Tidal stress influence on slow slip on the deep plate interface

Tectonic tremors and slow earthquakes, which are detected in many subduction zones and transform faults, are characterized by various quantities, such as duration, recurrence interval, and the sensitivity to tidal stress. These characteristic quantities are spatially variable among regions and even within an individual region. It is also known that the seismic energy release by tremor also spatially varies (e.g., Maeda and Obara, 2009; Ando et al., 2012; Yabe and Ide, in review). We interpret the spatial variation in seismic energy rate (amplitude) of tremors to represent the variation of plate-interface strength, in which case the spatial variation of tidal sensitivity might correlate with that of seismic energy rate. To investigate such relation, we have investigated the spatial variation of tidal sensitivity in Nankai and Cascadia subduction zones. Although the spatial variation of tidal sensitivity has been investigated using the frequency spectrum at specific tidal frequency, we calculate the time history of tidal stressing on the plate interface and compare it with tremor activity to investigate their relation. Both body tide and ocean tide are included in the calculation of tidal stress. We calculated Green functions for the spherical Earth based on the method by Okubo and Tsuji (2001). The time history of sea surface level is calculated with the SPOTL program (Agnew, 2012). The fault planes of VLF estimated by Ide and Yabe (2014) are used to calculate stress on the plate interface. Tremors are more likely to occur when tidal shear stress is larger, in the subduction direction. Tremor rate appears to increase exponentially with tidal shear stress, as previously seen by Houston (2013, AGU). Although tidal sensitivity can be seen in many regions, some regions show particularly strong sensitivity. In some regions, tidal sensitivity can be seen clearly during the later portions of large SSEs, consistent with behavior reported by Houston (2013, AGU), while other regions show tidal sensitivity without large SSEs as well. In the regions where tidal sensitivity can be seen, the rake direction of VLF is consistent with the direction of maximum tidal shear stress on that fault plane. This observation might imply that tidal stress significantly modulates the timing of slow slip on the deep plate interface.