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

世界上最大的地震和一些最具破坏性的地震发生在俯冲板块和俯冲板块之间的界面上，板块间耦合的程度在它们的地震成因中起着重要作用。从这个界面向上倾斜，外部隆起包括大洋岩石圈在下降到海沟之前的向上翘曲。伊萨克斯和莫尔纳(1969,1971)，伊萨克斯等人。(1969)和Oike(1971)建立了俯冲岩石圈内的应力状态特征，并认为板内地震是俯冲带大范围变形的应力计。Polet(2005年)最近编制了一个新的外部隆起地震活动目录(这里定义为位于海沟外的地震)，部分证实了早期的结果(Christensen和Ruff，1983；Lay等，)，表明正常的断层事件优先发生在大型板间推覆事件之后。挤压的外部上升事件显示出类似的时间模式，与早期的发现相反，早期的发现表明它们在大界面地震之前发生得更频繁，这暗示可能有比简单的弹性板块弯曲更复杂的物理机制在起作用(Ward，1983；Liu和McNally，1993)。Mueller等人对岩石圈应力分布的非弹性分析。(1996)预测的地震行为类似于Polet(2005)所观察到的。从这些时间和空间关系可以清楚地看出，俯冲板块中的应力状态可能会随着板间耦合的响应而改变。地震活动记录应该反映这一点，从而阐明起作用的物理过程。该项目的P.I.正在建立一个完整的实验室内地震活动目录，将Polet(2005)的分析扩展到海沟陆地以下的事件，更深(150公里)的事件，并将震级阈值从6.0降至5.0。研究的下一步是应用远震P波模拟技术来精炼板内事件的深度，以达到更高的精度和均质性。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。这些研究将为我们提供对无数问题的独特见解，包括：板内和外部隆起地震的破坏机制，俯冲岩石圈的应力状态，俯冲板块水化的来源和外部隆起的变形方式。横跨俯冲相关海沟的水深和地震剖面通常在外部海沟壁上显示出独特的正断层模式(Masson，1991)。Hilde(1983)回顾了外壕壁断层的发生，并发现它基本上是普遍存在的。这种类型的断层作用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)。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，例如海洋扩张造成的组构。这些和来自水深和地震沟剖面的类似观测可以与板内地震发生和地震矩释放有关。根据目前正在开发的目录，P.I.正在调查先前发现的张性外部隆起事件的震源机制在断层系统中的不对称性是否持续到更深的地方，测试先前存在的、在俯冲之前产生的薄弱带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、外部隆起变形的方式、地震耦合和板内地震活动之间的关系以及先前存在的薄弱带和脱水在板内地震发生中的作用的理解。进一步了解板块内地震活动的时间和空间特征，也将是开发新一代俯冲带动力学模型的重要第一步。板内事件在时间上的行为及其与地震周期的关系，也可能对中期地震危险性评估具有重要意义。

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.