Tidal sensitivity of tectonic tremors in Nankai and Cascadia subduction zones

Abstract

Tectonic tremors in subduction zones, which result from slip at the deep plate interface, are known to exhibit a 12.4 h periodicity in their activity, due to tidal influence. Because tidal stress can be calculated quantitatively, the response of the plate interface can yield quantitative information about its frictional property. The relation between tremor rate and tidal stress is investigated, and an exponential relation is widely confirmed, as observed by previous studies. This study particularly focuses on spatial variations of tidal sensitivity, which are compared with spatial variations of tremor duration and amplitude. The sensitivity is quantitatively defined by the exponent of the exponential relation, which can be related to the parameter aσ, or (a - b)σ in the rate-and-state friction law, where σ is effective normal stress. On the shallower tremor zone, short-duration and large-amplitude tremors occur followed by more sensitive tremors. Meanwhile, deeper tremors with longer duration and smaller amplitude show lower sensitivity, although along-strike variation also exists. Typical and maximum sensitivities estimated here imply values for aσ or (a - b)σ of about 3 and 1 kPa, respectively. These correlations are consistent with a model in which the plate interface consists of a velocity-strengthening background with embedded velocity-weakening regions. The frictional heterogeneity may be statistically characterized by cluster size and density of the velocity-weakening regions and controls the overall slip behavior. The observed depth dependency of tremor duration, amplitude, and sensitivity implies that frictional heterogeneity is controlled by physical quantities varying with depth, such as temperature or fluid amount.