Abstract
A long-standing question in Earth Science is the extent to which seismic and volcanic activity can be regulated by tidal stresses, a repeatable and predictable external excitation induced by the Moon-Sun gravitational force. Fortnightly tides, a ~14-day amplitude modulation of the daily tidal stresses that is associated to lunar cycles, have been suggested to affect volcano dynamics. However, previous studies found contradictory results and remain mostly inconclusive. Here we study how fortnightly tides have affected Ruapehu volcano (New Zealand) from 2004 to 2016 by analysing the rolling correlation between lunar cycles and seismic amplitude recorded close to the crater. The long-term (~1-year) correlation is found to increase significantly (up to confidence level of 5-sigma) during the ~3 months preceding the 2007 phreatic eruption of Ruapehu, thus revealing that the volcano is sensitive to fortnightly tides when it is prone to explode. We show through a mechanistic model that the real-time monitoring of seismic sensitivity to lunar cycles may help to detect the clogging of active volcanic vents, and thus to better forecast phreatic volcanic eruptions.
Highlights
We analyse whether fortnightly tides affect volcanoes by addressing the following questions: is the persistent seismicity recorded around active volcanic centres sensitive to fortnightly tidal modulation? If so, can we use this sensitivity to detect when a volcano is in a critical state and prone to erupt? We tackle these questions by exploring the correlation between lunar phases and seismic amplitude at Ruapehu volcano, New Zealand (Fig. 1)
We compute the logarithm of the daily seismic amplitude ln(ysam) from the raw seismic data, after applying a 20-day high-pass median filter to remove the influence of potential processes occurring over timescales greater than the average periodicity of spring tides, T = 14.7653 days (Supplementary Fig. S1)
We use backward windows of 1 year (i.e., >300 data pairs capturing more than 20 spring/neap tides), which allow us to explore the existence of long-term luni-seismic correlation and preserves a statically relevant sample size; the analysis is performed with backward windows from 100 to 600 days for the sake of comparison
Summary
E) we repeat steps b–d with different values of the cap permeability κ and porosity φ (related through equation (4)) to explore how overall cap sealing (e.g., due to pore mineralization or subsidence of the crater floor after the 2006 gas explosion) affects the luni-seismic correlation This analysis reveals that the luni-seismic correlation is significant when the permeability of the cap κ is below a threshold value; and it is negative as long as the phase shift δ meets π/2 < δ < 3π/2 and if the response time of the volcano to tidal stresses ranges between ~3 h and ~9 h (see Supplementary Discussion and Supplementary Fig. S13)
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