Factors Contributing to Multi-Segment Rupture in the 2010 M7.1 Darfield, New Zealand, Earthquake

Tuesday, 16 December 2014
Brad Aagaard1, Charles A Williams2 and Bill Fry2, (1)USGS, Earthquake Science Center, Menlo Park, CA, United States, (2)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand
We use dynamic prescribed slip (kinematic) modeling to examine the factors contributing to multi-segment rupture in the 2010 M7.1 Darfield earthquake. We consider fault geometry and slip distributions from inversions by Beavan et al. (2012) based on geodetic observations and by Elliott et al. (2012) based on geodetic and teleseismic observations. We invert for subevent origin times using strong-motion records and find complex rupture propagation across multiple fault segments. Our inversions suggest that the rupture began on one or two secondary faults with reverse/oblique slip near the hypocenter, consistent with the GNS first motion mechanism. The primary bilateral strike-slip rupture of the Greendale fault, consistent with centroid moment tensor solutions, occurred about 9-10 seconds after the origin time. The strong-motion records provide poor constraints on the timing of rupture of the reverse Hororata fault, which may have occurred about 16-17 seconds after the origin time. Denser strong-motion instrumentation would have provided stronger constraints on the timing of the complex rupture. The relative orientation of the regional stress field and the faulting regime explain the sense of motion and loading of these fault segments. Additionally, dynamic stress changes also created favorable conditions for triggering of the main rupture on the Greendale fault. Current work focuses on evaluating how well the UCERF3 (USGS Open File Report 2013-1165) criteria for forecasting multi-segment ruptures in California apply to this complex rupture in New Zealand.