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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS005 Oral Presentation 1907
Paleoearthquake surface rupture in a transition zone from strike-slip to
oblique-normal faulting
Dr. Vasiliki Mouslopoulou
School of Geological Sciences University College Dublin
A. Nicol, T. A. Little, J. G. Begg
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.
Keywords: earthquakeslip vectors, fault segmentation, fault trench