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Reconciling an Early Nineteenth‐Century Rupture of the Alpine Fault at a Section End, Toaroha River, Westland, New Zealand
Bulletin of the Seismological Society of America ( IF 3 ) Pub Date : 2021-02-01 , DOI: 10.1785/0120200116
Robert M. Langridge 1 , Pilar Villamor 1 , Jamie D. Howarth 2 , William F. Ries 3 , Kate J. Clark 1 , Nicola J. Litchfield 1
Affiliation  

The Alpine fault is a high slip‐rate plate boundary fault that poses a significant seismic hazard to southern and central New Zealand. To date, the strongest paleoseismic evidence for the onshore southern and central sections indicates that the fault typically ruptures during very large (⁠Mw≥7.7⁠) to great “full‐section” earthquakes. Three paleoseismic trenches excavated at the northeastern end of its central section at the Toaroha River (Staples site) provide new insights into its surface‐rupture behavior. Paleoseismic ruptures in each trench have been dated using the best‐ranked radiocarbon dating fractions, and stratigraphically and temporally correlated between each trench. The preferred timings of the four most recent earthquakes are 1813–1848, 1673–1792, 1250–1580, and ≥1084–1276 C.E. (95% confidence intervals using OxCal 4.4). These surface‐rupture dates correlate well with reinterpreted timings of paleoearthquakes from previous trenches excavated nearby and with the timing of shaking‐triggered turbidites in lakes along the central section of the Alpine fault. Results from these trenches indicate the most recent rupture event (MRE) in this area postdates the great 1717 C.E. Alpine fault rupture (the most recent full‐section rupture of the southern and central sections). This MRE probably occurred within the early nineteenth century and is reconciled as either: (a) a “partial‐section” rupture of the central section; (b) a northern section rupture that continued to the southwest; or (c) triggered slip from a Hope‐Kelly fault rupture at the southwestern end of the Marlborough fault system (MFS). Although, no single scenario is currently favored, our results indicate that the behavior of the Alpine fault is more complex in the north, as the plate boundary transitions into the MFS. An important outcome is that sites or towns near fault intersections and section ends may experience strong ground motions more frequently due to locally shorter rupture recurrence intervals.

中文翻译:

调和新西兰韦斯特兰Toaroha河段末的19世纪早期阿尔卑斯断层破裂

高山断层是高滑移率板块边界断层,对新西兰南部和中部构成重大地震危险。迄今为止,陆上南部和中部地区最强的古地震证据表明,断层通常在很大的地震(Mw≥7.7⁠)到大的“全断面”地震中破裂。在其Toaroha河(Staples站点)中部东北端开挖的三个古地震沟提供了有关其表面破裂行为的新见解。每个沟槽中的古地震破裂已使用排名最高的放射性碳测年分数确定了日期,并且每个沟槽之间在地层和时间上相关。最近四次地震的首选发生时间是1813-1848年,1673-1792年,1250-1580年和≥1084-1276276CE(使用OxCal 4.4的95%置信区间)。这些地表破裂日期与重新解释的附近附近以前开凿的沟渠的古时相以及与沿阿尔卑斯断裂中段的湖中的震动触发的浊积体的时间密切相关。这些沟槽的结果表明,该地区的最新破裂事件(MRE)早于1717年CE阿尔卑斯断层破裂(南部和中部的最新全断面破裂)之后。这种MRE可能发生在19世纪初期,并且由于以下原因而被调和:(a)中央部分的“局部断层”破裂;(b)继续向西南延伸的北部破裂;(c)由Marlborough断层系统(MFS)西南端的Hope-Kelly断层引起的滑动。尽管目前没有哪种方案受到青睐,我们的研究结果表明,随着板块边界向MFS过渡,北部断层的行为更加复杂。一个重要的结果是,由于局部较短的破裂重复间隔,断层相交处和断面末端附近的站点或城镇可能会更频繁地经历强烈的地面运动。
更新日期:2021-01-31
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