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

这项工作为地球物理学中一个长期存在的问题提供了一种新的方法：为什么中源和深源地震会发生？由于深部地震只发生在俯冲板块中，对其起源的建议解释，如转换断层和脱水脆化，与预期在板块中发生的过程有关。在脱水脆化假说中，海水渗透到洋壳俯冲前形成的外部隆起断层中。当板块俯冲时，弱区被保留下来，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，都提供了一个可检验的假设：在考虑俯冲角度后，外部隆起和更深层次事件的断层面方向是否相同？或者，深部事件的断层面方向是否与板片应力场更一致？以前支持预先存在的断层假说的证据通常来自于确定个别事件的断裂面的研究，这些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许多地震的节面方向的统计分析，但没有解决断裂面的模糊性。这项研究利用一种结合了这两种方法的优点的新方法，识别实际断裂面并分析大量事件，以检验深震发生在俯冲之前最初形成于大洋板块的断层上的假设。对1994年以来数百次Mw=5.7的外升、中源和深源地震的方向性进行了系统分析，以区分它们的断裂面。由于波形在破裂传播方向上变窄，在相反方向上变宽，因此可以将在震源球上观察到的破裂持续时间的差异与预测的单侧和/或双侧破裂的模式进行比较，从而可以识别破裂方向和断裂面。分析宽带地震记录，这项工作采用了一种最近开发的基于地震记录对之间的差异持续时间的方法。这一程序类似于测量差异走时，是解决这一问题的最佳方法，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)，而无需关于破裂复杂性或测量震源持续时间的先验假设。来自汤加俯冲带的初步结果表明，我们可以识别~1/3的深部大地震的断裂面，包括小至5.7兆瓦的地震，并且由此产生的断裂面方向显示出系统的图案，这将允许检验断层再激活假说。更广泛地说，150-200个断裂面的预期识别将有助于限制深地震破裂传播的物理基础：破裂传播是一个等压过程吗？温度对断裂特征有何影响？深部地震的物理机制是否存在随深度变化的现象？

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?