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

Abstract

The (1994) Arthur's Pass earthquake (Mw 6.7, South Island, New Zealand) had a complex aftershock sequence including events aligned with major mapped faults. To determine whether the major NE-SW-trending strike-slip faults in 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 strike-slip. We use the empirical Green's function method to obtain source time functions for four aftershocks (ML 4.1-5.1). We then invert for slip on each nodal plane and compare the variance reduction to determine which is the fault plane. The two largest earthquakes (ML 5.1 and ML 4.2) located close to the mapped trace of the Bruce fault both occurred on fault planes striking NNW-SSE, perpendicular to the strike of the Bruce and other regional strike-slip faults. The third earthquake studied (ML 4.1), located on a lineation of aftershocks parallel to the regional mapped trend, had a preferred fault plane with a NE-SW strike. The fourth aftershock (ML 4.1) was located close to the main-shock fault plane and had an oblique reverse mechanism. This earthquake exhibited northward directivity, but the fault plane could not be identified. The earthquake stress drops ranged from 1 to 10 MPa.