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（Otago）低地震活动区阿塔科勒断层（Akatore Fault）的古地震学研究成果。通过两条探槽揭示，该断层至少存在3次逆冲破裂事件，其发生时间分别限定为：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）；单次事件位移量为1.6~2.5米。探地雷达（Ground Penetrating Radar, GPR）剖面与沉积物分析结果显示，一处125 ka的海成阶地大概率仅受这三次事件影响，表明该断层此前至少经历了11万年的静止期，本次地震活动终结了这一静止阶段。因此该断层表现出显著的地震发生非周期性特征，这一特性此前已在阿塔科勒断层与其他研究较为充分的奥塔哥断层中被提出。当前活动期内的滑动速率与复发间隔分别为0.3~6.0毫米/年，以及450~5110年。本研究认为，在对附近达尼丁（Dunedin）开展地震危险性评估时，可合理假设阿塔科勒断层近期的高频地震活动将在短期内持续。