX file structure, of which the directory is a station code name. The station code names from the first to the last of the station sequence are assigned to the directory names from the root to the end. The epicenter data is saved in a file in each directory. This implementation reduces the searching time. The method was applied to the Hi-net P arrival time dataset. The mean difference of the estimated epicenter and that of the Hi-net hypocenter catalogue of all events around Japan is 0.609 degrees, when there is a maximum of 20 matching stations. The mean difference of selected events in the Chubu area, central Japan, is 0.076 degrees. The mean difference of events matching the given maximum station number only, are 0.165 degrees and 0.015 degrees, respectively, and are one fifth and one fourth of those obtained using only two stations. The mean difference can be efficiently reduced by increasing the maximum station number up to 10. It is possible to make the mean difference smaller by taking the disorder of the station sequence in the matching algorithm into account. A grid interval of 0.01 degrees is small enough for this dataset. An equation relating the arrival time error and estimated epicenter error is derived. The estimated epicenter error has the same order of amplitude as that obtained above in the Chubu area.

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