Paleoseismology of the Akatore Fault, Otago, New Zealand

We present the results of a paleoseismic study of the Akatore Fault in the low seismicity region of Otago, New Zealand. Two trenches reveal at least three reverse fault ruptures that are constrained to have occurred between 13,314 B.C. and 680 A.D. (antepenultimate event), 737 and 960 A.D. (penultimate event) and 1047 and 1278 A.D. (most recent event), with a single-event displacements of 1.6-2.5 m. Ground Penetrating Radar (GPR) profiles and sediment analyses show that a 125 ka marine terrace is likely also only displaced by these three events, suggesting these earthquakes have ended a minimum 110,000 year period of quiescence on the fault. The fault therefore appears to exhibit strong aperiodicity of earthquake occurrence, a characteristic previously suggested for the Akatore Fault and other well-studied Otago faults. Slip rate and recurrence interval for the current active period are 0.3-6.0 mm/yr and 450-5110 years respectively. We suggest that for seismic hazard assessments in nearby Dunedin it is prudent to assume that the high rates of recent earthquakes will continue into the immediate future on the Akatore Fault.