Mouslopoulou, V., Nicol, A.1, Little, T.A.2 & Begg, J.G.1
1 - GNS Science, Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand
2 - Victoria University of Wellington, New Zealand
Abstract - Landforms displaced by the North Island Fault System (in New Zealand) over the last c. 30 kyr indicate a gradual northward change from right-lateral strike-slip to oblique-normal faulting (c. 60° in the slip vector pitch) near its intersection with the Taupo Rift. We analyse fault data from 20 trenches and displacements along active traces to explore whether changes in late Quaternary fault slip vectors principally arise due to earthquake rupture arrest in the transition zone and/or to variations in slip vector pitch during individual earthquakes that span the transition zone. Results show that earthquake rupture arrest occurs along the strike of the North Island Fault System with, at least 80% of all events during the last 10-13 kyr terminating across the zone of late Quaternary (c. 30 kyr) kinematic transition from strike-slip to oblique-normal slip. The strike of the faults across the kinematic transition is unchanged, and we suggest that rupture was arrested there due to a 20-30° northward shallowing of the fault-dip across this zone. Rupture arrest decreases earthquake lengths and magnitudes which, when combined with recurrence intervals from trenching, locally decreases the seismic hazard in the region of the faults. Rupture arrest alone cannot account for the observed change in slip vectors and some northward steepening of slip vectors during individual earthquakes is required. Changes in coseismic slip vectors may arise due to the northward decrease in fault-dip and associated steepening of the principal compressive stress axis (?1) which, in turn, is due to fault interactions between the North Island Fault System and the adjacent active Taupo Rift.
Abstract of talk given to:
International Union of Geodesy and Geophysics XXIV, Perugia, July 2007.