Monitoring
Mauna Loa is an intensively monitored volcano. The Hawaiian Volcano Observatory (HVO) was established in 1912 to observe the Hawaiian volcanoes, and the HVO has developed many techniques to help predict when eruptions at Mauna Loa and other volcanoes are imminent.
One of the most important tools is seismometry. More than 60 seismometers around the Big Island enable scientists to measure the intensities and locations of hundreds of small earthquakes every week. Earthquakes can begin to increase years before an eruption actually starts: The 1975 and 1984 eruptions were both preceded by one to two years of increased seismic activity at depths of less than 8 miles (13 km).
Another type of seismic activity occurs in the hours preceding an eruption. So-called harmonic tremor is a continuous "rumble" which contrasts with the normal seismic activity of sudden shocks and is believed to be caused by the rapid movement of magma underground. Volcanic tremor normally indicates an imminent eruption, although it may also be caused by shallow intrusions of magma which do not reach the surface.
Another important indicator of what is happening underground is the shape of the mountain. Tiltmeters measure very small changes in the profile of the mountain, and sensitive equipment measures distances between points on the mountain. As magma fills the shallow reservoirs below the summit and rift zones, the mountain inflates. A survey line across the caldera measured a 3-inch (76 mm) increase in its width over the year preceding the 1975 eruption and a similar increase before the 1984 eruption.
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