Lunar and solar tides rhythmically stress Earth at the tidal periods. These cyclic variations, ranging from 10 to 100 hPa (Tanaka, 2010), appear small compared to earthquake stress drops, which lie between 1,000 and 10 5 hPa (Métivier et al., 2009). Nevertheless, tidal stressing is nonlinear in nature and reaches rates of up to 10 hPa/hour, often higher than tectonic stress rates between earthquakes, which reach ∼0.2 hPa/hour (Emter, 1997). As rapid tidal stresses are superimposed on the far slower, incremental tectonic stresses, they may provide a potential triggering or modulating effect on seismicity rates. As a result, a good deal of research has been carried out that studies the relationship between tides and earthquakes. A detailed review of this subject can be found in the supporting information (Text S1).

The volcanic eruption at La Palma started on September 19, 2021. The eruption was preceded by a seismic swarm that began on September 11, although anomalous seismicity has been observed on the island since 2017. During the co-eruptive phase of the seismic activity, hypocenters depth was generally less than 15 km, save for the period between November 10 and November 27, when hypocenters ranged in the depth from 15 to 40 km. The eruption ended on December 13, 2021. We compute tidal stress for each earthquake at the hypocenter depth and find statistically significant correlations between the occurrence times of the earthquakes and the confining tidal stress values and stress rates. The correlation is depth-dependent, and ocean-loading tides have a stronger effect than body tides. We find that tidal stress variations contribute to the eruption onset and that certain explosive events, typical in Strombolian type volcanoes, seem to occur preferentially when the tidal stress rate is high. Our analysis supports the hypothesis that tides may modulate earthquake activity in volcanic areas, specifically during magma migration at shallow depths. A conceptual model is proposed, which could have a general application in the Canary Islands archipelago and other volcanic islands.

Abstract.The interpretation of tidal gravity final residuals is an issue of high debate in geodesy. Whereas some authors suggest that final residuals are related to possible instrumental and measurement errors or to errors in the evaluation of the oceanic load computations, others propose certain empirical relationships between the gravity tidal residuals and the heat flow or the tectonothermal age of the lithosphere which, in turn, reveal structural variations with respect to a homogeneous elastic model.We examine such relationships in the Iberian Peninsula by studying correlations between the cosine component of the final residual vector for the M 2 and O 1 waves of the gravity tide potential and different structural parameters for a total of 21 gravity tide stations. The selected parameters, which are related to the capability of the lithosphere to deform, include surface heat flow as the main parameter and, to a lesser extend, lithospheric strength, Moho temperature and tectonothermal age of the lithosphere. Our study agrees with previous results by other authors and does not demonstrate the existence of empirical relationships between the gravity residuals and structural parameters. Actually, when we consider the most precise observations and recent ocean tide models the final residuals are mostly uncorrelated noise. Our results also agree with theoretical studies proposing that the distortions of the tidal gravity field from spherical symmetry are very small and hardly to observe.

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