Aftershocks and Earthquake Statistics(2) : Further Investigation of Aftershocks and Other Earthquake Sequences Based on a New Classification of Earthquake Sequences

Abstract

Earthquake sequences are usually classified into three types, 1: main shock -aftershock sequence, 2: foreshock sequence -main shock -aftershock sequence, and 3: earthquake swarm.However, it is pointed out in this paper that not a few sequences are multiple, i.e., they are characterized by the sequential occurrence of several simple sequences as illustrated by hourly frequency diagrams for many sequences in Japan.Taking such multiplicity into consideration, a new classification of earthquake sequences is proposed.According to this classification, it is seen that each of the terms "foreshocks", "aftershocks", and "earthquake swarm" used hitherto does not necessarily indicate only one type of earthquake groups.For example, earthquake swarms can be classified into two types.The first type, in which the activity is not triggered by large shocks, occurs mostly in volcanic areas and the largest magnitude rarely exceeds 6.The second type, which is a multiple occurrence of aftershock sequences triggered by several large shocks of comparable magnitudes, is found both in volcanic and nonvolcanic areas and the largest shock in non-volcanic areas occasionally reaches magnitude 8.The geographical distribution of the two types of swarms and the similar type of multiple sequences shows some characteristic features.The b values for some of the second type swarms are very small (b=e0.5).This is explained by the multiplicity of the sequence.The small b values reported for some foreshock sequences may be due to the structure of the sequence similar to the second type swarms.The mutual dependence among aftershocks in the same sequence is investigated by several statistical methods.It may be concluded that there is a noticeable tendency for aftershocks to cluster in relatively short intervals of time, but this tendency is not so remarkable that most aftershocks can be practically regarded as mutually independent events.In a multiple sequence, if the main shock of the earliest constitutent sequence is the largest, all later constituent sequences are usually included in the aftershock sequence of the initial main shock.For such sequences parameters Dr and b are rather small and paramters c and A/A are rather large.However, these parameters estimated for each constituent sequences do not seem to deviate systematically from their standard values, though only limited data are available to confirm this. 202T. UTSU Table 9.Some examples of intermidiate and deep earthquake clusters in and near Japan.