GEOMORPHIC MAPPING AND SCHMIDT HAMMER EXPOSURE AGE DATING USED TO CALCULATE THE SLIP RATE OF THE SOUTHERN TORLESSE FAULT, CASTLE HILL, NEW ZEALAND

The Torlesse fault is a poorly understood dextral-reverse fault that poses significant seismic hazard to the city of Christchurch and critical infrastructure networks in the South Island, New Zealand. This study uses a probabilistic approach to determine the slip rate of the Southern Torlesse fault (STF) where it crosses one of the island’s major thoroughfares. Detailed mapping, surveying and Schmidt Hammer (SH) exposure-age dating were used to provide the first ever constraints on net slip and slip rate for the STF. SH rebound values were inputs into a Bayesian age model used to determine the terraces ages, which range from ~5-50 kya. GPS scarp profiles were used to estimate the amount of vertical displacement, while displaced terrace riser-tread interfaces were mapped to determine the amount of horizontal displacement. Our results from the ~20 kya terrace suggests that the STF has a dip slip of ~0.8 m, a strike slip of ~17 m and a calculated net slip of ~17 m, most of which results from right lateral (dextral) displacement. The resulting ~1 mm/yr slip rate at this site is higher than previous estimates and demonstrates the need for further studies along the fault where it crosses rail, power lines, and through ski fields. The overall structure of the Torlesse range suggests that the Torlesse fault is an active backthrust of the longer and faster-slipping Porters-Pass fault, which begs the questions of whether these >40 km structures could rupture together in the same earthquake.