Determination of fault planes in a complex aftershock sequence basing two-dimensional slip inversion

The (1994) Arthur's Pass earthquake (M-W 6.7, South island, New Zealand) ha d a complex aftershock sequence including events aligned with major mapped faults. To determine whether the major NE-SW-trending strike-slip faults il l the region were activated during this aftershock sequence, we investigate the largest well-recorded aftershocks. The Arthur's Pass earthquake itself was a reverse-faulting event, but the majority of the aftershocks were str ike-slip. We use the empirical Green's function method to obtain source tim e functions for four aftershocks (M-L 4.1-5.1). We then invert for slip on each nodal plane and compare the variance reduction to determine which is t he fault plane. The two largest earthquakes (M-L 5.1 and M-L 4.2) located c lose to the mapped trace of the Bruce fault both occurred on fault planes s triking NNW-SSE, perpendicular to the strike of the Bruce and other regiona l strike-slip faults. The third earthquake studied (M-L 4.1), located on a lineation of aftershocks parallel to the regional mapped trend, had a prefe rred fault plane with a NE-SW strike. The fourth aftershock (M-L 4.1) was l ocated close to the main-shock fault plane and had an oblique reverse mecha nism. This earthquake exhibited northward directivity, but the fault plane could not be identified. The earthquake stress drops ranged from 1 to 10 MP a.