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This work presents evidence of seasonal and inter-annual variations of the earthquake occurrence rate in the Azores Triple Junction, on the Mid-Atlantic Ridge (MAR). Annual cycles in microearthquakes are relatively common in intraplate continental regions affected by large hydrological loads, but this is the first time that earthquake seasonality is recognized near mid-ocean ridges. First, we benchmark the methodology by matching the published results of earthquake seasonality in the intraplate New Madrid Seismic Zone (USA). Next, we analyze the Azores earthquake catalogue, from 2008 to 2018, separately for oceanic and island regions. The results demonstrate that the seasonal modulation of the seismicity rate is only observed in the ocean, especially in the vicinity of the triple junction, with more earthquakes occurring during the summer months from May to August. Monte Carlo simulations show that the probability of observing such seasonality by chance is less than 1% for the magnitude band from 3.3 to 4.5, well above the detection threshold and magnitude of completeness of the seismic catalogue. The methodology includes a Jack-Knife approach, which shows that the oceanic seasonality is not the consequence of abnormal or extreme events. Although we speculate about possible earthquake triggering processes, it remains a challenge to definitely establish the mechanism responsible for the observed earthquake seasonal modulation in the Azores. [ABSTRACT FROM AUTHOR]

Fluctuations in groundwater level in the Ria Formosa coastal aquifers, southern Portugal, owe 80% of the variability to climate-induced oscillations. Wavelet coherences computed between hydraulic heads and the North Atlantic Oscillation (NAO) and East Atlantic (EA) atmospheric teleconnections show nonstationary and spatially varying relationships. The NAO is the most important teleconnection and the main driver of long-term variability, inducing cycle periods of 6–10 years. The NAO fingerprint is ubiquitous and it accounts for nearly 50% of the total variance of groundwater levels. The influence of EA emerges coupled to NAO and is mainly associated with oscillations in the 2–4-year band. These cycles contribute to less than 5% of the variance in groundwater levels and are more evident further from the coast, in the northern part of the system near the main recharge area. Inversely, the power of the annual cycle increases towards the shoreline. The weight of the annual cycle (related to direct recharge) is greatest in the Campina de Faro aquifer, where it is responsible for 20–50% of the variance of piezometric levels. There, signals linked to atmospheric teleconnections (related to regional recharge) are low-pass filtered and have periods >8 years. This behavior (lack of power in the 2–8-year band) emphasizes the vulnerability of coastal groundwater levels to multi-year droughts, particularly in the already stressed Quinta do Lago region, where hydraulic heads are persistently below sea level. [ABSTRACT FROM AUTHOR]