ABSTRACT

During the Kaikōura earthquake, a paleoseismic trench was dextrally displaced ∼9 m and shortened by 1.3 ± 0.4 m – the largest globally recorded displacement of a trench. Analysis showed that two processes accommodated subequal amounts of slip at the surface: (1) discrete dextral-slip on two steeply-dipping faults bounding a <3.5 m wide central deformation zone; and (2) coseismic clockwise rotation of turf rafts and pervasive sediment deformation in that zone. The second (successive) process resulted in upward (<1 m) and outward (<2 m) bulging along low-angle thrusts, creating horizontal fault-perpendicular shortening that exceeds the heave (∼1.3 m). This discrepancy results from coseismic rotation of rafts, that shorten upon approaching perpendicularity with the fault – creating extra apparent shortening (in fault-orthogonal view). Comparison of pre- and post-earthquake trench logs indicates that strike-slip ruptures at the same site can be expressed differently over time; fault strands carrying major displacement in 2016 were not the locus of deformation in the previous earthquake(s), suggesting temporal unpredictability is important in defining fault zones. The last several paleoearthquakes at the trench produced cm-dm scale normal-sense dip separations across faults; however, the 2016 earthquake created compressive structures including up-bulging and low-angle reverse faulting, as well as fissuring. This contrast in deformation style likely resulted from an >8° clockwise rotation of the local slip vector in 2016 (becoming transpressive), highlighting that small changes in slip kinematics may affect rupture zone structures.

摘要

在凯库拉地震期间，一条古地震沟槽向右移动了约 9 米，并缩短了 1.3 ± 0.4 米——这是全球记录的最大沟槽位移。分析表明，两个过程在地表容纳了次等量的滑动：(1) 两个陡倾断层上的离散右旋滑动，边界 <3.5 m 宽的中央变形区；(2) 草皮筏的同震顺时针旋转和该区域普遍存在的沉积物变形。第二个（连续的）过程导致向上（<1 m）和向外（<2 m）沿低角度逆冲膨胀，产生超过隆起（~1.3 m）的水平断层垂直缩短。这种差异是由筏板的同震旋转引起的，在接近与断层垂直时会缩短——产生额外的明显缩短（在断层正交视图中）。地震前后沟槽测井的比较表明，同一地点的走滑破裂随着时间的推移可能会有不同的表现；2016 年发生主要位移的断层线并不是之前地震的变形轨迹，这表明时间的不可预测性对于定义断层带很重要。在海沟发生的最后几次古地震在断层上产生了厘米-分米尺度的常感倾角分离；然而，2016 年的地震造成了压缩结构，包括上凸和低角度逆断层以及裂隙。这种变形风格的对比可能是由于 2016 年局部滑移矢量顺时针旋转 >8°（变为压变）造成的，突出表明滑移运动学的微小变化可能会影响破裂带结构。