Strain rate of Boso slow slip events related to seismic activity after 2011 using GNSS data

The March 11, 2011, M_W9.0 Tohoku-Oki earthquake, in Japan, caused rapid strain release near the epicenter, while the Boso Peninsula, located farther away, experienced stress redistribution, leading to changes in the recurrence interval of slow slip events (SSEs) and regional strain. This study focuses on three detected post-2011 Boso SSEs, utilizing a segmented model displacement time series measured by Global Navigation Satellite System (GNSS) to calculate velocity and strain rate fields for eight periods before, during, and after the SSEs. Results show that the 2011 earthquake and the three SSEs significantly alter the velocity field in the Boso region, with SSE velocities predominantly oriented southeast, reaching maximum values of 26.9 cm/a, 10.6 cm/a, and 38.5 cm/a—nearly opposite to non-SSE periods. After the third SSE, the velocity field nearly returns to its pre-earthquake state, with a maximum of 1.8 cm/a. The maximum shear strain rates during the three SSEs are 25.88 × 10^-7 a^-1, 11.38 × 10^-7 a^-1, and 29.02 × 10⁻⁷ a⁻¹ (i.e., per annum), significantly higher than those during non-slow slip periods, with principal strain rates following a similar pattern. The spatial distribution of strain rates during the SSEs indicates greater deformation compared to the non-slip periods, dominated by northwest-southeast extension and southwest-northeast compression. Spatiotemporal analysis reveals a strong correlation between seismic frequency and strain rate during the SSEs, with time correlation coefficients of 0.85, 0.88, and 0.9. Although larger accumulated strain results in stronger strain release during the latter two SSEs, not all strain is fully released, suggesting that earthquake swarms accompanying the SSEs may contribute to the partial release of unreleased strain. This study, through the analysis of GNSS data, evaluates the spatiotemporal distribution of strain fields during periodic SSEs, contributing to further research on strain accumulation and release, and aiding in the analysis of this regional seismic activity.