Detection of Long- and Short-Term Slow Slip Events Using a Network Inversion Filter and Dense Global Navigation Satellite System Network in Shikoku, Japan

The study of slow slip events is essential to elucidating the slip mechanisms and frictional properties of plate interfaces. We imaged the aseismic slip at the plate interface between the subducting Philippine Sea Plate and overriding Amur Plate beneath Shikoku Island, Japan, by applying a network inversion filter to the position time series obtained from Global Navigation Satellite System data spanning January 2012 to April 2022. The results show long-term slow slip events in the Bungo Channel, central Shikoku, and the Kii Channel, which started from 2018. We also identified 39 short-term slow slip events associated with low-frequency tremors. The estimated moment magnitudes of the Bungo Channel, central Shikoku, and Kii Channel slow slip events were Mw7.0, Mw6.4, and Mw6.3, respectively, with a rigidity of 30 GPa. The estimated moment magnitudes of the short-term slow slip events ranged from Mw5.7 to Mw6.4. Most of the detected short-term slow slip events are associated with low-frequency tremor, and many showed propagation through the spatial domain. Events with magnitudes of >=;Mw6.30 typically consist of subevents. The slip rate of short-term slow slip events ranges from 1 to 7 mm/day, which is considerably larger than the plate convergence rate. The short-term slow slip events are 6 to 40 days, and follow the accepted scaling law between slip event duration and total magnitude. The short-term slow slip event zone is consistent with a three-segment hypothesis. The short-term slow slip event in northwest Shikoku and long-term slow slip event in central Shikoku may be correlated.