Anatomy of an eruption: The inflation-deflation cycle

During inflation the rocks surrounding the reservoir become stressed, and this stress is partly relieved by increasing numbers of earthquakes, too small to be felt, but easily recorded by seismometers at Kilauea summit. These earthquakes (called short- period or tectonic) are recorded as high- frequency features on a seismograph. During deflation the stress is completely relieved. The short-period earthquakes stop, but their place is taken by low-frequency earthquakes (called long-period or volcanic), which reflect adjustments related to the exit of magma from the summit reservoir to feed the eruption. The long-period earthquakes are related to harmonic tremor, the continuous seismic record of underground magma movement.

Kilauea's distinctive inflation-deflation pattern is seen for nearly every eruption, regardless of the amount of tilt change observed. For example, the pattern is dramatically shown for the Kilauea Iki eruption in 1959, which involved the largest tilt change observed to date

Forecasting eruptions

Accurate short-term forecasts of Hawaiian eruptions are based primarily on analyses of inflation-deflation patterns, made possible through decades of study of ground deformation (tilt) and seismicity (earthquakes and harmonic tremor). When the level of inflation and the short-period earthquake counts are high, the volcano is ready to erupt. Sometimes there is~ a delay of days or even weeks before eruption occurs, but scientists can be ready to study the activity when it occurs. Eruption is signaled by the beginning of sharp deflation accompanied by either harmonic tremor or earthquakes close to the site of eruptive outbreak. These signals are usually seen an hour to several hours before lava breaks the surface and allow scientists enough time to travel to the likely site of activity.