The Macquarie Ridge earthquake of 1989

Abstract

SUMMARY A study of the seismicity on the central portion of the Macquarie Ridge before and after the 1989 May 23 Macquarie Ridge earthquake (Mw = 8.2) and the solution of the inverse problem to obtain the moment release time history during the earthquake are presented. The aftershock study shows that there were remarkably few and small aftershocks on the main fault. The aftershocks were distributed along a 220 km portion of the India/Australia-Pacific plate boundary and indicate that the motion was bilateral on a vertical fault. In addition, the earthquake reactivated a 175 km section of a fault to its west which had been dormant since at least 1964 and 44 per cent of the aftershocks occurred on this feature. Moreover, the largest aftershocks in the five month period following the event occurred not on the main fault plane but on the reactivated one. Based on available bathymetry, gravity and magnetic anomaly data from the region, this reactivated feature is interpreted as being an old oceanic fracture zone and hence a pre-existing zone of weakness. The aftershock distribution and their focal mechanisms are interpreted in terms of the tectonics of the region and suggest that a triangular piece of crust to the west of the Macquarie Ridge is being ‘squeezed’ to the north and west. The centroid-moment tensor solution was obtained using both GDSN and GEOSCOPE data simultaneously. It shows that the motion was primarily strike-slip on a vertical fault with a moment of Mo= 1.5 x lo2’ N m and a centroid depth of 19.5 km. The moment release distribution and time history were obtained by inversion of broad-band body waves recorded digitally by the GDSN and GEOSCOPE networks. The rupture is found to be bilateral with the propagation first starting towards the north-east and a few seconds later towards the south-west. The moment release was low in the hypocentral area with the higher areas of moment located towards the north-east and south-west of this. The average rupture speed was approximately the shear wave speed. No simple relation between the moment release pattern and the distribution of aftershocks on the fault plane was found. Instead, the aftershock distribution is seen to be dominated by the intersection of the reactivated fault and the main fault plane with most aftershocks occurring to the north of this intersection.