Influence of tidal effects on the periodicity of earthquake activity in diverse geological settings with particular emphasis on submarine hydrothermal systems

Hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids. It is suggested that they correspond to the cracking of the ocean crust at the brittle–ductile transition that marks the downward limit of seawater circulation into the ocean crust. Semi-diurnal variations in submarine hydrothermal discharge in backarc settings and at the crests of mid-ocean ridges have been detected by hydrophone but no influence of tidal effects on the periodicity of microearthquakes around submarine hydrothermal vents has been observed so far. Although tidal stresses are too small to initiate earthquakes themselves, they may trigger earthquake activity when imposed on slowly increasing tectonic stress. These effects therefore tend to be observed in geologically active regions where stress levels in the earth's crust are high. The influence of tides on the periodicity of microearthquakes tends to be complex because the effects of ocean and earth tides are not uniform at the surface of the earth and because the microearthquakes occur in a wide variety of geological settings. Earth tides cause compression and dilatation of the oceanic crust which may be the most significant factor in initiating changes in fluid flow rate and microseismicity in submarine hydrothermal systems. Much of the information now available on the tidal periodicity of microearthquakes is based on relatively short-term monitoring which lacks statistical validity. The development and installation of deep-sea monitoring stations will permit rigorous long-term monitoring programmes to be carried out, which will lead to a better understanding of the nature and periodicity of microearthquakes around submarine hydrothermal vents.