Detection of a recent large Hyuga-nada long-term slow slip event and estimation of its spatiotemporal slip distributions

Abstract

Using horizontal and vertical GNSS time series data from the GSI in sotheastern Kyushu from January 1, 2017 to June 30, 2022, we detected a recent long-term slow slip event (L-SSE) that occurred in the Hyuga-nada region, southwest Japan, and estimated its spatiotemporal slip distribution. We performed such analysis considering the piecewise evolution of slip over time with time windows of 0.8 years to ensure a good signal-to-noise ratio in the horizontal displacements in each time window. The results showed that a slip of more than 2 cm occurred in the central part of Miyazaki Prefecture from 2018.5 to 2019.3 (the units were 0.1 years = 36.5 days). Then, the amount of slip increased and expanded slightly to the southern part of Miyazaki Prefecture from 2019.3 to 2020.1, and the amount of slip reached a maximum of 3.9 cm in the subsequent period (2020.1–2020.9). A smaller slip occurred at almost the same location in the following 0.8 years. Therefore, the duration of the L-SSE was approximately 3.2 years from approximately 2018.5 to 2021.7. The annual average maximum slip rate was approximately 4.9 cm/yr (3.9 cm/0.8 yr) during the period from 2020.1 to 2020.9. The maximum total slip was estimated to be approximately 12.9 cm, and the equivalent release moment was 4.9 × 1019 Nm, corresponding to Mw7.1. Compared to previous L-SSEs that occurred in the Hyuga-nada region, the annual average maximum slip rate was relatively low, and the main total slip was estimated at almost the same location on the plate interface. On the other hand, the slip duration of this L-SSE was the longest, and the release moment and moment magnitude were the greatest. The total slip area of more than approximately 10 cm in the Hyuga-nada L-SSE estimated in this study almost overlapped with the afterslip area of the December 3, 1996 Hyuga-nada earthquake, with a depth range of approximately 30–40 km. Short-term SSEs also occurred in the same depth range. These coincidental same depth ranges of interplate seismic events stem from large thermal gradients. The large thermal gradients play an important role in narrowing the depth range of frictional parameters (a–b) at the plate interface. In addition, the pore pressure and normal stress are closely related to the critical stiffness. In particular, contrasting values of pore pressure and normal stress at the shallow and deep sides of the mantle wedge corner may also contribute to transient aseismic slip within the same depth range.