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

汇聚的板块边缘因引发历史上最大的火山喷发和最大规模的地震而臭名昭著。然而，板块构造会聚、弹性应变积累地震旋回与同震破裂之间的关系，以及会聚边缘特征的地形和构造之间的关系，仍然存在许多基本问题。该项目主要研究弹性应变累积和释放的地震周期如何转化为较长期的构造净变形的问题。传统观点认为，在地震周期的同震阶段，前弧以某种方式向上和向下移动。然而，如果板间滑动产生较长期的净变形，那么其机制不太可能局限于板间推力的孕震部分的同震滑动。可能是凹凸体之间的滑动、板间逆冲带上比粘滑孕震带更深的滑移、或孕震带内的无震滑动都可能导致上板块的永久构造变形。该项目的主要目标是确定西所罗门弧前一段从上一次大型逆冲破裂到现在的垂直运动历史，并推断永久垂直形变分量与暂时弹性应变现象分量的地理位置和时间。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弧形方向约100公里的大部分俯冲板块。浅层生物礁珊瑚被用作活潮仪，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。通过铀系列同位素分析，被隆起杀死的珊瑚的年龄不到其日历年龄的百分之一，以便能够确定较早的垂直形变的日期和绘制地图。珊瑚形态、珊瑚年生长带和稳定的同位素测量将提供垂直运动的细节。除了具有垂直运动的地质历史外，合适的地点还在会聚边缘地区覆盖着珊瑚礁边缘的海岸，垂直构造运动史将解决这样的观点，即前弧的垂直隆起发生在地震和造山旋回之间的时间尺度上。在岛弧(如所罗门群岛)或大陆边缘(如南美洲的太平洋边缘)的会聚板块边缘，发生了有记录以来最大的地震和一些最猛烈的火山喷发。印度尼西亚和日本最近的例子表明，如果基于相对较少的古地震研究，汇聚的边缘地震比已知的特定地区更大、更具破坏性，科学家们可能会感到惊讶。因此，更多地了解这种地震的产生是很重要的。发生在会聚边缘的大地震之间和期间的地壳运动是地震孕育过程的一个窗口，它可以帮助我们理解构造板块汇聚是如何通过边缘的永久变形来适应的，而不是那些将产生大地震的构造板块，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。这项工作对解释目前在地震危险地区如美国西北部卡斯卡迪亚边缘监测到的地壳运动具有重要意义。此外，与板块会聚相关的暂时性弹性动力地震旋回和地壳永久运动对会聚板块边缘的构造演化和大陆岩石圈的发展具有基础性意义。

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.