The Earthquake Cycle and its Role in Permanent Vertical Deformation in the Western Solomons Arc from Coral Paleogeodesy of the Past Few Centuries

从过去几个世纪的珊瑚古测地学来看，地震周期及其在西所罗门岛弧永久垂直变形中的作用

• 批准号: 1119211
• 负责人: Frederick Taylor
• 金额: $ 34.35万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-08-01 至 2014-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1119211&HistoricalAwards=false
• 关键词: Earthquake; Cycle; its; Role; Permanent

Convergent plate margins are notorious for generating the largest historic volcanic eruptions and the largest magnitude earthquakes. However, many fundamental questions remain about the relationships between plate tectonic convergence, the earthquake cycle of elastic strain accumulation and coseismic rupture, and construction of the topography and structures characteristic of convergent margins. This project focuses on the question of how the earthquake cycle of elastic strain accumulation and release translates into longer-term net tectonic deformation. The traditional view is that the forearc is somehow ratcheted up and down during the coseismic phase of the earthquake cycle. However, if interplate slip produces longer-term net deformation, then it is unlikely that the mechanism is limited to coseismic slip on the seismogenic part of the interplate thrust. Perhaps slip between asperities, slip on the interplate thrust zone deeper than the stick-slip seismogenic zone, or aseismic slip within the seismogenic zone may lead to permanent tectonic deformation of the upper plate. The primary goal of this project is to determine the history of vertical movements across a segment of the Western Solomons forearc from the time of the last full-scale megathrust rupture until present and to infer the geography and timing of components of vertical deformation that are permanent versus those that are temporary elastic strain phenomena. The idea is being tested in the Western Solomon Islands where coral-fringed islands occupy much of the overriding plate from near the trench to approximately 100 km arc-ward. Shallow living reef corals are used as living tide gauges that can record the vertical changes that raise and lower corals through sea level either gradually or abruptly. Corals killed by uplift are dated to within one per cent of their calendar ages by uranium-series isotope analysis so that older vertical deformation can be dated and mapped. Coral morphology, annual growth bands in corals, and stable isotopic measurements will provide the details of vertical motions. Besides having a geological history of vertical motions, a suitable site has reef-fringed coasts overlying the area of a convergent margin for which the history of vertical tectonic movements would address the idea that vertical uplift in forearcs occurs at time-scales between seismic and orogenic cycles.Convergent plate margins at island arcs, such as the Solomon Islands, or at the edges of continents, as along the Pacific margin of South America, have produced the largest earthquakes and some of the most violent volcanic eruptions on record. Recent examples in Indonesia and Japan demonstrate that scientists can be taken by surprise when convergent margin earthquakes are even larger and more destructive than known for a particular area based on the relatively sparse paleoseismic studies. Thus is it important to understand more about the generation of such earthquakes. The crustal motions that occur between and during large earthquakes at convergent margins represent a window on the earthquake generation process that can help us understand how tectonic plate convergence is accommodated by permanent deformation of the margins vs. that which goes to generate large earthquakes, including unusually large earthquakes produced when several adjacent seismogenic units rupture together at the same time. This work has implications for interpretation of crustal motions presently being monitored in great detail at seismically hazardous areas such as the Cascadia margin of the northwestern US. Furthermore, the temporary elastically powered earthquake cycle and permanent crustal motions associated with plate convergence are a matter of fundamental significance to the tectonic evolution of convergent plate margins and the development of continental lithosphere.

会聚板块边缘因产生历史上最大的火山爆发和最大规模的地震而臭名昭著。然而，关于板块构造会聚、弹性应变积累和同震破裂的地震周期以及会聚边缘的地形和构造特征之间的关系，仍然存在许多基本问题。这个项目的重点是弹性应变积累和释放的地震周期如何转化为长期的净构造变形的问题。传统的观点认为，在地震周期的同震阶段，前弧以某种方式上下移动。然而，如果板间滑动产生长期的净变形，那么该机制不太可能仅限于板间逆冲断层孕震部分的同震滑动。可能是微凸体之间的滑动、板间逆冲带上比粘滑孕震带更深的滑动或孕震带内的不稳定滑动导致了上板块的永久性构造变形。该项目的主要目标是确定从最后一次全面的巨型逆冲断层破裂到现在的西所罗门前弧段的垂直运动历史，并推断出永久性垂直变形与临时弹性应变现象的地理和时间。这一想法正在西所罗门群岛进行试验，那里珊瑚环绕的岛屿占据了从海沟附近到大约100公里弧形方向的大部分覆盖板块。浅的活珊瑚礁被用作活的潮汐测量仪，可以记录珊瑚在海平面上逐渐或突然上升和下降的垂直变化。通过铀系同位素分析，被抬升杀死的珊瑚的年龄在其日历年龄的百分之一以内，因此可以确定更古老的垂直变形的年龄并绘制地图。珊瑚形态、珊瑚的年生长带和稳定同位素测量将提供垂直运动的详细情况。除了具有垂直运动的地质历史外，一个合适的地点具有覆盖会聚边缘区域的礁缘海岸，对于会聚边缘区域，垂直构造运动的历史将解决前弧的垂直隆起发生在地震和造山周期之间的时间尺度上的想法。在岛弧（如所罗门群岛）或大陆边缘的会聚板块边缘，像沿着南美洲的太平洋边缘一样，产生了有记录以来最大的地震和一些最猛烈的火山爆发。最近在印度尼西亚和日本的例子表明，当会聚边缘地震比基于相对稀疏的古地震研究的特定地区已知的地震更大、更具破坏性时，科学家可能会感到惊讶。因此，更好地了解这类地震的发生是很重要的。发生在会聚边缘的大地震之间和期间的地壳运动代表了地震生成过程的一个窗口，可以帮助我们了解构造板块会聚是如何通过边缘的永久变形与产生大地震的变形相适应的，包括当几个相邻的孕震单元同时破裂时产生的异常大地震。这项工作的影响，目前正在监测的地壳运动的解释非常详细的地震危险区，如美国西北部的卡斯卡迪亚边缘。 与板块会聚有关的弹性动力地震周期和永久性地壳运动对会聚板块边缘的构造演化和大陆岩石圈的发育具有重要意义。