Variation of frequency—magnitude relationship during Taupo earthquake swarm of 1964–65

Abstract

A swarm of 1126 earthquakes with magnitudes M _L between 2·7 and 4·5 began in the north-western part of Lake Taupo in 1964 December. Later it migrated south-eastwards across the Lake and continued until 1965 February. The relationship between the magnitude of the shocks and their frequency of occurrence changed during the course of the swarm, the value of the regression coefficient of b falling from 1·2 to 0·6 in a linear manner with logarithm of time. The average value for the sequence was 0·82. The swarm exhibited two phases, marked by a sharp increase in the number of shocks per day 7·5 days after the swarm began. In both phases the rate of occurrence varied logarithmically with time, which is known to be the case in aftershock sequences. Relationships describing the rate of occurrence and the total number of earthquakes as a function of magnitude and time are derived. The variation of energy release with time is inversely correlated with variations in b.