Comparative Study on Source Processes of Recurrent Earthquakes

为了提取与重发地震有关的凹凸体(大滑移区)，对日本气象厅几个台站的历史地震记录进行了波形反演。历史地震图通常被记录在烟熏纸上。由于影印和图像处理技术的最新发展，我们可以通过追踪、数字化和校正倾斜杠杆引起的弧效应来重建可行的波形数据。我们获得了非常有趣的结果：(1)1968年的东京地震由几个凹陷组成，其中一个几乎与1994年三陆地震的凹陷重合。该凹凸体上的断层滑动相当于两次地震之间的相对板块运动。这表明，微凸体上的耦合率接近100%。推测8级大地震是由多个凹凸体同时触发引起的，单个凹凸体引起7级地震。(2)在Hyuga-nada地区，小尺度凹凸体分布稀疏。因此，没有发生8级大地震，而只发生了7级大地震。(3)1944年托南凯地震基本上由单一的凹凸体组成。这一区域没有分段结构。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.