Does the Tidal Sensitivity of Tectonic Tremors Constrain Local Stress Orientation?

The activity of tectonic tremor, which is high-frequency endmember of slow earthquakes, is useful for gaining insights into the physical processes that govern slow slips and geodynamic activities along the plate boundary. While the focal mechanism of tremors is estimated from seismic waveforms, the stress states that trigger tremors are largely unknown in most areas. An exponential relationship exists between tremor rate and tidal shear stress, and the solution for a tidal sensitivity parameter can be determined using the maximum likelihood method. The likelihood function includes stress orientation, which can also be optimized. Therefore, the optimized stress orientation may have relation to their focal mechanism. In this study, we initially present a method for obtaining the optimal stress orientation with a double-couple constraint and illustrate its effectiveness by applying it to tectonic tremors in western Japan from 2004 to 2009. Our results show that, without any geometric constraints, the stress orientations derived from tidal sensitivity do not match those suggested by focal mechanisms. When we limit the analysis to a plane aligned with the local plate interface, however, some of the preferred orientations become consistent with the focal-mechanism solutions. This indicates that tidal sensitivity on its own cannot reliably determine the slip or stress orientations of slow deformation, because fault slip is guided by pre-existing weak planes rather than being free to occur in any direction. This approach introduces a novel perspective for investigating geodynamic processes occurring within active plate boundaries.