Synthesis of strong motion records using slip distributions from historical New Zealand earthquakes

Abstract

Slip Inversion

We have investigated four aftershocks of the 1994 Arthur's Pass earthquake (Mw 6.7), South Island, New Zealand) to determine whether the major strike-slip faults in the region, or faults at right angles to them, were activated during the aftershock sequence. The Arthur's Pass earthquake itself involved mainly vertical faulting, but the majority of the aftershocks, including all the largest, involved horizontal movement. The earthquake occurred only 25 km SE of the Alpine fault, the largest of the NE-SW trending predominantly strike-slip faults which dominate this region of oblique compression. We obtained details of the fault ruptures for 4 aftershocks (ML 4.1-5.1). We then determined the slip over each of the two possible faults and determined which one moved. Two of these earthquakes (M£, 5.1 and M 4.2), located closed to the mapped trace of the Bruce fault, occurred on faults striking NNW-SSE perpendicular to the predominant strike of the regional horizontal-motion faults. The former ruptured northwards and the latter to
the south. The largest earthquake which could have occurred on the Bruce fault in this sequence cannot, therefore, have been larger than about At, 4. A third event (ML 4.1) was located on a lineation of aftershocks parallel to the regional mapped trend. The preferred fault has a NE-SW strike, providing evidence of activation of horizontal-movement faults parallel to the Alpine fault. The fourth earthquake studied here (ML 4.1) was located close to the mainshock and involved both horizontal and vertical movement. It ruptured towards the north, but this could be matched with slip on either of the two possible fault planes. All four earthquakes released relatively high amounts of stress within the crust. These results confirm that previously unknown horizontal-movement faults trending NNW-SSE, and at right angles to the predominant regional trend of the mapped faults, slipped in large (Mz, 25.1) aftershocks of the Arthur's Pass earthquake.

Strong Motion Modelling

The 1993 Tikokino, New Zealand earthquake (M 6.1) is modelled as a rupture directed to the south. The earthquake was recorded by four strong motion stations with 30km: 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) of shaking 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 fault with both horizontal and vertical movement, and probably represents movement at the interface between the Pacific and Australian plates. A high rupture velocity is required to match the distribution of observed ground shaking, and the rupture area is constrained to be c. 7 x 2km: The magnitude derived from the preferred model is 6.0. 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.

The Mi 6.3 1987 Edgecumbe earthquake involved vertical faulting and was located at the
northern end of the Taupo Volcanic Zone. There was only one close, free field, strong motion record suitable for modelling. A pure vertical fault with either a single or double burst of energy was modelled. The relative amplitudes of vertical to horizontal motions were found to be quite sensitive to near-surface velocities used in the modelling, which made it difficult to get a unique solution. In the single and double fault models we could match the amplitude, frequency content and duration of the data well, which is encouraging for future seismic hazard work. With suitable adjustment of the many free parameters, such as the velocity model, fault orientation, size, and slip distribution it would be possible to fit the data with either a double or a single fault model, but the problem is too poorly constrained for the solutions so obtained to be unique.
Better azimuthal coverage of recorded data is required to constrain these parameters.