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

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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.