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In the course of examining the wide dynamic range records of the aftershocks of the Tokachi-oki Earthquake of 1968, the seventy two shocks having similar wave forms out of about 50, 000 aftershocks were found by means of a simple but systematic data sorting. High correlations in P and later phases are clear not only among the events of the same order of magnitude but also among the events with considerably different magnitudes (M= 1=4. 6). The recurrent occurrence of such events can be traced for the length of about 50 days, which is much longer duration compared with so-called twin or triple shocks reported so far. The activity is, as a whole, a swarm type; however, it is composed of several foreshock-main shock-aftershock sequences. The determination of hypocenters for the several large shocks shows that the events are closely distributed in space within the range of 5 km, which suggests that they have common spatial and dynamical characters. Hereby, the group of earthquakes having a similar wave character can be called an Earthquake Family, which may be the most basic unit of sequence of shocks. The nature of this family is characterized by a small b-value of 0. 43 (total number= 68) in the Gutenberg-Richter's formula: this result is in qualitative harmony with Mogi's explanation on b-value. The variations of P wave spectral amplitude ratio with magnitude are compared with the w2and w3-theoretical source models and the a2-model agrees well with the observed data in the frequency range between 4. 6 and 16. 2 Hz. The average Qa in the crust is estimated to be 400 from the frequency dependence of the difference between the observed and theoretical amplitude ratio.

Recurrent Occurrence of the Earthquakes with Similar Wave Forms and Its Related Problems