Strong motion modelling of the 1993 Tikokino earthquake, southern Hawke's Bay, New Zealand

Abstract

The 1993 Tikokino, New Zealand earthquake (M_L 6.1) is modelled as a unilateral rupture, exhibiting clear source directivity to the south. The earthquake was recorded by four strong motion stations within 30 km: Waipawa to the south, and three sites in Napier and Hastings to the northeast. The shorter duration and greater amplitudes (by a factor of 10) observed at Waipawa with respect to the other stations provide clear evidence for the southward rupture direction.

The Tikokino earthquake occurred on a shallow dipping, oblique reverse fault, and probably represents movement at the plate interface. A high rupture velocity is required to match the distribution of observed ground shaking, and the rupture area is constrained to be c. 7 × 2 km². The moment of the preferred model is 1.1 × 10¹⁸ Nm (M_w 6.0) and the stress drop about 35 MPa. This high average stress drop is consistent with the rupture being confined to an isolated asperity.

The model used consists of a finite, rectangular fault rupturing with prescribed velocity and direction, and with uniform slip. The fault is embedded in a planar layered seismic velocity structure. The ability of the model to match the principal features of the observed seismograms suggests that it will be a useful tool in the prediction of strong ground motion for seismic hazard studies in the region.

Keywords:

• earthquake source directivity
• strong ground motion
• plate interface thrust
• extended fault model