Palaeoearthquake surface rupture in a transition zone from strike-slip to oblique-normal slip and its implications to seismic hazard, North Island Fault System, New Zealand
Vasiliki Mouslopoulou, Andrew Nicol, Timothy A. Little, John G. Begg, 2009. "Palaeoearthquake surface rupture in a transition zone from strike-slip to oblique-normal slip and its implications to seismic hazard, North Island Fault System, New Zealand", Palaeoseismology: Historical and Prehistorical Records of Earthquake Ground Effects for Seismic Hazard Assessment, K. Reicherter, A. M. Michetti, P. G. Silva
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The North Island Fault System (NIFS) is the longest and highest slip-rate active strike-slip fault system within the Hikurangi subduction margin in New Zealand, accommodating up to 10 mm/a of the margin-parallel plate motion. Displacement of landforms over the last c. 30 ka indicates a gradual northward change from right-lateral strike-slip to oblique-normal slip along the northern NIFS and within 60 km of its intersection with the active Taupo Rift. This change is expressed by a c. 60° increase in the pitch of the slip vectors. We use fault data from 20 trenches and displacements along active traces to explore whether changes in late Quaternary fault kinematics principally arise due to earthquake rupture arrest and/or variations in slip vector pitch during individual earthquakes that span the kinematic transition zone. Results show that earthquake rupture arrest occurs along the strike of the NIFS, with at least 60–80% of all events during the last 10–13 ka terminating across the zone of late Quaternary (c. 30 ka) 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 fault-dip across this zone. Rupture arrest limits earthquake lengths and magnitudes which, when combined with recurrence intervals from trenching, locally decreases the seismic hazard in the region of the faults. Simple kinematic earthquake slip models, which simulate the addition of slip vectors during individual earthquakes, suggest that rupture arrest was accompanied by a northward steepening of slip vectors during individual earthquakes. Changes in coseismic slip vectors may arise due to the northward decrease in fault dip and associated steepening of the principal compressive stress axis (σ1) which, in turn, is due to fault interactions between the NIFS and the adjacent active Taupo Rift.