Favorable tidal stress triggers more tremors with higher energies

Siyuan Zhang, Heidi Houston, Shuye Huang, & Binhao Wang

Submitted September 7, 2025, SCEC Contribution #14375, 2025 SCEC Annual Meeting Poster #TBD

Tidal stress has been proposed to modulate the occurrence of tremors, low-frequency earthquakes, and even regular earthquakes in certain tectonic settings. Precisely computed tidal loading offers a unique window into earthquake physics. Variations of just a few kilopascals in tidal stress can typically affect seismic events such as tremors whose effective stress is of the same order. We further hypothesize that tidal stress may also alter event stress drops, thereby modifying radiated energies. Analyzing over 8 years of the Cascadia tremor catalog, which includes estimates of radiated energies, we observe an apparently weak positive correlation between the radiated energy and tidal stress. However, the routine application of a fixed energy-of-completeness (Ec) leads to an underestimation of this relationship: tremors with energies suppressed by unfavorable tidal stress are more likely to fall below the Ec, skewing the energy distribution across tidal stress. As a result, the energy modulation is consistently underestimated, while the rate modulation is overestimated. We develop an iterative algorithm to recover the unbiased modulations, which differ substantially from the apparent values. Applying this method reveals that during the slip of large tremor swarms, the sensitivity of tremor energy to tidal stress increases more markedly than the sensitivity of tremor occurrence. Because energy modulation has rarely been accounted for in earthquake-triggering studies, we propose that stress-induced variations in radiated energy introduce a previously unrecognized, universal bias. This suggests that retrieving the unbiased energy and rate modulations can significantly advance our understanding of fault mechanics and properties.

Citation
Zhang, S., Houston, H., Huang, S., & Wang, B. (2025, 09). Favorable tidal stress triggers more tremors with higher energies. Poster Presentation at 2025 SCEC Annual Meeting.

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Seismology