Stress transfer cycle from slow to fast earthquakes across the southernmost Ryukyu subduction thrust during deep slow slip

Tectonic stresses at convergent plate boundaries are primarily managed through seismic ruptures and aseismic slips, both vital for maintaining lithospheric balance. Stress drops in fault nucleation zones can occur at varying rates during fast and slow earthquakes, leading to permanent stress loads and nearby structural disruptions. This stress transfer often correlates with earthquake swarms triggered by slow slips and changes in principal stress axes following seismic events. Recent studies emphasize the intricate interactions between slow and fast earthquakes, highlighting the need to understand these dynamics for crustal stress management. 2018 we commenced a decade-long research plan after observing slow slip events (SSEs) in the Ryukyu subduction zone, offshore northeastern Taiwan. These SSEs likely initiate the downdip of a seismogenic locked zone, with potential M_w ≥ 8.0 megathrust earthquake and tsunami threats. The SSE source zone is separate from the coseismic slip area of the 2002 M_w 7.1 Hualien earthquake and overlaps with the afterslip, contributing to irregular SSE recurrence patterns. A significant correlation exists between SSEs and earthquake swarms, which typically occur every few years, featuring M_w ≥ 4.0 events above the SSE source zone. Notable swarms occurred in 2005, 2009, and 2015, intensifying occurrences of these significant earthquakes. We analyzed the Central Weather Administration (CWA) events in northeastern Taiwan from 2000 to 2016, calculating temporal variations in b-values and stress orientations using data from the Broadband Array for Seismology in Taiwan (BATS). The b-values showed a marked drop during the 2009 SSE, while stress orientations exhibited a rotation in σ₁. This stress rotation likely initiated the SSEs, facilitating a cyclic transfer of stress to the earthquake swarms.