Do Old Faults Ever Die? Deep Earthquakes, Directivity, and Differential Duration

这项工作对地球物理学中一个长期存在的问题采取了新的方法：为什么会发生中源和深源地震？ 由于深源地震只发生在俯冲板块中，因此对其起源的解释，如转换断层和脱水脆化，与预期发生在板块中的过程有关。 在脱水脆化假说中，海水渗透到洋壳在俯冲之前形成的外隆断层中。 当板块俯冲时，软弱带被保存下来，然后通过脱水反应重新活化。 因此，脱水脆化机制，以及任何机制，需要重新激活的表面断层，提供了一个可检验的假设：是断层面的取向外隆和更深的事件后，俯冲的角度已被占相同的？ 或者，深部地震的断层面方向是否与板块应力场更加一致？ 先前支持先存断层假说的证据通常来自于识别单个事件断层面的研究，通常是对破裂过程进行更全面分析的副产品，或者是对给定俯冲带内众多地震的断层平面方向的统计分析，但没有解决断层面的模糊性。 这项研究利用了一种新的方法，结合了这两种方法的优点，识别实际的断层面和分析大量的事件，以测试的假设，深地震发生在断层，最初形成的海洋板块俯冲之前。 对1994年以来发生的数百次MW=5.7的外隆、中源和深源地震的方向性进行了系统的分析，以区分它们的断层面。 由于波形在破裂传播的方向上变窄，在相反的方向上变宽，在震源球上观察到的破裂持续时间的差异可以与单侧和/或双侧破裂预测的模式进行比较，从而可以识别破裂方向和断层面。分析宽带地震图，这项工作采用了最近开发的程序之间的差分持续时间的地震图对的基础上。 这个过程，类似于测量差分走时，代表了解决这个问题的最佳方法，因为它允许相对快速地确定数百次地震的所需属性（断层面），而无需对破裂复杂性或测量震源持续时间的需要进行先验假设。 从汤加俯冲带的初步结果表明，我们可以确定的断层面的大，深地震，包括小到MW 5.7的事件的约1/3，和由此产生的断层面的方向显示系统的模式，将允许测试的故障复活假说。 更一般地说，预计识别150-200个断层面将有助于限制深震破裂传播的物理基础：破裂传播是等压过程吗？温度如何影响破裂特性？在深源地震的物理机制中是否存在深度依赖性变化？

This work takes a new approach to a long-standing question in geophysics: Why do intermediate- and deep-focus earthquakes occur? Since deep earthquakes only occur in subducting slabs, suggested explanations for their origin, such as transformational faulting and dehydration embrittlement, are related to processes expected to occur in the slab. In the dehydration embrittlement hypothesis, seawater permeates into outer-rise faults that form in the oceanic crust prior to subduction. As the slab subducts, the weak zone is preserved and then reactivated by dehydration reactions. Thus, the dehydration-embrittlement mechanism, as well as any mechanism that requires the reactivation of surface faults, offers a testable hypothesis: are the fault-plane orientations of outer-rise and deeper events the same after the angle of subduction has been accounted for? Alternatively, are the fault-plane orientations of deep events more consistent with the slab stress field? Previous evidence supporting the pre-existing fault hypothesis has generally come from either studies that identify the fault planes of individual events, often as a by-product of a more comprehensive analysis of the rupture process, or analyses of the statistics of the nodal-plane orientations of numerous earthquakes within a given subduction zone, but without resolving the fault plane ambiguity. This study utilizes a novel method that combines the advantages of both of these approaches , identifying the actual fault planes and analyzing large numbers of events , to test the hypothesis that deep earthquakes occur on faults that initially formed in the oceanic plate prior to subduction. The directivity of hundreds of outer-rise, intermediate- and deep-focus earthquakes with MW=5.7 since 1994 will be systematically analyzed to distinguish their fault planes. Since waveforms are narrowed in the direction of rupture propagation and broadened in the opposite direction, the differences in rupture duration observed over the focal sphere can be compared with the patterns predicted for unilateral and/or bilateral ruptures and, thereby, the rupture direction and fault plane can be identified. Analyzing broadband seismograms, this work employs a recently developed procedure based on the differential duration between pairs of seismograms. This procedure, similar to measuring differential travel times, represents an optimal method for addressing this problem, since it permits the relatively-fast determination of the desired property (the fault plane) for hundreds of earthquakes without a priori assumptions about rupture complexity or the need to measure source duration. Preliminary results from the Tonga subduction zone show that we can identify the fault plane for ~1/3 of large, deep earthquakes, including events as small as MW 5.7, and that the resulting fault plane orientations show systematic patterns that will allow the testing of the fault-reactivation hypothesis. More generally, the expected identification of 150-200 fault planes will help constrain the physical basis for deep-earthquake rupture propagation: Is rupture propagation an isobaric process? How does temperature influence rupture characteristics? Is there a depth-dependent variation in the physical mechanism of deep earthquakes?