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.本研究报道了新西兰奥塔哥低地震活动性区域内阿卡托尔断层（Akatore Fault）的古地震研究结果。通过两处探槽发掘，共识别出至少3次逆断层破裂事件，其发生时间分别被限定为：公元前13314年至公元680年（倒数第三次事件）、公元737年至960年（倒数第二次事件），以及公元1047年至1278年（最新一次事件），单次地震位移量介于1.6~2.5米之间。探地雷达（Ground Penetrating Radar，GPR）剖面与沉积物分析结果表明，一处形成于125 ka的海成阶地大概率仅受上述三次事件错动，由此推断这些地震终结了该断层至少11万年的地震静息阶段。由此可见，该断层的地震发生具有显著的非周期性，这一特征此前已在阿卡托尔断层及其他研究较为深入的奥塔哥断层中被证实。当前活动期内的断层滑动速率与地震复发间隔分别为0.3~6.0毫米/年及450~5110年。我们建议，在对邻近的达尼丁地区开展地震危险性评估时，应合理假定阿卡托尔断层近期的高频地震活动将在短期内持续。