High‐Rate GNSS Analysis of the Onset, Growth, and Termination of Slow Slip Events and Their Along‐Dip Variability Offshore the Boso Peninsula, Japan (2011–2024)

Abstract Recurring slow slip events (SSEs) with moment magnitudes in the mid‐6 range have occurred every few years since the 1990s offshore the Boso Peninsula, Japan. These events occur near the source area of ∼M8 megathrust earthquakes beneath the Tokyo metropolitan area and are accompanied by earthquake swarms. A previous study using daily Global Navigation Satellite System (GNSS) data identified two distinct nucleation styles. Here, we investigate spatiotemporal variability of four SSEs (2011, 2013–2014, 2018, and 2024) using 5‐minute GNSS data. To leverage the high temporal resolution, we apply sparse change‐point analysis to identify candidate onset and termination phases of the events from GNSS time series. These trend segments are then used to derive displacement vectors. Fault slip evolution is then estimated through Bayesian inversion using a horseshoe prior to allow for spatially localized slip. The four SSEs exhibit substantial variability. The 2011 event nucleated closest to land at shallow depth and propagated up‐dip, showing abrupt onset and gradual termination. The 2013–2014 and 2024 events initiated offshore at greater depths and displayed near‐symmetric temporal evolution, though the former was smaller and slower. The 2018 event began at greater depth near the 2013–2014 source area and propagated toward shallower regions, overlapping the 2011 source area, with a sharper termination than onset. Despite quasi‐periodic recurrence, these SSEs differ markedly in depth, propagation, and temporal pattern. These differences may reflect quasi‐static frictional heterogeneity at spatial scales of about 10 km. Enhanced monitoring, especially offshore, is essential to resolve these dynamics and event durations accurately.