Variation of the Frequency‐magnitude Relation During the 1931 Hawkes Bay, 1934 pahiatua, and 1942 Wairarapa Aftershock Sequences

Four New Zealand earthquake sequences are studied statistically. The magnitude stability law of Lomnitz does not hold. During the sequences the coefficient b , governing the frequency‐magnitude relationship, is found to increase rapidly after the main shock, and then to decrease until the occurrence of the largest aftershock, when it again begins to increase. This can be explained in terms of stress changes, and is consistent with laboratory studies on rock deformations. One event contains a secondary sequence, which is not usually observed, except in micro‐earthquakes.