Abstract
Reverse faulting in Otago, southern New Zealand, accommodates distributed tectonic convergence on the eastern side of the Australian‐Pacific plate boundary. Paleoearthquake records from some of the faults in the region show highly variable earthquake recurrence times, with long periods of quiescence separated by periods of earthquake activity. Here we develop a longer‐term context for these records, using cosmogenic radionuclide dating of faulted alluvial fan surfaces to characterize the late Quaternary slip rates on two significant faults within the system, the Hyde and Dunstan faults. We determine an average slip rate of 0.24 mm/yr (0.19–0.29 mm/yr at 95% confidence) for the Hyde Fault since about 115 ka, and an average slip rate of 0.16 mm/yr (0.12–0.21 mm/yr) for the Dunstan Fault since about 320 ka. Both faults show increases in slip rate of a factor of three to five times the average long‐term rate over timescales of 10 kyr. Increases in slip rate are out of phase on the two faults, supporting a hypothesis that strain is shared within the fault system over timescales on the order of 10 kyr. Over longer timescales, on the order of 100 kyr, slip rates can be well‐approximated by a linear fit, providing an upper limit on the variability of fault slip rates with time.
Key Points
Cosmogenic radionuclide dating of faulted alluvial fans constrains slip rates for two reverse faults in southern New Zealand
Slip rates are 0.24 mm/yr (0.19–0.29 mm/yr) for the Hyde Fault and 0.16 mm/yr (0.12–0.21 mm/yr) for the Dunstan Fault
Strain sharing is suggested by changes in slip rates on 10 kyr timescales, while over longer timescales this variability averages out