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USGS Seismograph of magmatic earthquakes caused by the dome growth of Mt St Helens in 1980  A Seismograh or seismometer is an instrument for measuring the movement of the earth caused by earthquakes. Volcanoes produce magmatic quakes as magma moves within the crust. Monitoring the depth,frequency and strength of these quakes can help predict when a volcanic eruption may occur. In the past seismographs consisted of a trace drawn on a roll of paper nowadays more modern examples are all digital and on a computer.