Investigation on short-term slow slip events in the northeast Japan subduction zones using decadal GNSS data

Since their discovery over 25 years ago, slow slip events (SSEs) have been regarded as key phenomena for better understanding the characteristics and kinematics of faults. Although ordinary and seismic slow earthquake activities indicate numerous SSE occurrences, the number of SSEs detected by geodetic measurements remains limited in northeast Japan, where the Pacific plate subducts beneath the Okhotsk and Philippine Sea plates. In this study, we focus on short-term SSEs (S-SSEs) with a duration of several days to weeks and investigate their activity by a systematic detection method and a time series stacking technique using data from global navigation satellite system (GNSS). By applying the systematic detection method to ~ 27-year data, we identified 71 S-SSEs. Most of them are located in the southernmost part of the analyzed region. These isolated distributions are likely attributable to the GNSS station distribution and the subduction of the Philippine Sea plate. In addition, we elucidate the Sanriku and Tokachi-Oki SSEs, which are synchronized with repeating and slow earthquake activities, respectively. We conduct the time series stacking with reference to very low-frequency earthquakes in Tokachi-Oki and average fault model estimation using displacements obtained from the stacked series to discuss their possible location range with their uncertainty. The average displacement field exhibits southeastward displacements in the coastal area, which indicates the occurrence of interplate slip. Although the estimated fault size has a large uncertainty, the average fault model is located offshore Hokkaido and overlaps with the source area of very low-frequency earthquakes, tectonic tremors, and the afterslip of the 2003 Tokachi-Oki earthquake. Our scrupulous data processing and techniques to emphasize deformation signals demonstrate the overlap between the source area of the SSEs and those of other interplate slip phenomena including repeating earthquakes, seismic slow earthquakes, and afterslip.