![]() Second, we create a synthetic periodic time series to parameterize lunar cycles: y lun = − cos(2 π( t − t low)/ T), where t is time in days and t low is a reference day of the calendar with neap tide (i.e., quarter moon). ![]() First, we compute the logarithm of the daily seismic amplitude ln( y sam) 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 (i.e., the average time between full moon and the next new moon) (Supplementary Fig. The analysis we undertake here consists of five main steps. Ruapehu is a good candidate for this study as it has displayed a broad spectrum of behaviour over the last decade, including several episodes of unrest, periods of quiescence, a small gas explosion (October 4, 2006), and a large phreatic eruption which occurred without warning 32, 33 (September 25, 2007). ![]() In particular, we use data from a seismic station installed at the summit of Ruapehu because: (a) the processes that are more likely to respond to tidal forcing are expected to take place at shallow levels beneath the volcanic crater 5, 31, and (b) nearly-continuous data are openly available for the last 13 years (we use data from 22-February-2004 to 15-November-2016, Geonet-GNS archive, DRZ station, vertical component 32). To overcome these limitations, 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 (hereafter called luni-seismic correlation) at Ruapehu volcano, New Zealand (Fig. This may be a statistical bias due to the unknown start time of many historical eruptions, and therefore to the small number of events considered 30. The aforementioned studies appear to yield contradictory conclusions as to the nature or existence of correlation between lunar phases (i.e., fortnightly tidal modulation) and volcanic activity. On the other hand, fortnightly tidal modulation does not apparently affect the onset of eruptions at Mauna Loa 25, 29 (Hawai’i). The same correlation with spring tides was reported for Kilauea (Hawai’i, US), Fuego (Guatemala), Ngauruhoe (New Zealand), and Mayon (Philippines) volcanoes 25, 26, 27, 28. In contrast, Sottili and Palladino 24 suggested that the frequency of small explosive events in Stromboli increases during fortnightly tidal maxima (spring tides), that is, close to full or new moon. The same correlation with neap tides was reported for five of the six dome extrusions occurring between 18 in Islas Quemadas volcano 23 (El Salvador). For example, Johnston and Mauk 22 suggested that major eruptions at Stromboli (Italy) start preferentially close to the fortnightly tidal minima (neap tides), that is, when the moon phase is close to the first or third quarter. The most intriguing debate lies on whether fortnightly tides, a ~14-day amplitude modulation of the daily tidal stresses that is related to lunar phases, affect volcanic activity or even force eruptions to occur some specific days instead of others. The possibility that Moon-Sun gravitational forces can influence terrestrial volcanism has been widely debated over the last century 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21. 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. 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. 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. However, previous studies found contradictory results and remain mostly inconclusive. ![]() 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. 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.
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