<Li> Likely activity is signalled by swelling of the land around the volcano as magma below begins to build up . At Sakurajima, this is marked by a rise in the seabed in Kagoshima Bay--tide levels rise as a result . </Li> <Li> As magma begins to flow, melting and splitting base rock can be detected as volcanic earthquakes . At Sakurajima, they occur two to five kilometres beneath the surface . An underground observation tunnel is used to detect volcanic earthquakes more reliably . </Li> <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>

Which of the following are methods used by geologists to predict a volcanic eruption