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

Abstract

Abstract Recent observations beneath central Japan have shown that periodic fluid drainage occurs during slow‐slip events (SSEs) based on temporal variations in QP−1 above the megathrust boundary of the subducting Philippine Sea slab. However, no previous studies have estimated associated QS−1. A comparison of QP−1 and QS−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 and QS−1 via spectral analyses of waveform data from November 2009 to August 2021 period. The results indicate that both QP−1 and QS−1 exhibit temporal variations at about 1‐year periodicity and there are systematic differences between QP−1 and QS−1 that QS−1 have smaller values, less insignificant variation, and weaker correlations with SSEs. Furthermore, QP−1/QS−1 increases concurrent with SSEs. These differences suggest that attenuation is caused by the wave‐induced fluid flow. QP−1/QS−1 could be an important parameter for detecting the presence of fluid.

Keywords

• slow‐slip events
• attenuation
• fluid