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

震颤和慢滑移对潮汐应力的响应：断层摩擦和弱化的约束

• 批准号: 1447005
• 负责人: Heidi Houston
• 金额: $ 17万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-02-01 至 2019-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1447005&HistoricalAwards=false
• 关键词: Response; tremor; slow; slip; tidal

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.

在过去的10年里，地震学家发现了一种新型的断层行为--以板块边界上的缓慢滑动地震为缩影，在这种情况下，断层上的滑动发生在比常规地震大得多的区域上，持续时间长得多。这些事件无法被感觉到，但通常可以用GPS检测到。在这里，PI研究了小的，但相对知名的，应力变化对缓慢滑动地震的影响。太阳和月亮对地球的引力使地球产生压力并使其轻微变形，同时也引起了水潮的流动并增加了变化的压力。这些小的应力变化影响深断裂带并调制慢滑动和震颤，这是最近发现的与慢滑动相关的弱地震辐射。可以计算潮汐应力，因此可以评估已知的小应力变化对震颤产生的影响，这是缓慢滑动的标志。 结果涉及如何缓慢的滑动是增强和抑制潮汐应力，在俯冲带深处的岩石摩擦水平，以及如何在大的慢滑地震过程中，断层界面减弱滑动积累。这项工作提供了第一个这样的估计，在原位内摩擦板边界深处。如初步结果所示，将深俯冲带的摩擦力限制在非常低的水平，具有潜在的变革性，并对俯冲带科学的各个方面产生影响，远远超出了地震学，包括岩石学，板应力，幕式地震和滑动（ETS）是最近发现的一种现象，其中称为震颤的弱地震信号缓慢地伴随着-在慢地震中，板块边界界面上的剪切滑移传播，通常持续数周。在华盛顿北方的卡斯卡迪亚俯冲带深处，大约每14个月就会发生一次6.6至6.8级的大规模ETS事件，这种事件对于地震现象来说具有不寻常的规律性。所提出的方法利用校准良好的潮汐应力探测该地区的响应容纳幕式缓慢变形在深俯冲带。对来自6次ETS事件的33，000次地震的时间和位置的潮汐应力的分析揭示了潮汐应力的强烈影响，特别是，潮汐应力的地震敏感性会随着给定位置的缓慢滑动积累而逐渐增加。这种灵敏度的增加意味着故障的逐渐减弱。这项工作将扩展潮汐应力的影响的研究，1）震颤之间发生的主要ETS事件，2）大ETS事件的所有沿着卡斯卡迪亚，和3）ETS事件在日本。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。其目的是更好地约束摩擦和断层弱化和再加强的板块边界在ETS周期。初步的研究已经产生了第一个明确的发现，即在给定地点的ETS滑动的几天内，不断变化的震颤对潮汐应力的敏感性，这可能是由于进行性断层弱化。 这些结果限制了ETS期间断层弱化的时间和程度，而拟议的工作将解决ETS之间的14个月间隔期间重新加强的时间，以及沿着卡斯卡迪亚俯冲带和沿着南海纪伊俯冲带在不同地质和构造环境中的震颤潮汐敏感性的相对行为。此外，初步研究还首次对深俯冲界面的内摩擦进行了现场估计，而拟议的工作将为ETS地震周期的更广泛的位置和部分采样提供类似的结果。这种方法已经显示出限制深俯冲环境中的关键属性和过程的潜力：内摩擦，断层弱化和随后的再强化。此外，慢滑产生的应力对俯冲带的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，最近的2011年东北大地震似乎是在缓慢滑动之前发生的。