Their findings reveal clear periodic patterns in seismic velocity changes on daily, semi-daily, and monthly timescales (Figure 2, C-F), with these variations being most pronounced within the fault zone. Comparing these results with theoretical tidal strain models, the researchers observed strong correlations across all three timescales (Figure 2, D-F), indicating that tidal forces are a primary driver of these temporal fluctuations.
Further analysis using a spectral ratio method from teleseismic waveforms showed that the fault fracture zone exhibited significantly higher spectral ratio values (higher amplification) than surrounding areas, indicating a higher degree of subsurface fracturing (Figure 2B). Tidal forces affect seismic velocity by causing tiny underground fractures to open and close periodically—velocity decreases when fractures open while increasing when they close. Due to its highly damaged nature, the fault fracture zone is particularly sensitive to these tidal influences (Figure 3).
This study, utilizing an ambient noise-based monitoring approach, provides new insights into how tidal forces impact fault systems. It offers valuable information for understanding fault dynamics, earthquake nucleation processes, and seismic hazards.
