抄録
The relation between surface-wave magnitude MS and body-wave magnitude mb is studied on the basis of measurents of MS and Haskell's deterministic fault model. The fifty-four measurements of MS are made, over a wide range of magnitude for three large shocks and their aftershocks which occurred in 1968, 1969 and 1973 along the Southern Kurile trench. For a constant mb, MS values of the aftershocks of the 1968 earthquake are systematically larger by 0.5 to 1.0 than those of the aftershocks of the 1973 earthquake. This systematic difference can be explained in terms of the difference in stress drop; stress drops of the 1968 events are considered to be about four to five times lower than those of 1973 events. This is comparable to the difference of stress drops between the two main shocks. There are abnormal aftershocks which show a remarkable deviation from the average MS-mb relation. It is suggested that this deviation represents a breakdown of the assumption of one similarity law. Among various parameters such as stress drop, rupture velocity and rise time of dislocation function, the ratio of rise time to fault length is found most convenient to account for the abnormal values of magnitude. This ratio tends to increase with an increase of MS for some aftershocks of the 1973 earthquake.
