NASA measures sea level around the globe using satellites. The Jason-3 satellite uses radio waves and other instruments to measure the height of the ocean's surface – also known as sea level. It does this for the entire Earth every 10 days, studying how global sea level is changing over time
Scientists use seismographs that record the length and strength of each earthquake to try to determine if an eruption is imminent. Magma and gas can push the volcano's slope upward. ... But ground swelling may sometimes create huge changes in the shape of a volcano.