Comparative Study on Source Processes of Recurrent Earthquakes

为了提取与重复性地震有关的凹凸体（大滑动区），对日本气象厅几个观测站的历史地震记录进行了波形反演。历史上的地震图通常记录在烟熏纸上。由于近年来影印技术和图象处理技术的发展，我们可以通过对波形数据的跟踪、数字化和倾斜杠杆的弧效应的校正来重建波形数据，得到了以下有趣的结果：（1）1968年十胜冲地震由几个凹凸体组成，其中一个凹凸体与1994年三陆冲地震的凹凸体几乎重合。在两次地震之间的时间间隔内，断层在这个凹凸体上的滑动可以与相对板块运动相比较。这表明在粗糙面上的耦合率接近100%。推测8级大地震是由多个微凸体同时触发引起的，而单个微凸体则导致7级地震。(2)在日向那田地区，小规模的凹凸分布稀疏。因此，没有发生8级大地震，而只有7级地震。(3)1944年托南凯地震基本上由一个单一的粗糙体组成。此区域中没有段结构。因此，该地区不太可能发生7级甚至6级地震。

In an attempt to extract asperities (large slip area) associated with recurrent earthquakes, a waveform inversion was carried out for the historical seismograms at several observatories of Japan Meteorological Agency. Historical seismograms were usually recorded on smoked paper. Owing to the recent development of photocopy and image processing techniques, we can reconstruct feasible waveform data by tracing, digitizing, and correcting the are effect due to inclined lever.We obtained very interesting results as follows :(1) The 1968 Tokachi-oki earthquake consists of a few asperities, one of which is almost coincident with the asperity of the 1994 Sanriku-oki earthquake. The fault slip on this asperity is comparable to the relative plate motion during the time interval between two earthquakes. This indicates that the coupling rate on the asperity is nearly 100%. It is inferred that a great earthquake of M8 class is caused by a simultaneous trigger of more than one asperities, while individual asperities result in M7 class earthquakes.(2) In the Hyuga-nada region, small-scale asperities are distributed sparsely. As a result, a great earthquake of M8 class does not occur but only M7 class events.(3) The 1944 Tonankai earthquake basically consists of a single asperity. There is no segment structure in this area. As a result, M7 or even M6 class earthquakes are unlikely to occur in this region.