On a planetary forcing of global seismicity

Abstract: We have explored the temporal variability of the seismicity at global scale over the last 124 years, as well as its potential drivers. To achieve this, we constructed and analyzed an averaged global seismicity curve for earthquakes of magnitude equal or greater than 6 since 1900. Using Singular Spectrum Analysis, we decomposed this curve and compared the extracted pseudo-cycles with two global geophysical parameters associated with Earth's tides: length-of-day variations and sea-level changes. Our results reveal that these three geophysical phenomena can be be explained with 90% accuracy, as the sum of up to seven periodic components, largely aligned with planetary ephemerides: 1 year, 3.4 years (Quasi-Biennial Oscillation, QBO), $\sim$11 years, $\sim$14 years, $\sim$18.6 years (lunar nodal cycle), $\sim$33 years, and $\sim$60 years. We discuss these results in the framework of Laplace's theory, with a particular focus on the phase relationships between seismicity, length-of-day variations, and sea-level changes to further elucidate the underlying physical mechanisms. Finally,integrating observations from seismogenic regions, we propose a trigger mechanism based on solid Earth-hydrosphere interactions, emphasizing the key role of water-rock interactions in modulating earthquake occurrence.

On a Planetary Forcing of Global Seismicity: A Summary for Experts

The paper "On a Planetary Forcing of Global Seismicity" by Dumont et al. investigates the potential influence of planetary and astronomical forces on the temporal variability of global seismicity over the last 124 years. The authors aim to establish a connection between seismic activities of magnitude 6 or greater and global geophysical parameters associated with Earth's tides, such as length-of-day (LOD) variations and sea-level changes.

Methodology

The research leverages singular spectrum analysis (SSA) to decompose a global seismicity curve into periodic components, which are then compared with cycles derived from LOD and sea-level records at the Brest tide gauge. The study identifies seven significant periodic components, including a 1-year seasonal cycle, a 2.3-year Quasi-Biennial Oscillation (QBO), and cycles of approximately 11 years, 14 years, 19 years (lunar nodal cycle), 33 years, and 60 years.

Key Findings

1. Periodic Components: The three geophysical phenomena (seismic activity, LOD, and sea level) can be explained with up to 90% accuracy when considering periodic components. The identified cycles correspond to commensurable periods tied to planetary positions, implying a possible astronomical influence on seismicity.

2. Consistent Phase Relationships: Most cycles showed a consistent phase relationship that aligns with planetary ephemerides, notably a phase quadrature or phase opposition. These relationships suggest a system influenced by planetary gravitational forces, which are consistent over time despite the chaotic nature of earthquake occurrences.

3. Presence of Known Oscillations: The QBO, a critical atmospheric oscillation of around 2.3 years, is notably detected in both seismic activity and LOD data, marking its first clear appearance in seismic analyses.

Theoretical Implications

The findings suggest that global seismic activity might be modulated by external astronomical forcing, mediated through angular momentum exchanges that affect Earth's hydrosphere. This proposition is consistent with Laplace's theory, which predicts such a quadrature phase shift as a result of dynamic interactions within Earth's rotational mechanics.

The paper challenges traditional views that dismiss external astronomical influences on seismicity, proposing that hydrosphere-solid Earth interactions could serve as a trigger for seismic events. Such interactions could modulate the effective stresses within Earth's crust, a conclusion supported by similar oscillation patterns observed in global volcanism records.

Practical Implications and Future Directions

If validated, this hypothesis provides a framework for predicting seismic events at a global scale, not based solely on local geological conditions but as part of a broader, predictable system modulated by long-term geophysical cycles. Future research could expand on the mechanisms of water-rock interactions in seismic triggering, perhaps focusing on the role of groundwater and tidal forces in long-term stress accumulation and release.

Overall, Dumont et al. present a compelling case for reevaluating how we consider astronomical influences in seismology, pointing to a synthesis of geophysical data that links Earth's seismic activity to broader celestial mechanics. The paper calls for further investigation into the precise mechanisms driving these interactions, potentially paving the way for advancements in earthquake forecasting.