The Matata Earthquake Sequence of 1977, Bay of Plenty, New Zealand

Abstract

An ML. 5.4 earthquake and an associated sequence of smaller earthquakes, including foreshocks, were well recorded in 1977 by a network of 10 seismographs set for a microearthquake survey in the Bay of Plenty region, which is transitional between back-are spreading regions of the Havre Trough and the continental North Island. Upper crustal aftershock origins clustered and migrated within an area 7 km by 15 km elongated east-west. The aftershocks were relatively swarm-like, producing a b- value of 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 mainshock, 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, helped to constrain the solution of focal mechanism for the mainshock. 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 a prevailing traction across the volcanic front. The volcanic region is characterised by a low Poisson's ratio, suggested by the Wadati method to be v= 0.19 ±0.01 in contrast to v =0.27±0.01 for the greywacke region to the southeast; this difference is attributed to contrasting rock types and other conditions either side of the volcanic front. The multiplicity of earthquake sequences in the volcanic region indicates a high degree of heterogeneous structure. A low stress drop of 2.8 MPa inferred for the Matata mainshock suggests that the faulting occurs on pre-existing planes. Off-fault aftershocks occurred where the failure stress increased as a result of the mainshock rupture. A concurrent sequence of earthquakes originating near 50km depth indicated thrusting on the lithospheric plate interface underlying the North Island; thrusting on the interface apparently extends to about 70km depth, where the plates become decoupled. Oblique plate convergence and stick-slip motion on the weakly coupled interface provides the regional dextral shear component observed in the volcanic region for the Matata mainshock. An extensional component is therefore a necessary addition for the observed normal component of faulting, which predominated for the 1987 Edgecumbe mainshock. Wave mode conversions inferred for subcrustal earthquakes and the Matata sequence mainshock indicate that the Moho shallows from 28.5 km to 22 km northwestwards across the volcanic front, suggesting that new crust in the Bay of Plenty region is being created over a wide region rather than by active rifting along a sharp margin.