Comparative Study on Source Processes of Recurrent Earthquakes

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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.