<Li> Groundwater levels begin to change, the temperature of hot springs may rise and the chemical composition and amount of gases released may alter . Temperature sensors are placed in bore holes which are used to detect ground water temp . Remote sensing is used on Sakurajima since the gases are highly toxic--the ratio of HCl gas to SO gas increases significantly shortly before an eruption . </Li> <Li> As an eruption approaches, tiltmeter systems measure minute movements of the mountain . Data is relayed in real - time to monitoring systems at SVO . </Li> <Li> Seismometers detect earthquakes which occur immediately beneath the crater, signaling the onset of the eruption . They occur 1 to 1.5 seconds before the explosion . </Li> <Li> With the passing of an explosion, the tiltmeter system records the settling of the volcano . </Li>

One of the most important warning signs of a volcanic eruption is