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

这项工作采用了一种新的方法来解决地球物理学中一个长期存在的问题：为什么会发生中震源和深震源地震？由于深地震只发生在俯冲板块中，对其起源的解释，如转换断层作用和脱水脆化作用，都与板块中可能发生的过程有关。在脱水脆化假说中，海水渗透到洋壳在俯冲之前形成的外升断层中。随着板块的俯冲，弱带被保存下来，然后通过脱水反应重新激活。因此，脱水脆化机制，以及任何需要地表断层重新激活的机制，提供了一个可验证的假设：在考虑了俯冲角度后，外隆起和更深的断层面方向是相同的吗？或者，深部事件的断面方向是否与板块应力场更加一致？先前支持预先存在断层假说的证据通常来自于确定单个事件的断层面（通常是对破裂过程进行更全面分析的副产品）的研究，或者来自于对给定俯冲带内许多地震的节面方向的统计分析，但没有解决断层面模糊问题。本研究采用了一种结合了这两种方法的优点的新方法，识别实际的断层面并分析大量事件，以验证深地震发生在最初形成于海洋板块的断层上的假设，这些断层在俯冲之前形成。对1994年以来发生的数百次震级为5.7级的外隆起、中、深源地震的指向性进行系统分析，以区分其断层面。由于波形在破裂传播方向上变窄，而在相反方向上变宽，因此可以将在焦点球上观察到的破裂持续时间的差异与单侧和/或双侧破裂的预测模式进行比较，从而可以识别破裂方向和断层面。分析宽带地震记录，这项工作采用了最近开发的一种基于地震记录对之间的差时长的程序。这个程序，类似于测量差分传播时间，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先验假设。汤加俯冲带的初步结果表明，我们可以识别出约1/3的大、深地震的断裂面，包括小至5.7 MW的地震事件，并且由此得出的断裂面方向显示出系统的模式，这将允许测试断层再激活假说。更一般地说，150-200个断层面的预期识别将有助于约束深震破裂传播的物理基础：破裂传播是一个等压过程吗？温度如何影响破裂特性？深地震的物理机制是否存在与深度相关的变化？

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?