The Seismology of Shallow Intraplate Subduction Earthquakes: From Outer Rise to Interface

浅层板内俯冲地震的地震学：从外隆到界面

• 批准号: 0538263
• 负责人: Jarmila Polet
• 金额: $ 10.7万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-01-01 至 2008-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0538263&HistoricalAwards=false
• 关键词: Seismology; Shallow; Intraplate; Subduction; Earthquakes

The world's largest and some of its most destructive earthquakes occur on the interface between subducting and overriding plates, with the degree of interplate coupling playing an important role in their seismogenesis. Updip from this interface, the outer rise comprises an upwarping of the oceanic lithosphere just before it descends into the trench. Isacks and Molnar (1969, 1971), Isacks et al. (1969) and Oike (1971) established the characteristics of the stress regime within the subducting lithosphere and suggested that intraplate earthquakes serve as stress gauges for the large-scale deformation involved in subduction zones. Polet (2005) recently compiled a new catalog of outer rise seismicity (here defined as earthquakes located seawards from the trench), partly confirming earlier results (Christensen and Ruff, 1983; Lay et al., 1989) by showing that normal faulting events occur preferentially after large interplate thrust events. Compressional outer rise events were found to show a similar temporal pattern, in contrast to earlier findings suggesting they occurred more frequently prior to large interface earthquakes, which hints that a more complex physical mechanism may be at work than simple elastic plate bending (Ward, 1983; Liu and McNally, 1993). An inelastic analysis of lithospheric stress distributions by Mueller et al. (1996) predicts seismic behavior similar to that observed by Polet (2005). From these temporal and spatial relationships, it is clear that the state of stress in the subducting plate may change in response to interplate coupling. The seismicity record should reflect this and thus shed light on the physical processes at work. The P.I. of this project is constructing a complete catalog of intraslab seismicity, extending the analysis of Polet (2005) to events landwards of the trench, greater depths (150 km) and lowering the magnitude threshold from 6.0 to 5.0. The next step of the research is to apply a teleseismic P-wave modeling technique to refine the depths of the intraplate events to a higher precision and homogeneity. Subsequently a source spectral analysis of the Mw=6.5 earthquakes can be carried out to determine dynamic rupture parameters. These investigations will provide us with unique insight into a myriad of issues, including: the failure mechanism of intraplate and outer rise earthquakes, the state of stress in the subducting lithosphere, the origin of the hydration of the subducting plate and the mode of deformation of the outer rise. Bathymetric and seismic profiles across subduction-related trenches commonly show distinctive patterns of normal faulting on the outer trench wall (Masson, 1991). Hilde (1983) reviewed the occurrence of outer trench wall faulting and found it to be essentially ubiquitous. The relationship between this type of faulting and the structure and geometry of the subduction zone is still not clear (Hilde, 1983; Aubouin et al, 1984; Scholl et al., 1982). Obvious factors that may control the strike of these faults include the strike of the trench and any weakness in the subducting plate, such as the fabric resulting from oceanic spreading. These and similar observations from bathymetric and seismic trench profiles can be related to intraplate earthquake occurrences and seismic moment release. Based on the catalog currently in development, the P.I. is investigating whether the asymmetry in the fault system previously found for the source mechanisms of tensional outer rise events persists to greater depths, testing the hypothesis that reactivation of preexisting weak zones, created prior to subduction, is responsible for these events.The main interest of this research is in improving our understanding of the role of intraplate seismicity in the earthquake cycle, the mode of outer rise deformation, the relationship between seismic coupling and intraplate seismicity and the role of pre-existing weak zones and dehydration in the generation of intraplate earthquakes. Improved knowledge of the temporal and spatial character of intraplate seismicity will also be an important first step towards the development of a new generation of models of subduction zone dynamics. The behavior of intraplate events in time, and their relationship to the earthquake cycle, may also be significant for intermediate term earthquake hazard assessment.

世界上最大的地震和一些最具破坏性的地震发生在俯冲和覆盖板块之间的界面上，板间耦合的程度在地震成因中起着重要作用。从这个界面上倾，外部隆起包括海洋岩石圈在下降到海沟之前的隆起。Isacks和Molnar（1969，1971）、Isacks等人（1969）和Oike（1971）建立了俯冲岩石圈内应力状态的特征，并提出板内地震可作为俯冲带大规模变形的应力计。Polet（2005年）最近编制了一份新的外隆地震活动目录（此处定义为从海沟向海的地震），部分证实了早期的结果（Christensen和Ruff，1983年; Lay等人，1989年），通过显示正常断层活动优先发生在大型板间逆冲事件之后。压缩性外隆事件被发现显示出类似的时间模式，与早期的研究结果相反，早期的研究结果表明，它们更频繁地发生在大的界面地震之前，这暗示着一个更复杂的物理机制可能比简单的弹性板弯曲起作用（Ward，1983; Liu和McNally，1993）。Mueller等人（1996）对岩石圈应力分布的非弹性分析预测了与Polet（2005）观察到的类似的地震行为。从这些时间和空间的关系，很明显，在俯冲板块的应力状态可能会改变，以响应板间耦合。地震活动记录应该反映出这一点，从而揭示出起作用的物理过程。私家侦探该项目的第一步是建立一个完整的实验室内地震活动目录，将Polet（2005年）的分析扩展到海沟向陆地、更深（150公里）的地震，并将震级阈值从6.0级降低到5.0级。研究的下一步是应用一种快速P波模拟技术，将板内同相轴的深度细化到更高的精度和均匀性。随后，可以对Mw=6.5地震进行震源谱分析，以确定动态破裂参数。这些调查将为我们提供独特的洞察无数的问题，包括：板内和外隆地震的破坏机制，在俯冲岩石圈的应力状态，俯冲板块的水化的起源和变形模式的外隆。 横跨与俯冲有关的海沟的测深剖面和地震剖面通常显示出海沟外壁上独特的正断层模式（Masson，1991年）。Hilde（1983）回顾了外沟壁断层的发生，发现它基本上无处不在。这种类型的断层与俯冲带的结构和几何形状之间的关系仍然不清楚（Hilde，1983; Aubouin等人，1984; Scholl等人，1982年）。可能控制这些断层走向的明显因素包括海沟的走向和俯冲板块的任何弱点，如海洋扩张造成的组构。这些和类似的观察水深和地震海沟剖面可以与板内地震发生和地震矩释放。根据目前正在开发的目录，P.I.正在研究以前发现的断裂系统中的不对称性是否持续到更深的深度，以检验在俯冲之前产生的先前存在的弱带的重新激活是这些事件的原因的假设。这项研究的主要兴趣是提高我们对板内地震活动在地震周期中的作用，外隆变形的模式，地震耦合与板内地震活动的关系，以及先存弱带和脱水作用在板内地震发生中的作用。更好地了解板内地震活动的时间和空间特征也将是朝着发展新一代俯冲带动力学模型迈出的重要的第一步。板内事件在时间上的行为，以及它们与地震周期的关系，对于中期地震危险性评估也可能是重要的。