Tracking short-term slow slip along Nankai with GNSS: temporal evolution of slip rate and interaction with the seismogenic zone

Short-term slow slip events (S-SSEs) occur at depths greater than ~30 km along the Nankai subduction zone in southwest Japan. Previous studies have successfully detected these events using GNSS data and characterized their spatial distribution along the subduction interface. However, fundamental questions remain regarding their role in the slip budget: what fraction of accumulated interseismic strain do S-SSEs release, and does their behavior remain stationary over time? Addressing these questions requires resolving the temporal evolution of S-SSE slip patterns — an aspect that has remained largely unexplored.

Here, we analyze nearly two decades of GNSS data to quantify the slip contribution of S-SSEs and to investigate their temporal evolution. To resolve these small-amplitude, short-lived signals, we leverage the exceptional density of the GNSS network (>700 stations) by stacking time series from triplets of nearby stations, enhancing the signal-to-noise ratio. Using tremor and LFE timing to synchronize the detection, we measure offsets before and after each episode and infer local slip rates. By applying this approach across multiple successive time windows, which has not been done in Nankai or other subduction zones, we track how slip patterns evolve through time.

We find that S-SSEs release a significant fraction of accumulated slip within the tremor/LFEs zone (between 40 and 30%, which is consistent with the long-term coupling of 60%). Still, this contribution and the associated spatial slip pattern vary across different time periods. Our results reveal that S-SSE behavior is not stationary: the along-strike slip distribution and slip rates show systematic changes over multi-year timescales. We also observe that S-SSE slip occasionally extends to shallower depths, approaching the base of the seismogenic zone.

These spatio-temporal variations in slow slip provide new constraints on the evolution of interplate coupling and on how stress accumulation in the seismogenic zone may be modulated by deeper slow slip processes.