Historical seismicity, ground deformation, thermal activity, and eruptions at large Quaternary calderas of the world are common, spatially and temporally complex, and usually caused by combinations of tectonic, magmatic, and hydrologic processes. Study of historical unrest helps to define what is ordinary and what is extraordinary, and to document processes of unrest that act over periods of hours to millennia.
At least 1,299 episodes of unrest have occurred at 138 calderas >5 km in diameter during historical time. The most commonly reported unrest includes local earthquakes or earthquake swarms; volcanic tremor; uplift; subsidence; tilt; ground fissuring; changes in the temperature of soil, water, or gas; changes in fumarolic activity; and eruptions. Of 94 well-reported episodes of unrest within the past 30 years, 45 culminated in eruptions. However, at silicic calderas that had not erupted for at least 100 years, only 10 of 60 episodes of unrest led to eruptions.
Most unrest involves interaction between the local tectonic stress field, subcaldera magma, and ground water. In most unrest, the subcaldera environment gains or loses energy; often, magma and volatiles also are added or lost. A few processes of unrest occur within a closed subcaldera environment, with neither import nor export of energy or mass. We do not attempt to prove that specific processes have acted in specific episodes of unrest. Instead, we demonstrate the general plausibility of many processes and argue that any given episode of unrest likely involves a combination of these processes.
The subcaldera environment is dynamic and in delicate balance between quiet and unrest. The stimuli for unrest need not be large; most often, they are everyday geologic processes such as regional tectonic strain or small magmatic intrusions.
Many, perhaps most, episodes of unrest end without an eruption, and eruptions that do occur are generally small. Nonetheless, caldera unrest is worrisome because the largest eruptions from calderas devastate large areas and, in populous countries, could kill several million people. Caldera systems are complex, and forecasting the outcome of unrest is more difficult than at smaller volcanic centers. Residents of restless calderas will continue to be faced with difficult uncertainties and must learn to cope with false alarms.
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