The Tikokino earthquake of 11 April 1993 : Movement at the plate interface in southern Hawkes Bay

The nature of faulting that took place during the M L 5.9 Tikokino earthquake of 11 April 1993 has been determined using data from six temporary seismographs installed immediately after the event. The rupture initiated at 25 km depth, within the thrust zone between the subducted Pacific and overlying Australian plates. The earthquake had surprisingly few aftershocks, but those that did occur define a rupture zone which parallels the plate interface. When combined with the focal mechanism of the mainshock, this rupture zone indicates that the earthquake involved thrusting at the plate interface. The earthquake ruptured unilaterally to the south, and this explains the strong directivity seen in both strong motion accelerograph and seismograph records. Movement of the plate interface during the mainshock did not lead to significant triggering of other earthquakes in the subducted or overlying plates. A plausible explanation for the very few aftershocks is that the rupture initiated at an asperity at the plate interface and then propagated into subducted sediment lying in the conditionally stable frictional field. The nearby M L 6.1 Ashley Clinton earthquake of 1958, which also had a conspicuous absence of aftershocks, may have involved a similar process.