Temporal Variations in QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ Above a Megathrust Following Episodic Slow‐Slip Events.

Recent observations beneath central Japan have shown that periodic fluid drainage occurs during slow‐slip events (SSEs) based on temporal variations in QP−1 ${Q}_{P}^{-1}$ above the megathrust boundary of the subducting Philippine Sea slab. However, no previous studies have estimated associated QS−1 ${Q}_{S}^{-1}$. A comparison of QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ can provide clues to the mechanism of seismic attenuation because of different propagation characteristics of the two waves. We estimate temporal variations in QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ via spectral analyses of waveform data from November 2009 to August 2021 period. The results indicate that both QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ exhibit temporal variations at about 1‐year periodicity and there are systematic differences between QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ that QS−1 ${Q}_{S}^{-1}$ have smaller values, less insignificant variation, and weaker correlations with SSEs. Furthermore, QP−1/QS−1 ${Q}_{P}^{-1}/{Q}_{S}^{-1}$ increases concurrent with SSEs. These differences suggest that attenuation is caused by the wave‐induced fluid flow. QP−1/QS−1 ${Q}_{P}^{-1}/{Q}_{S}^{-1}$ could be an important parameter for detecting the presence of fluid. Plain Language Summary: Q−1 ${Q}^{-1}$ value, which represents energy dissipation of seismic wave, is an important parameter for detecting the anelastic heterogeneity in subsurface rocks. However, few observational studies have revealed the characteristics of both P‐ and S‐wave attenuation (QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$, respectively) related to the presence of fluids in the crust and the uppermost mantle. This study reveals that both QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ show temporal variations associated with the inferred periodic drainage from the upper surface of a subducting slab but QS−1 ${Q}_{S}^{-1}$ is less fluctuated with time. We discuss a possible attenuation mechanism focusing on the differences in the observed QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ values and propose that the wave‐induced fluid flow (WIFF) may be a dominant control on the increases in attenuation concurrent with the drainage from the megathrust. The obtained results will provide important constraints on attenuation mechanism that is enhanced by the presence of fluid in rocks. Key Points: We estimated the long‐term temporal variations in QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ at 20–45 Hz above the megathrust boundary of the Philippine Sea PlateBoth QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ exhibited temporal variations with about 1‐year periodicity and spatiotemporal correlations with slow‐slip eventsSystematic differences between QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ may provide important constraints on attenuation mechanism enhanced by fluids [ABSTRACT FROM AUTHOR]