Long-Term Behaviour of an Aftershock Sequence: The Inangahua, New Zealand, Earthquake of 1968

The behaviour of the aftershock sequence of the Inangahua, New Zealand, earthquake (magnitude 7·1, depth 12km), as determined 3·6 years after the main event, can be compared with the behaviour during the first 40 days of the sequence. The b-value (slope of the magnitude- frequency relationship) remains near unity and the rate of aftershock occurrence is consistent with a decay proportional to (time)-1·05. Epicentres of the late aftershocks (magnitudes less than 3·3) occupy roughly the same area as the epicentres of the early aftershocks (magnitudes greater than 3·8), an elliptical region elongated along the trace of the Glasgow Fault. No fault plane is defined by the hypocentres of the late aftershocks. There has been a radical change in mechanisms from the thrusting of the early aftershocks (and main event) to the normal faulting mechanism of the late aftershocks. This change can be interpreted as due to outflow of pore fluids if the dilatancy hypothesis of earthquake occurrence is correct.