Numerical Estimation of Confidence Region of Fault-Plane Solutions
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概要
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A numerical method for determining fault-plane solutions and estimating confidence regions from P-wave first motion data was studied. Coverage of first motion data on the focal sphere influences fault-plane solution in the conventional methods, especially in graphical methods. There are sometimes inconsistent directions of first motion which are opposite to adjacent ones though located within common areas of most directions. In the new numerical method such inconsistent first motion data are discarded in an automatic and objective way, and the confidence regions of alternative solutions of the maximum pressure (P) and (T) with scores of 95% or greater are estimated. The new method was applied to 358 earthquakes which occurred in and around the Kanto region, Central Japan. Fault-plane solutions and their confidence regions with the scores of 95% or greater could be obtained for 294 earthquakes (84%) in the first run. For the remaining earthquakes, the preliminary discarding of inconsistent first motion data made it possible to determine the fault-plane solutions. Significant differences of the fault-plane solutions were observed between the best fit solutions in previous methods and those by the new method. Equally probable fault-plane solutions for the same earthquake are sometimes separated into several groups and are also observed in a wide range on the focal sphere.P波初動データによる地震メカニズム解とその信頼域の数値的解析法が改良された.従来の方法,特に図式法では,特定の初動データがメカニズム解を左右する傾向がある.初動方向データの中で,周囲と比較して,明らかに誤まりと思われるものがみられる場合がある.又与えられた初動データを満足する他の解か存在する場合もある.今回の数値解法では,与えられた初動データが客観的・自動的に判別され不良データが省かれる.又初動データの95%以上を満足する主圧力軸(P,T)の信頼域も得られる.この数値解法は関東地方周辺の358個の地震に応用され,294個の地震に対して解が得られた,残りの地震(16%)では最良のメカニズム解に対する初動データのスコアは95%以下である.これらの地震に対する観測点別のスコアにもとづいて,初動データを取拾選択して,メカニズム解を得ることができる.従来の方法による"最良解"と今回の数値解には違いがみられることがある.又代用解がいくつかのグループに分れたり,広範囲に分布する場合もみられる.
- 1982-09-30