The Matata earthquake of 1977 May 31: A recent event near Edgecumbe, Bay of Plenty, New Zealand

Abstract

An M _L 5. 4 earthquake and associated sequence of smaller earthquakes were well recorded in 1977 in the Bay of Plenty region. Upper crustal aftershock origins clustered and migrated within an area 7 × 15 km elongated east-west The aftershocks were relatively swarm-like, producing a slope value for the frequency-magnitude relationship of b = 1. 29 ± 0. 13, and were apparently of long-sequence duration, with decay coefficient p = 0. 67 ± 0. 03. A northeast-trending rupture fitted for the main shock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side. Teleseismic waveforms, in addition to providing a 10. 5 km estimate of focal depth, help constrain the solution of focal mechanism for the main shock. The preferred solution is for mainly right-lateral slip on a northeast-striking plane but with a normal component The slip trend parallels the front of recent volcanism. Mechanisms for related events range from normal to strike-slip, on parallel and intersecting planes, and are indicative of the complexity of geological structure where north-trending faults of the North Island Shear Belt meet with the front of recent volcanism, as well as of the prevailing traction across the volcanic front.