Some Investigations of Aftershocks (Part I)

Local Concentration of Aftershock Energy and Crustal Deformation Accompanying the Main Shock

Abstract

It was pointed out by some seismologists that the investigation of occurrence of aftershocks might supply important clues for solving the mechanics of main shock occurrence itself. This paper is the preliminary investigation for this purpose, and deals with the regional concentration of aftershock energy and its relation with the crustal deformation caused by the main shock, chiefly making use of seismological data of the Japan Meteorological Agency (J. M. A.) during the period between 1926 and 1963, with some reference to other data.
First, the accuracy of seismological data of J. M. A. is examined, and according to the degree of accuracy, the following classification is introduced.
1) The first period (1926-1950) in which the average accuracy of epicenters may exceed several tenths of degree, and only great shocks of magnitude 8 and above can be treated for the present purpose.
2) The second period (1951-1960) in which the accuracy of many epicenters can be estimated about one tenth degree, and shocks of magnitude 7 1/2 can be treated.
3) The third period (1961-1963) in which the accuracy itself is shown in the seismological Bulletin of J. M. A. and the accuracy of many epicenters is less than 1 or 2 minutes. In this period shocks of magnitude 7 can be treated for the present purpose.
Then earthquakes in each of the three periods, such as the Nankaido earthquake of Dec. 21, 1946, the earthquake off Boso peninsula of Nov. 26, 1953, and the earthquake off Echizen-misaki of May 27, 1963, are treated.
It is found that aftershock energy is not distributed equally in so-called aftershock area, but there is regional concentration of energy. This energy concentrated area is near the epicenter of main shock, or on a line including the epicenter of main shock. This energy concentration is also found in the case of so-called earthquake swarms, such as the swarm off Ibaragi Pref. in January and February of 1961.
In the case where data of precise leveling are available before and after main shocks, the energy concentrated area or line corresponds to the zone of upheaval or the boundary between upheaval and subsidence caused by the main shock.
In the case of earthquakes of magnitude 6 1/2, such as the north Miyagi Pref. earthquake of April 30, 1962, the regional concentration of aftershock energy is not so clear, but it is found that the aftershock area coincides with the zone of upheaval caused by the main shock.
It is suggested that there seems to be the difference between the aftershock area determined from the routine observation and that from special observations of aftershocks.