Earthquakes is caused by movemont of tetonic plates
volunic eruptions are caused by when there is enough of heat  inside the earth and it cannot escape.its the natures way of reducing temprature of the earth.the aprox. temprature inside should be between 3000-5000 degree celsius..
so,the relationship is that they both do harm to humans every year.

Earthquake happens whenever their is movement of tectonic plates deep down the earth. and volcanic eruption happen when inside a volcano a deep crack forms through which hot molten lava comes comes out. It is so hot that when it comes out with a great speed it also causes the movement of tectonic plates and therefore the places surrounding the volcano have to overcome earthquake till the lava coming out stops.
sometimes due to earthquake the cracks are formed where volcanic eruption takes place. so we get to know that Earthquake can cause volcanic eruption and Volcanic eruption can also cause earthquake.
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