Response of tremor and slow slip to tidal stress: Constraints on fault friction and weakening

在过去的10年里，地震学家发现了一种新型的断层行为——以板块边界上的慢滑地震为代表，在这种地震中，断层上的滑动比常规地震发生的范围大得多，时间长得多。这些事件是感觉不到的，但通常可以用GPS探测到。在这里，PI研究了小的，但相对知名的应力变化对慢滑地震的影响。太阳和月球对地球的引力作用会使地球产生压力并轻微变形，同时也会引起潮汐的涨落，增加压力的变化。这些小的应力变化影响深断裂带，并调节慢滑和震颤，这是最近发现的与慢滑相关的弱地震辐射。潮汐应力可以计算出来，因此人们可以评估已知的小应力变化对地震产生的影响，这是缓慢滑动的标志。研究结果涉及到潮汐应力如何增强和抑制慢滑，俯冲带深处的岩石摩擦水平，以及在大的慢滑地震期间，断层界面如何随着滑动的积累而减弱。这项工作提供了第一个在板块边界深处的原位固有摩擦的估计。正如初步结果所表明的那样，将深俯冲带的摩擦限制在非常低的水平可能具有变革性，并对俯冲带科学的各个方面产生影响，远远超出地震学的范畴，包括岩石学、板块应力和地球动力学。间歇性震颤和滑动（ETS）是最近发现的一种现象，在这种现象中，称为震颤的弱地震信号伴随着缓慢传播的板块边界界面上的剪切滑动，通常持续数周。在华盛顿北部的卡斯卡迪亚俯冲带深处，6.6到6.8级的大型ETS活动以非同寻常的规律性发生，大约每14个月发生一次。所提出的方法利用校准良好的潮汐应力来探测深俯冲带中容纳幕式缓慢变形的区域的响应。对来自6次ETS事件的3.3万次地震的时间和地点的潮汐应力分析揭示了潮汐应力的强烈影响，特别是，地震对潮汐应力的敏感性在演变，随着慢滑在给定地点的积累，在几天内增加。这种灵敏度的增加意味着断层的逐渐减弱。这项工作将扩展潮汐应力对1)主要ETS事件之间发生的震颤的影响的研究，2)沿卡斯卡迪亚的大型ETS事件，以及3)日本的ETS事件。PI还将确定提供震颤和潮汐应力之间最佳相关性的首选孔隙弹性模型。目的是更好地约束ETS周期期间板块边界上的摩擦和断层的减弱和再加强。初步研究首次明确发现，在给定地点，随着ETS滑动数天，地震对潮汐应力的敏感性不断变化，这可能是由于断层逐渐减弱。这些结果限制了ETS期间断层减弱的时间和程度，而拟议的工作将解决ETS之间14个月间隔内重新加强的时间，以及卡斯卡迪亚俯冲带和南开- kii俯冲带不同地质和构造背景下地震潮汐敏感性的相对行为。此外，初步研究已经对深俯冲界面的内在摩擦进行了首次现场估计，而拟议的工作将为ETS地震周期的更广泛采样地点和部分提供类似的结果。这种方法已经显示出在深俯冲环境中约束关键属性和过程的潜力：内在摩擦、断层弱化和随后的再强化。此外，慢滑产生的应力对俯冲带相邻危险锁定部分的可能影响要求更好地理解慢滑现象。例如，最近发生的2011年日本东北大地震似乎是在缓慢滑动之前发生的。

In the past 10 years seismologists have discovered a new type of fault behavior - epitomized by slow slip earthquakes on plate boundaries, in which slip on a fault occurs over a much greater area for a much longer time than for regular earthquakes. These events cannot be felt, but are often detectable with GPS. Here the PI studies the effects of small, but relatively well-known, stress changes on slow slip earthquakes. The gravitational attraction of the Sun and Moon on the Earth stress and slightly deform the Earth and also raise water tides that flow and add to the changing stress. These small stress changes affect deep fault zones and modulate slow slip and tremor, which is recently-discovered weak seismic radiation associated with slow slip. The tidal stresses can be calculated, so one can assess the effect of small known stress changes on the generation of tremor, which is a marker for slow slip. The results bear on how slow slip is enhanced and suppressed by tidal stressing, on levels of rock friction deep in subduction zones, and on how the fault interface weakens as slip accumulates during large slow slip earthquakes. This work provides the first such estimate of in-situ intrinsic friction deep in a plate boundary. Constraining the friction in deep subduction zones to a very low level, as preliminary results indicate, is potentially tranformative, and has implications for aspects of subduction zone science well beyond seismology, including petrology, slab stresses, and geodynamics.Episodic Tremor and Slip (ETS) is a recently-discovered phenomenon in which weak seismic signals called tremor accompany slowly-propagating shear slip on a plate boundary interface in slow earthquakes, often with durations of several weeks. Deep on the Cascadia subduction zone under northern Washington, large M6.6 to 6.8 ETS events occur with unusual regularity for a seismic phenomenon, roughly every 14 months. The proposed approach utilizes the well-calibrated tidal stresses to probe the response of the region accommodating episodic slow deformation in the deep subduction zone. Analysis of tidal stresses at the time and location of 33,000 tremors from 6 ETS events reveals the strong influence of tidal stress, and particularly, that tremor sensitivity to tidal stress evolves, increasing over several days as slow slip accumulates at a given location. This increase in sensitivity implies progressive weakening of the fault. This work will extend the study of the influence of tidal stressing to 1) tremor that occurs between the major ETS events, 2) large ETS events all along Cascadia, and 3) ETS events in Japan. The PI will also determine preferred poroelastic models that provide the best correlation between tremor and tidal stresses. The goal is to better constrain friction and fault weakening and restrengthening on the plate boundary during the ETS cycle. The preliminary research has yielded the first clear finding of evolving tremor sensitivity to tidal stress over days of ETS slip at a given spot, likely due to progressive fault weakening. These results constrain the timing and degree of fault weakening during ETS, while the proposed work will address the timing of restrengthening during the 14-month intervals between ETSs, as well as the relative behavior of tremor tidal sensitivity in different geologic and tectonic settings along the Cascadia subduction zone, and along the Nankai-Kii subduction zone. Additionally, the preliminary research has yielded the first in-situ estimate of intrinsic friction on the deep subduction interface, while the proposed work will deliver similar results for a wider sampling of locations and portions of the ETS seismic cycle. This approach has shown the potential to constrain key properties and processes in the deep subduction environment: intrinsic friction, fault weakening, and subsequent restrengthening. Furthermore, the possible effects of slow-slip-generated stress on the adjacent hazardous locked portions of subduction zones mandate better understanding of slow slip phenomena. For example, it appears that the recent devastating 2011 Tohoku earthquake was preceded by slow slip.